Section 7: Packages
1 [{Package} [Glossary Entry]Packages are program units that allow the
specification of groups of logically related entities. Typically, a package
contains the declaration of a type (often a private type or private extension)
along with the declarations of primitive subprograms of the type, which can be
called from outside the package, while their inner workings remain hidden from
outside users. {information hiding: See package} {encapsulation: See package}
{module: See package} {class: See also package} ]
7.1 Package Specifications and Declarations
1 [A package is generally provided in two parts: a package_specification and
a package_body. Every package has a package_specification, but not all
packages have a package_body.]
Syntax
2 package_declaration ::= package_specification;
3 package_specification ::=
package defining_program_unit_name is
{basic_declarative_item}
[private
{basic_declarative_item}]
end [[parent_unit_name.]identifier]
4 If an identifier or parent_unit_name.identifier appears at the end of
a package_specification, then this sequence of lexical elements shall
repeat the defining_program_unit_name.
Legality Rules
5/2 {AI95-00434-01} {requires a completion (package_declaration) [partial]}
{requires a completion (generic_package_declaration) [partial]} A
package_declaration or generic_package_declaration requires a completion [(a
body)] if it contains any basic_declarative_item that requires a completion,
but whose completion is not in its package_specification.
5.a To be honest: If an implementation supports it, a pragma Import
may substitute for the body of a package or generic package.
Static Semantics
6/2 {AI95-00420-01} {AI95-00434-01}
{visible part (of a package (other than a generic formal package)) [partial]}
The first list of basic_declarative_items of a package_specification of a
package other than a generic formal package is called the visible part of the
package. [{private part (of a package) [partial]} The optional list of
basic_declarative_items after the reserved word private (of any
package_specification) is called the private part of the package. If the
reserved word private does not appear, the package has an implicit empty
private part.] Each list of basic_declarative_items of a
package_specification forms a declaration list of the
package.{declaration list (package_specification) [partial]}
6.a Ramification: This definition of visible part does not apply to
generic formal packages - 12.7 defines the visible part of a
generic formal package.
6.b The implicit empty private part is important because certain
implicit declarations occur there if the package is a child
package, and it defines types in its visible part that are derived
from, or contain as components, private types declared within the
parent package. These implicit declarations are visible in
children of the child package. See 10.1.1.
7 [An entity declared in the private part of a package is visible only
within the declarative region of the package itself (including any child units
- see 10.1.1). In contrast, expanded names denoting entities declared in the
visible part can be used even outside the package; furthermore, direct
visibility of such entities can be achieved by means of use_clauses (see
4.1.3 and 8.4).]
Dynamic Semantics
8 {elaboration (package_declaration) [partial]} The elaboration of a
package_declaration consists of the elaboration of its
basic_declarative_items in the given order.
NOTES
9 1 The visible part of a package contains all the information that
another program unit is able to know about the package.
10 2 If a declaration occurs immediately within the specification of a
package, and the declaration has a corresponding completion that is a
body, then that body has to occur immediately within the body of the
package.
10.a Proof: This follows from the fact that the declaration and
completion are required to occur immediately within the same
declarative region, and the fact that bodies are disallowed (by
the Syntax Rules) in package_specifications. This does not apply
to instances of generic units, whose bodies can occur in
package_specifications.
Examples
11 Example of a package declaration:
12 package Rational_Numbers is
13 type Rational is
record
Numerator : Integer;
Denominator : Positive;
end record;
14 function "="(X,Y : Rational) return Boolean;
15 function "/" (X,Y : Integer) return Rational; -- to construct a rational number
16 function "+" (X,Y : Rational) return Rational;
function "-" (X,Y : Rational) return Rational;
function "*" (X,Y : Rational) return Rational;
function "/" (X,Y : Rational) return Rational;
end Rational_Numbers;
17 There are also many examples of package declarations in the predefined
language environment (see Annex A).
Incompatibilities With Ada 83
17.a {incompatibilities with Ada 83} In Ada 83, a library package is
allowed to have a body even if it doesn't need one. In Ada 95, a
library package body is either required or forbidden - never
optional. The workaround is to add pragma Elaborate_Body, or
something else requiring a body, to each library package that has
a body that isn't otherwise required.
Wording Changes from Ada 83
17.b We have moved the syntax into this clause and the next clause from
RM83-7.1, "Package Structure", which we have removed.
17.c RM83 was unclear on the rules about when a package requires a
body. For example, RM83-7.1(4) and RM83-7.1(8) clearly forgot
about the case of an incomplete type declared in a
package_declaration but completed in the body. In addition, RM83
forgot to make this rule apply to a generic package. We have
corrected these rules. Finally, since we now allow a pragma Import
for any explicit declaration, the completion rules need to take
this into account as well.
Wording Changes from Ada 95
17.d/2 {AI95-00420-01} Defined "declaration list" to avoid ambiguity in
other rules as to whether packages are included.
7.2 Package Bodies
1 [In contrast to the entities declared in the visible part of a package,
the entities declared in the package_body are visible only within the
package_body itself. As a consequence, a package with a package_body can be
used for the construction of a group of related subprograms in which the
logical operations available to clients are clearly isolated from the internal
entities.]
Syntax
2 package_body ::=
package body defining_program_unit_name is
declarative_part
[begin
handled_sequence_of_statements]
end [[parent_unit_name.]identifier];
3 If an identifier or parent_unit_name.identifier appears at the end of
a package_body, then this sequence of lexical elements shall repeat
the defining_program_unit_name.
Legality Rules
4 A package_body shall be the completion of a previous package_declaration
or generic_package_declaration. A library package_declaration or library
generic_package_declaration shall not have a body unless it requires a body[;
pragma Elaborate_Body can be used to require a library_unit_declaration to
have a body (see 10.2.1) if it would not otherwise require one].
4.a Ramification: The first part of the rule forbids a package_body
from standing alone - it has to belong to some previous
package_declaration or generic_package_declaration.
4.b A nonlibrary package_declaration or nonlibrary
generic_package_declaration that does not require a completion may
have a corresponding body anyway.
Static Semantics
5 In any package_body without statements there is an implicit
null_statement. For any package_declaration without an explicit completion,
there is an implicit package_body containing a single null_statement. For a
noninstance, nonlibrary package, this body occurs at the end of the
declarative_part of the innermost enclosing program unit or block_statement;
if there are several such packages, the order of the implicit package_bodies
is unspecified. {unspecified [partial]} [(For an instance, the implicit
package_body occurs at the place of the instantiation (see 12.3). For a
library package, the place is partially determined by the elaboration
dependences (see Section 10).)]
5.a Discussion: Thus, for example, we can refer to something happening
just after the begin of a package_body, and we can refer to the
handled_sequence_of_statements of a package_body, without worrying
about all the optional pieces. The place of the implicit body
makes a difference for tasks activated by the package. See also
RM83-9.3(5).
5.b The implicit body would be illegal if explicit in the case of a
library package that does not require (and therefore does not
allow) a body. This is a bit strange, but not harmful.
Dynamic Semantics
6 {elaboration (nongeneric package_body) [partial]} For the elaboration of a
nongeneric package_body, its declarative_part is first elaborated, and its
handled_sequence_of_statements is then executed.
NOTES
7 3 A variable declared in the body of a package is only visible within
this body and, consequently, its value can only be changed within the
package_body. In the absence of local tasks, the value of such a
variable remains unchanged between calls issued from outside the
package to subprograms declared in the visible part. The properties of
such a variable are similar to those of a "static" variable of C.
8 4 The elaboration of the body of a subprogram explicitly declared in
the visible part of a package is caused by the elaboration of the body
of the package. Hence a call of such a subprogram by an outside
program unit raises the exception Program_Error if the call takes
place before the elaboration of the package_body (see 3.11).
Examples
9 Example of a package body (see 7.1):
10 package body Rational_Numbers is
11 procedure Same_Denominator (X,Y : in out Rational) is
begin
-- reduces X and Y to the same denominator:
...
end Same_Denominator;
12 function "="(X,Y : Rational) return Boolean is
U : Rational := X;
V : Rational := Y;
begin
Same_Denominator (U,V);
return U.Numerator = V.Numerator;
end "=";
13 function "/" (X,Y : Integer) return Rational is
begin
if Y > 0 then
return (Numerator => X, Denominator => Y);
else
return (Numerator => -X, Denominator => -Y);
end if;
end "/";
14 function "+" (X,Y : Rational) return Rational is ... end "+";
function "-" (X,Y : Rational) return Rational is ... end "-";
function "*" (X,Y : Rational) return Rational is ... end "*";
function "/" (X,Y : Rational) return Rational is ... end "/";
15 end Rational_Numbers;
Wording Changes from Ada 83
15.a The syntax rule for package_body now uses the syntactic category
handled_sequence_of_statements.
15.b The declarative_part of a package_body is now required; that
doesn't make any real difference, since a declarative_part can be
empty.
15.c RM83 seems to have forgotten to say that a package_body can't
stand alone, without a previous declaration. We state that rule
here.
15.d RM83 forgot to restrict the definition of elaboration of
package_bodies to nongeneric ones. We have corrected that omission.
15.e The rule about implicit bodies (from RM83-9.3(5)) is moved here,
since it is more generally applicable.
7.3 Private Types and Private Extensions
1 [The declaration (in the visible part of a package) of a type as a private
type or private extension serves to separate the characteristics that can be
used directly by outside program units (that is, the logical properties) from
other characteristics whose direct use is confined to the package (the details
of the definition of the type itself). See 3.9.1 for an overview of type
extensions. {private types and private extensions}
{information hiding: See private types and private extensions}
{opaque type: See private types and private extensions}
{abstract data type (ADT): See private types and private extensions}
{ADT (abstract data type): See private types and private extensions} ]
Language Design Principles
1.a A private (untagged) type can be thought of as a record type with
the type of its single (hidden) component being the full view.
1.b A private tagged type can be thought of as a private extension of
an anonymous parent with no components. The only dispatching
operation of the parent is equality (although the Size attribute,
and, if nonlimited, assignment are allowed, and those will
presumably be implemented in terms of dispatching).
Syntax
2 private_type_declaration ::=
type defining_identifier [discriminant_part
] is [[abstract] tagged] [limited] private;
3/2 {AI95-00251-01} {AI95-00419-01} {AI95-00443-01}
private_extension_declaration ::=
type defining_identifier [discriminant_part] is
[abstract] [limited | synchronized] new ancestor_subtype_indication
[and interface_list] with private;
Legality Rules
4 {partial view (of a type)}
{requires a completion (declaration of a partial view) [partial]} A
private_type_declaration or private_extension_declaration declares a partial
view of the type; such a declaration is allowed only as a declarative_item of
the visible part of a package, and it requires a completion, which shall be a
full_type_declaration that occurs as a declarative_item of the private part of
the package. [ The view of the type declared by the full_type_declaration is
called the full view.] A generic formal private type or a generic formal
private extension is also a partial view.
4.a To be honest: A private type can also be completed by a pragma
Import, if supported by an implementation.
4.b Reason: We originally used the term "private view," but this was
easily confused with the view provided from the private part,
namely the full view.
4.c/2 Proof: {AI95-00326-01} Full view is now defined in 3.2.1, "
Type Declarations", as all types now have them.
5 [A type shall be completely defined before it is frozen (see 3.11.1 and
13.14). Thus, neither the declaration of a variable of a partial view of a
type, nor the creation by an allocator of an object of the partial view are
allowed before the full declaration of the type. Similarly, before the full
declaration, the name of the partial view cannot be used in a
generic_instantiation or in a representation item.]
5.a Proof: This rule is stated officially in 3.11.1, "
Completions of Declarations".
6/2 {AI95-00419-01} {AI95-00443-01} [A private type is limited if its
declaration includes the reserved word limited; a private extension is limited
if its ancestor type is a limited type that is not an interface type, or if
the reserved word limited or synchronized appears in its definition.] If the
partial view is nonlimited, then the full view shall be nonlimited. If a
tagged partial view is limited, then the full view shall be limited. [On the
other hand, if an untagged partial view is limited, the full view may be
limited or nonlimited.]
7 If the partial view is tagged, then the full view shall be tagged. [On the
other hand, if the partial view is untagged, then the full view may be tagged
or untagged.] In the case where the partial view is untagged and the full view
is tagged, no derivatives of the partial view are allowed within the immediate
scope of the partial view; [derivatives of the full view are allowed.]
7.a Ramification: Note that deriving from a partial view within its
immediate scope can only occur in a package that is a child of the
one where the partial view is declared. The rule implies that in
the visible part of a public child package, it is impossible to
derive from an untagged private type declared in the visible part
of the parent package in the case where the full view of the
parent type turns out to be tagged. We considered a model in which
the derived type was implicitly redeclared at the earliest place
within its immediate scope where characteristics needed to be
added. However, we rejected that model, because (1) it would imply
that (for an untagged type) subprograms explicitly declared after
the derived type could be inherited, and (2) to make this model
work for composite types as well, several implicit redeclarations
would be needed, since new characteristics can become visible one
by one; that seemed like too much mechanism.
7.b Discussion: The rule for tagged partial views is redundant for
partial views that are private extensions, since all extensions of
a given ancestor tagged type are tagged, and limited if the
ancestor is limited. We phrase this rule partially redundantly to
keep its structure parallel with the other rules.
7.c To be honest: This rule is checked in a generic unit, rather than
using the "assume the best" or "assume the worst" method.
7.d/2 Reason: {AI95-00230-01} Tagged limited private types have certain
capabilities that are incompatible with having assignment for the
full view of the type. In particular, tagged limited private types
can be extended with components of a limited type, which works
only because assignment is not allowed. Consider the following
example:
7.e package P1 is
type T1 is tagged limited private;
procedure Foo(X : in T1'Class);
private
type T1 is tagged null record; -- Illegal!
-- This should say "tagged limited null record".
end P1;
7.f/1 package body P1 is
type A is access T1'Class;
Global : A;
procedure Foo(X : in T1'Class) is
begin
Global := new T1'Class'(X);
-- This would be illegal if the full view of
-- T1 were limited, like it's supposed to be.
end Foo;
end P1;
7.g/2 {AI95-00230-01} with P1;
package P2 is
type T2(D : access Integer)
is new P1.T1 with
record
My_Task : Some_Task_Type; -- Trouble!
end record;
end P2;
7.h/1 with P1;
with P2;
procedure Main is
Local : aliased Integer;
Y : P2.T2(D => Local'Access);
begin
P1.Foo(Y);
end Main;
7.i/2 {AI95-00230-01} If the above example were legal, we would have
succeeded in doing an assignment of a task object, which is
supposed to be a no-no.
7.j This rule is not needed for private extensions, because they
inherit their limitedness from their ancestor, and there is a
separate rule forbidding limited components of the corresponding
record extension if the parent is nonlimited.
7.k Ramification: A type derived from an untagged private type is
untagged, even if the full view of the parent is tagged, and even
at places that can see the parent:
7.l package P is
type Parent is private;
private
type Parent is tagged
record
X: Integer;
end record;
end P;
7.m/1 with P;
package Q is
type T is new P.Parent;
end Q;
7.n with Q; use Q;
package body P is
... T'Class ... -- Illegal!
Object: T;
... Object.X ... -- Illegal!
... Parent(Object).X ... -- OK.
end P;
7.o The declaration of T declares an untagged view. This view is
always untagged, so T'Class is illegal, it would be illegal to
extend T, and so forth. The component name X is never visible for
this view, although the component is still there - one can get
one's hands on it via a type_conversion.
7.1/2 {AI95-00396-01} If a full type has a partial view that is tagged, then:
7.2/2 * the partial view shall be a synchronized tagged type (see 3.9.4) if
and only if the full type is a synchronized tagged type;
7.o.1/2 Reason: Since we do not allow record extensions of synchronized
tagged types, this property has to be visible in the partial view
to avoid privacy breaking. Generic formals do not need a similar
rule as any extensions are rechecked for legality in the
specification, and extensions of tagged formals are always illegal
in a generic body.
7.3/2 * the partial view shall be a descendant of an interface type (see
3.9.4) if and only if the full type is a descendant of the interface
type.
7.p/2 Reason: Consider the following example:
7.q/2 package P is
package Pkg is
type Ifc is interface;
procedure Foo (X : Ifc) is abstract;
end Pkg;
7.r/2 type Parent_1 is tagged null record;
7.s/2 type T1 is new Parent_1 with private;
private
type Parent_2 is new Parent_1 and Pkg.Ifc with null record;
procedure Foo (X : Parent_2); -- Foo #1
7.t/2 type T1 is new Parent_2 with null record; -- Illegal.
end P;
7.u/2 with P;
package P_Client is
type T2 is new P.T1 and P.Pkg.Ifc with null record;
procedure Foo (X : T2); -- Foo #2
X : T2;
end P_Client;
7.v/2 with P_Client;
package body P is
...
7.w/2 procedure Bar (X : T1'Class) is
begin
Pkg.Foo (X); -- should call Foo #1 or an override thereof
end;
7.x/2 begin
Pkg.Foo (Pkg.Ifc'Class (P_Client.X)); -- should call Foo #2
Bar (T1'Class (P_Client.X));
end P;
7.y/2 This example is illegal because the completion of T1 is descended
from an interface that the partial view is not descended from. If
it were legal, T2 would implement Ifc twice, once in the visible
part of P, and once in the visible part of P_Client. We would need
to decide how Foo #1 and Foo #2 relate to each other. There are
two options: either Foo #2 overrides Foo #1, or it doesn't.
7.z/2 If Foo #2 overrides Foo #1, we have a problem because the client
redefines a behavior that it doesn't know about, and we try to
avoid this at all costs, as it would lead to a breakdown of
whatever abstraction was implemented. If the abstraction didn't
expose that it implements Ifc, there must be a reason, and it
should be able to depend on the fact that no overriding takes
place in clients. Also, during maintenance, things may change and
the full view might implement a different set of interfaces.
Furthermore, the situation is even worse if the full type
implements another interface Ifc2 that happens to have a
conforming Foo (otherwise unrelated, except for its name and
profile).
7.aa/2 If Foo #2 doesn't override Foo #1, there is some similarity with
the case of normal tagged private types, where a client can
declare an operation that happens to conform to some private
operation, and that's OK, it gets a different slot in the type
descriptor. The problem here is that T2 would implement Ifc in two
different ways, and through conversions to Ifc'Class we could end
up with visibility on both of these two different implementations.
This is the "diamond inheritance" problem of C++ all over again,
and we would need some kind of a preference rule to pick one
implementation. We don't want to go there (if we did, we might as
well provide full-fledged multiple inheritance).
7.bb/2 Note that there wouldn't be any difficulty to implement the first
option, so the restriction is essentially methodological. The
second option might be harder to implement, depending on the
language rules that we would choose.
7.cc/2 Ramification: This rule also prevents completing a private type
with an interface. A interface, like all types, is a descendant of
itself, and thus this rule is triggered. One reason this is
necessary is that a client of a private extension should be able
to inherit limitedness without having to look in the private part
to see if the type is an interface (remember that limitedness of
interfaces is never inherited, while it is inherited from other
types).
8 {ancestor subtype (of a private_extension_declaration)} The ancestor
subtype of a private_extension_declaration is the subtype defined by the
ancestor_subtype_indication; the ancestor type shall be a specific tagged
type. The full view of a private extension shall be derived (directly or
indirectly) from the ancestor type. In addition to the places where
Legality Rules normally apply (see 12.3), the requirement that the ancestor be
specific applies also in the private part of an instance of a generic unit.
8.a Reason: This rule allows the full view to be defined through
several intermediate derivations, possibly from a series of types
produced by generic_instantiations.
8.1/2 {AI95-00419-01} {AI95-00443-01} If the reserved word limited appears in
a private_extension_declaration, the ancestor type shall be a limited type. If
the reserved word synchronized appears in a private_extension_declaration, the
ancestor type shall be a limited interface.
9 If the declaration of a partial view includes a known_discriminant_part,
then the full_type_declaration shall have a fully conforming [(explicit)]
known_discriminant_part [(see 6.3.1, "Conformance Rules")].
{full conformance (required)} [The ancestor subtype may be unconstrained; the
parent subtype of the full view is required to be constrained (see 3.7).]
9.a Discussion: If the ancestor subtype has discriminants, then it is
usually best to make it unconstrained.
9.b Ramification: If the partial view has a known_discriminant_part,
then the full view has to be a composite, non-array type, since
only such types may have known discriminants. Also, the full view
cannot inherit the discriminants in this case; the
known_discriminant_part has to be explicit.
9.c That is, the following is illegal:
9.d package P is
type T(D : Integer) is private;
private
type T is new Some_Other_Type; -- Illegal!
end P;
9.e even if Some_Other_Type has an integer discriminant called D.
9.f It is a ramification of this and other rules that in order for a
tagged type to privately inherit unconstrained discriminants, the
private type declaration has to have an
unknown_discriminant_part.
10 If a private extension inherits known discriminants from the ancestor
subtype, then the full view shall also inherit its discriminants from the
ancestor subtype, and the parent subtype of the full view shall be constrained
if and only if the ancestor subtype is constrained.
10.a Reason: The first part ensures that the full view has the same
discriminants as the partial view. The second part ensures that if
the partial view is unconstrained, then the full view is also
unconstrained; otherwise, a client might constrain the partial
view in a way that conflicts with the constraint on the full view.
10.1/2 {AI95-00419-01} If the full_type_declaration for a private extension is
defined by a derived_type_definition, then the reserved word limited shall
appear in the full_type_declaration if and only if it also appears in the
private_extension_declaration.
10.b/2 Reason: The word limited is optional (unless the ancestor is an
interface), but it should be used consistently. Otherwise things
would be too confusing for the reader. Of course, we only require
that if the full type is defined by a derived_type_definition, as
we want to allow task and protected types to complete extensions
of synchronized interfaces.
11 [If a partial view has unknown discriminants, then the
full_type_declaration may define a definite or an indefinite subtype, with or
without discriminants.]
12 If a partial view has neither known nor unknown discriminants, then the
full_type_declaration shall define a definite subtype.
13 If the ancestor subtype of a private extension has constrained
discriminants, then the parent subtype of the full view shall impose a
statically matching constraint on those discriminants.
{statically matching (required) [partial]}
13.a Ramification: If the parent type of the full view is not the
ancestor type, but is rather some descendant thereof, the
constraint on the discriminants of the parent type might come from
the declaration of some intermediate type in the derivation chain
between the ancestor type and the parent type.
13.b Reason: This prevents the following:
13.c package P is
type T2 is new T1(Discrim => 3) with private;
private
type T2 is new T1(Discrim => 999) -- Illegal!
with record ...;
end P;
13.d The constraints in this example do not statically match.
13.e If the constraint on the parent subtype of the full view depends
on discriminants of the full view, then the ancestor subtype has
to be unconstrained:
13.f type One_Discrim(A: Integer) is tagged ...;
...
package P is
type Two_Discrims(B: Boolean; C: Integer) is new One_Discrim with private;
private
type Two_Discrims(B: Boolean; C: Integer) is new One_Discrim(A => C) with
record
...
end record;
end P;
13.g The above example would be illegal if the private extension said
"is new One_Discrim(A => C);", because then the constraints would
not statically match. (Constraints that depend on discriminants
are not static.)
Static Semantics
14 {private type [partial]} A private_type_declaration declares a private
type and its first subtype. {private extension [partial]} Similarly, a private_-
extension_declaration declares a private extension and its first subtype.
14.a Discussion: {package-private type} A package-private type is one
declared by a private_type_declaration; that is, a private type
other than a generic formal private type.
{package-private extension} Similarly, a package-private extension
is one declared by a private_extension_declaration. These terms
are not used in the RM95 version of this document.
15 A declaration of a partial view and the corresponding
full_type_declaration define two views of a single type. The declaration of a
partial view together with the visible part define the operations that are
available to outside program units; the declaration of the full view together
with the private part define other operations whose direct use is possible
only within the declarative region of the package itself. {characteristics}
Moreover, within the scope of the declaration of the full view, the
characteristics of the type are determined by the full view; in particular,
within its scope, the full view determines the classes that include the type,
which components, entries, and protected subprograms are visible, what
attributes and other predefined operations are allowed, and whether the first
subtype is static. See 7.3.1.
16/2 {AI95-00401} A private extension inherits components (including
discriminants unless there is a new discriminant_part specified) and
user-defined primitive subprograms from its ancestor type and its progenitor
types (if any), in the same way that a record extension inherits components
and user-defined primitive subprograms from its parent type and its progenitor
types (see 3.4).
16.a To be honest: If an operation of the parent type is abstract, then
the abstractness of the inherited operation is different for
nonabstract record extensions than for nonabstract private
extensions (see 3.9.3).
Dynamic Semantics
17 {elaboration (private_type_declaration) [partial]} The elaboration of a
private_type_declaration creates a partial view of a type.
{elaboration (private_extension_declaration) [partial]} The elaboration of a
private_extension_declaration elaborates the ancestor_subtype_indication, and
creates a partial view of a type.
NOTES
18 5 The partial view of a type as declared by a
private_type_declaration is defined to be a composite view (in 3.2).
The full view of the type might or might not be composite. A private
extension is also composite, as is its full view.
19/2 6 {AI95-00318-02} Declaring a private type with an
unknown_discriminant_part is a way of preventing clients from creating
uninitialized objects of the type; they are then forced to initialize
each object by calling some operation declared in the visible part of
the package.
19.a Discussion: {generic contract/private type contract analogy}
Packages with private types are analogous to generic packages with
formal private types, as follows: The declaration of a
package-private type is like the declaration of a formal private
type. The visible part of the package is like the generic formal
part; these both specify a contract (that is, a set of operations
and other things available for the private type). The private part
of the package is like an instantiation of the generic; they both
give a full_type_declaration that specifies implementation details
of the private type. The clients of the package are like the body
of the generic; usage of the private type in these places is
restricted to the operations defined by the contract.
19.b In other words, being inside the package is like being outside the
generic, and being outside the package is like being inside the
generic; a generic is like an "inside-out" package.
19.c This analogy also works for private extensions in the same
inside-out way.
19.d Many of the legality rules are defined with this analogy in mind.
See, for example, the rules relating to operations of [formal]
derived types.
19.e The completion rules for a private type are intentionally quite
similar to the matching rules for a generic formal private type.
19.f This analogy breaks down in one respect: a generic actual subtype
is a subtype, whereas the full view for a private type is always a
new type. (We considered allowing the completion of a
private_type_declaration to be a subtype_declaration, but the
semantics just won't work.) This difference is behind the fact
that a generic actual type can be class-wide, whereas the
completion of a private type always declares a specific type.
20/2 7 {AI95-00401} The ancestor type specified in a
private_extension_declaration and the parent type specified in the
corresponding declaration of a record extension given in the private
part need not be the same. If the ancestor type is not an interface
type, the parent type of the full view can be any descendant of the
ancestor type. In this case, for a primitive subprogram that is
inherited from the ancestor type and not overridden, the formal
parameter names and default expressions (if any) come from the
corresponding primitive subprogram of the specified ancestor type,
while the body comes from the corresponding primitive subprogram of
the parent type of the full view. See 3.9.2.
20.1/2 8 {AI95-00401} If the ancestor type specified in a
private_extension_declaration is an interface type, the parent type
can be any type so long as the full view is a descendant of the
ancestor type. The progenitor types specified in a
private_extension_declaration and the progenitor types specified in
the corresponding declaration of a record extension given in the
private part need not be the same - the only requirement is that the
private extension and the record extension be descended from the same
set of interfaces.
Examples
21 Examples of private type declarations:
22 type Key is private;
type File_Name is limited private;
23 Example of a private extension declaration:
24 type List is new Ada.Finalization.Controlled with private;
Extensions to Ada 83
24.a {extensions to Ada 83} The syntax for a private_type_declaration
is augmented to allow the reserved word tagged.
24.b In Ada 83, a private type without discriminants cannot be
completed with a type with discriminants. Ada 95 allows the full
view to have discriminants, so long as they have defaults (that
is, so long as the first subtype is definite). This change is made
for uniformity with generics, and because the rule as stated is
simpler and easier to remember than the Ada 83 rule. In the
original version of Ada 83, the same restriction applied to
generic formal private types. However, the restriction was removed
by the ARG for generics. In order to maintain the "generic
contract/private type contract analogy" discussed above, we have
to apply the same rule to package-private types. Note that a
private untagged type without discriminants can be completed with
a tagged type with discriminants only if the full view is
constrained, because discriminants of tagged types cannot have
defaults.
Wording Changes from Ada 83
24.c RM83-7.4.1(4), "Within the specification of the package that
declares a private type and before the end of the corresponding
full type declaration, a restriction applies....", is subsumed
(and corrected) by the rule that a type shall be completely
defined before it is frozen, and the rule that the parent type of
a derived type declaration shall be completely defined, unless the
derived type is a private extension.
Extensions to Ada 95
24.d/2 {AI95-00251-01} {AI95-00396-01} {AI95-00401-01}
{extensions to Ada 95} Added interface_list to private extensions
to support interfaces and multiple inheritance (see 3.9.4).
24.e/2 {AI95-00419-01} A private extension may specify that it is a
limited type. This is required for interface ancestors (from which
limitedness is not inherited), but it is generally useful as
documentation of limitedness.
24.f/2 {AI95-00443-01} A private extension may specify that it is a
synchronized type. This is required in order so that a regular
limited interface can be used as the ancestor of a synchronized
type (we do not allow hiding of synchronization).
7.3.1 Private Operations
1 [For a type declared in the visible part of a package or generic package,
certain operations on the type do not become visible until later in the
package - either in the private part or the body. {private operations} Such
private operations are available only inside the declarative region of the
package or generic package.]
Static Semantics
2 The predefined operators that exist for a given type are determined by the
classes to which the type belongs. For example, an integer type has a
predefined "+" operator. In most cases, the predefined operators of a type are
declared immediately after the definition of the type; the exceptions are
explained below. Inherited subprograms are also implicitly declared
immediately after the definition of the type, except as stated below.
3/1 {8652/0019} {AI95-00033-01} For a composite type, the characteristics (see
7.3) of the type are determined in part by the characteristics of its
component types. At the place where the composite type is declared, the only
characteristics of component types used are those characteristics visible at
that place. If later immediately within the declarative region in which the
composite type is declared additional characteristics become visible for a
component type, then any corresponding characteristics become visible for the
composite type. Any additional predefined operators are implicitly declared at
that place.
4/1 {8652/0019} {AI95-00033-01} The corresponding rule applies to a type
defined by a derived_type_definition, if there is a place immediately within
the declarative region in which the type is declared where additional
characteristics of its parent type become visible.
5/1 {8652/0019} {AI95-00033-01} {become nonlimited}
{nonlimited type (becoming nonlimited)} {limited type (becoming nonlimited)}
[For example, an array type whose component type is limited private becomes
nonlimited if the full view of the component type is nonlimited and visible at
some later place immediately within the declarative region in which the array
type is declared. In such a case, the predefined "=" operator is implicitly
declared at that place, and assignment is allowed after that place.]
6/1 {8652/0019} {AI95-00033-01} Inherited primitive subprograms follow a
different rule. For a derived_type_definition, each inherited primitive
subprogram is implicitly declared at the earliest place, if any, immediately
within the declarative region in which the type_declaration occurs, but after
the type_declaration, where the corresponding declaration from the parent is
visible. If there is no such place, then the inherited subprogram is not
declared at all. [An inherited subprogram that is not declared at all cannot
be named in a call and cannot be overridden, but for a tagged type, it is
possible to dispatch to it.]
7 For a private_extension_declaration, each inherited subprogram is declared
immediately after the private_extension_declaration if the corresponding
declaration from the ancestor is visible at that place. Otherwise, the
inherited subprogram is not declared for the private extension, [though it
might be for the full type].
7.a/1 Reason: There is no need for the "earliest place immediately
within the declarative region" business here, because a
private_extension_declaration will be completed with a
full_type_declaration, so we can hang the necessary private
implicit declarations on the full_type_declaration.
7.b Discussion: The above rules matter only when the component type
(or parent type) is declared in the visible part of a package, and
the composite type (or derived type) is declared within the
declarative region of that package (possibly in a nested package
or a child package).
7.c Consider:
7.d package Parent is
type Root is tagged null record;
procedure Op1(X : Root);
7.e type My_Int is range 1..10;
private
procedure Op2(X : Root);
7.f type Another_Int is new My_Int;
procedure Int_Op(X : My_Int);
end Parent;
7.g with Parent; use Parent;
package Unrelated is
type T2 is new Root with null record;
procedure Op2(X : T2);
end Unrelated;
7.h package Parent.Child is
type T3 is new Root with null record;
-- Op1(T3) implicitly declared here.
7.i package Nested is
type T4 is new Root with null record;
private
...
end Nested;
private
-- Op2(T3) implicitly declared here.
...
end Parent.Child;
7.j with Unrelated; use Unrelated;
package body Parent.Child is
package body Nested is
-- Op2(T4) implicitly declared here.
end Nested;
7.k type T5 is new T2 with null record;
end Parent.Child;
7.l Another_Int does not inherit Int_Op, because Int_Op does not "
exist" at the place where Another_Int is declared.
7.m/1 Type T2 inherits Op1 and Op2 from Root. However, the inherited Op2
is never declared, because Parent.Op2 is never visible immediately
within the declarative region of T2. T2 explicitly declares its
own Op2, but this is unrelated to the inherited one - it does not
override the inherited one, and occupies a different slot in the
type descriptor.
7.n T3 inherits both Op1 and Op2. Op1 is implicitly declared
immediately after the type declaration, whereas Op2 is declared at
the beginning of the private part. Note that if Child were a
private child of Parent, then Op1 and Op2 would both be implicitly
declared immediately after the type declaration.
7.o/1 T4 is similar to T3, except that the earliest place immediately
within the declarative region containing T4 where Root's Op2 is
visible is in the body of Nested.
7.p If T3 or T4 were to declare a type-conformant Op2, this would
override the one inherited from Root. This is different from the
situation with T2.
7.q T5 inherits Op1 and two Op2's from T2. Op1 is implicitly declared
immediately after the declaration of T5, as is the Op2 that came
from Unrelated.Op2. However, the Op2 that originally came from
Parent.Op2 is never implicitly declared for T5, since T2's version
of that Op2 is never visible (anywhere - it never got declared
either).
7.r For all of these rules, implicit private parts and bodies are
assumed as needed.
7.s It is possible for characteristics of a type to be revealed in
more than one place:
7.t package P is
type Comp1 is private;
private
type Comp1 is new Boolean;
end P;
7.u package P.Q is
package R is
type Comp2 is limited private;
type A is array(Integer range <>) of Comp2;
private
type Comp2 is new Comp1;
-- A becomes nonlimited here.
-- "="(A, A) return Boolean is implicitly declared here.
...
end R;
private
-- Now we find out what Comp1 really is, which reveals
-- more information about Comp2, but we're not within
-- the immediate scope of Comp2, so we don't do anything
-- about it yet.
end P.Q;
7.v package body P.Q is
package body R is
-- Things like "xor"(A,A) return A are implicitly
-- declared here.
end R;
end P.Q;
7.v.1/1 {8652/0019} {AI95-00033-01} We say immediately within the
declarative region in order that types do not gain operations
within a nested scope. Consider:
7.v.2/1 package Outer is
package Inner is
type Inner_Type is private;
private
type Inner_Type is new Boolean;
end Inner;
type Outer_Type is array(Natural range <>) of Inner.Inner_Type;
end Outer;
7.v.3/1 package body Outer is
package body Inner is
-- At this point, we can see that Inner_Type is a Boolean type.
-- But we don't want Outer_Type to gain an "and" operator here.
end Inner;
end Outer;
8 [The Class attribute is defined for tagged subtypes in 3.9. In addition,]
for every subtype S of an untagged private type whose full view is tagged, the
following attribute is defined:
9 S'Class Denotes the class-wide subtype corresponding to the full view
of S. This attribute is allowed only from the beginning of the
private part in which the full view is declared, until the
declaration of the full view. [After the full view, the Class
attribute of the full view can be used.]
NOTES
10 9 Because a partial view and a full view are two different views of
one and the same type, outside of the defining package the
characteristics of the type are those defined by the visible part.
Within these outside program units the type is just a private type or
private extension, and any language rule that applies only to another
class of types does not apply. The fact that the full declaration
might implement a private type with a type of a particular class (for
example, as an array type) is relevant only within the declarative
region of the package itself including any child units.
11 The consequences of this actual implementation are, however, valid
everywhere. For example: any default initialization of components
takes place; the attribute Size provides the size of the full view;
finalization is still done for controlled components of the full view;
task dependence rules still apply to components that are task objects.
12/2 10 {AI95-00287-01} Partial views provide initialization, membership
tests, selected components for the selection of discriminants and
inherited components, qualification, and explicit conversion.
Nonlimited partial views also allow use of assignment_statements.
13 11 For a subtype S of a partial view, S'Size is defined (see 13.3).
For an object A of a partial view, the attributes A'Size and A'Address
are defined (see 13.3). The Position, First_Bit, and Last_Bit
attributes are also defined for discriminants and inherited
components.
Examples
14 Example of a type with private operations:
15 package Key_Manager is
type Key is private;
Null_Key : constant Key; -- a deferred constant declaration (see 7.4
)
procedure Get_Key(K : out Key);
function "<" (X, Y : Key) return Boolean;
private
type Key is new Natural;
Null_Key : constant Key := Key'First;
end Key_Manager;
16 package body Key_Manager is
Last_Key : Key := Null_Key;
procedure Get_Key(K : out Key) is
begin
Last_Key := Last_Key + 1;
K := Last_Key;
end Get_Key;
17 function "<" (X, Y : Key) return Boolean is
begin
return Natural(X) < Natural(Y);
end "<";
end Key_Manager;
NOTES
18 12 Notes on the example: Outside of the package Key_Manager, the
operations available for objects of type Key include assignment, the
comparison for equality or inequality, the procedure Get_Key and the
operator "<"; they do not include other relational operators such as
">=", or arithmetic operators.
19 The explicitly declared operator "<" hides the predefined operator "<"
implicitly declared by the full_type_declaration. Within the body of
the function, an explicit conversion of X and Y to the subtype Natural
is necessary to invoke the "<" operator of the parent type.
Alternatively, the result of the function could be written as not (X
>= Y), since the operator ">=" is not redefined.
20 The value of the variable Last_Key, declared in the package body,
remains unchanged between calls of the procedure Get_Key. (See also
the NOTES of 7.2.)
Wording Changes from Ada 83
20.a The phrase in RM83-7.4.2(7), "...after the full type
declaration", doesn't work in the presence of child units, so we define that
rule in terms of visibility.
20.b The definition of the Constrained attribute for private types has
been moved to "Obsolescent Features." (The Constrained attribute
of an object has not been moved there.)
Wording Changes from Ada 95
20.c/2 {8652/0018} {AI95-00033-01} Corrigendum: Clarified when additional
operations are declared.
20.d/2 {AI95-00287-01} Revised the note on operations of partial views to
reflect that limited types do have an assignment operation, but
not assignment_statements.
7.4 Deferred Constants
1 [Deferred constant declarations may be used to declare constants in the
visible part of a package, but with the value of the constant given in the
private part. They may also be used to declare constants imported from other
languages (see Annex B).]
Legality Rules
2 [{deferred constant declaration} A deferred constant declaration is an
object_declaration with the reserved word constant but no initialization
expression.] {deferred constant} The constant declared by a deferred constant
declaration is called a deferred constant.
{requires a completion (deferred constant declaration) [partial]} A deferred
constant declaration requires a completion, which shall be a full constant
declaration (called the full declaration of the deferred constant), or a
pragma Import (see Annex B). {full declaration}
2.a Proof: The first sentence is redundant, as it is stated officially
in 3.3.1.
3 A deferred constant declaration that is completed by a full constant
declaration shall occur immediately within the visible part of a
package_specification. For this case, the following additional rules apply to
the corresponding full declaration:
4 * The full declaration shall occur immediately within the private part
of the same package;
5/2 * {AI95-00385-01} The deferred and full constants shall have the same
type, or shall have statically matching anonymous access subtypes;
5.a/2 Ramification: {AI95-00385-01} This implies that both the deferred
declaration and the full declaration have to have a
subtype_indication or access_definition rather than an
array_type_definition, because each array_type_definition would
define a new type.
6/2 * {AI95-00385-01} If the deferred constant declaration includes a
subtype_indication that defines a constrained subtype, then the
subtype defined by the subtype_indication in the full declaration
shall match it statically.[ On the other hand, if the subtype of the
deferred constant is unconstrained, then the full declaration is still
allowed to impose a constraint. The constant itself will be
constrained, like all constants;]
7/2 * {AI95-00231-01} If the deferred constant declaration includes the
reserved word aliased, then the full declaration shall also;
7.a Ramification: On the other hand, the full constant can be aliased
even if the deferred constant is not.
7.1/2 * {AI95-00231-01} If the subtype of the deferred constant declaration
excludes null, the subtype of the full declaration shall also exclude
null.
7.a.1/2 Ramification: On the other hand, the full constant can exclude
null even if the deferred constant does not. But that can only
happen for a subtype_indication, as anonymous access types are
required to statically match (which includes any null_exclusion).
8 [A deferred constant declaration that is completed by a pragma Import need
not appear in the visible part of a package_specification, and has no full
constant declaration.]
9/2 {AI95-00256-01} The completion of a deferred constant declaration shall
occur before the constant is frozen (see 13.14).
Dynamic Semantics
10 {elaboration (deferred constant declaration) [partial]} The elaboration of
a deferred constant declaration elaborates the subtype_indication or (only
allowed in the case of an imported constant) the array_type_definition.
NOTES
11 13 The full constant declaration for a deferred constant that is of a
given private type or private extension is not allowed before the
corresponding full_type_declaration. This is a consequence of the
freezing rules for types (see 13.14).
11.a Ramification: Multiple or single declarations are allowed for the
deferred and the full declarations, provided that the equivalent
single declarations would be allowed.
11.b Deferred constant declarations are useful for declaring constants
of private views, and types with components of private views. They
are also useful for declaring access-to-constant objects that
designate variables declared in the private part of a package.
Examples
12 Examples of deferred constant declarations:
13 Null_Key : constant Key; -- see 7.3.1
14 CPU_Identifier : constant String(1..8);
pragma Import(Assembler, CPU_Identifier, Link_Name => "CPU_ID");
-- see B.1
Extensions to Ada 83
14.a {extensions to Ada 83} In Ada 83, a deferred constant is required
to be of a private type declared in the same visible part. This
restriction is removed for Ada 95; deferred constants can be of
any type.
14.b In Ada 83, a deferred constant declaration was not permitted to
include a constraint, nor the reserved word aliased.
14.c In Ada 83, the rules required conformance of type marks; here we
require static matching of subtypes if the deferred constant is
constrained.
14.d A deferred constant declaration can be completed with a pragma
Import. Such a deferred constant declaration need not be within a
package_specification.
14.e The rules for too-early uses of deferred constants are modified in
Ada 95 to allow more cases, and catch all errors at compile time.
This change is necessary in order to allow deferred constants of a
tagged type without violating the principle that for a dispatching
call, there is always an implementation to dispatch to. It has the
beneficial side-effect of catching some Ada-83-erroneous programs
at compile time. The new rule fits in well with the new
freezing-point rules. Furthermore, we are trying to convert
undefined-value problems into bounded errors, and we were having
trouble for the case of deferred constants. Furthermore,
uninitialized deferred constants cause trouble for the shared
variable / tasking rules, since they are really variable, even
though they purport to be constant. In Ada 95, they cannot be
touched until they become constant.
14.f Note that we do not consider this change to be an upward
incompatibility, because it merely changes an erroneous execution
in Ada 83 into a compile-time error.
14.g The Ada 83 semantics are unclear in the case where the full view
turns out to be an access type. It is a goal of the language
design to prevent uninitialized access objects. One wonders if the
implementation is required to initialize the deferred constant to
null, and then initialize it (again!) to its real value. In Ada
95, the problem goes away.
Wording Changes from Ada 83
14.h Since deferred constants can now be of a nonprivate type, we have
made this a stand-alone clause, rather than a subclause of 7.3
, "Private Types and Private Extensions".
14.i Deferred constant declarations used to have their own syntax, but
now they are simply a special case of object_declarations.
Extensions to Ada 95
14.j/2 {AI95-00385-01} {extensions to Ada 95} Deferred constants were
enhanced to allow the use of anonymous access types in them.
Wording Changes from Ada 95
14.k/2 {AI95-00231-01} Added matching rules for subtypes that exclude
null.
7.5 Limited Types
1/2 {AI95-00287-01} [A limited type is (a view of) a type for which copying
(such as for an assignment_statement) is not allowed. A nonlimited type is a
(view of a) type for which copying is allowed.]
1.a Discussion: The concept of the value of a limited type is
difficult to define, since the abstract value of a limited type
often extends beyond its physical representation. In some sense,
values of a limited type cannot be divorced from their object. The
value is the object.
1.b/2 {AI95-00318-02} In Ada 83, in the two places where limited types
were defined by the language, namely tasks and files, an implicit
level of indirection was implied by the semantics to avoid the
separation of the value from an associated object. In Ada 95, most
limited types are passed by reference, and even return-ed by
reference. In Ada 2005, most limited types are built-in-place upon
return, rather than returned by reference. Thus the object "
identity" is part of the logical value of most limited types.
1.c/2 To be honest: {AI95-00287-01} {AI95-00419-01} For a limited
partial view whose full view is nonlimited, copying is possible on
parameter passing and function return. To prevent any copying
whatsoever, one should make both the partial and full views
limited.
1.d/2 Glossary entry: {Limited type} A limited type is a type for which
copying (such as in an assignment_statement) is not allowed. A
nonlimited type is a type for which copying is allowed.
Legality Rules
2/2 {AI95-00419-01} If a tagged record type has any limited components, then
the reserved word limited shall appear in its record_type_definition. [If the
reserved word limited appears in the definition of a derived_type_definition,
its parent type and any progenitor interfaces shall be limited.]
2.a.1/2 Proof: {AI95-00419-01} The rule about the parent type being
required to be limited can be found in 3.4. Rules about progenitor
interfaces can be found in 3.9.4, specifically, a nonlimited
interface can appear only on a nonlimited type. We repeat these
rules here to gather these scattered rules in one obvious place.
2.a Reason: This prevents tagged limited types from becoming
nonlimited. Otherwise, the following could happen:
2.b package P is
type T is limited private;
type R is tagged
record -- Illegal!
-- This should say "limited record".
X : T;
end record;
private
type T is new Integer; -- R becomes nonlimited here.
end P;
2.c/2 package Q is
type R2 is new R with
record
Y : Some_Task_Type;
end record;
end Q;
2.d/2 {AI95-00230-01} If the above were legal, then assignment would be
defined for R'Class in the body of P, which is bad news, given the
task.
2.1/2 {AI95-00287-01} {AI95-00318-02} In the following contexts, an
expression of a limited type is not permitted unless it is an aggregate, a
function_call, or a parenthesized expression or qualified_expression whose
operand is permitted by this rule:
2.2/2 * the initialization expression of an object_declaration (see 3.3.1)
2.3/2 * the default_expression of a component_declaration (see 3.8)
2.4/2 * the expression of a record_component_association (see 4.3.1)
2.5/2 * the expression for an ancestor_part of an extension_aggregate (see
4.3.2)
2.6/2 * an expression of a positional_array_aggregate or the expression of
an array_component_association (see 4.3.3)
2.7/2 * the qualified_expression of an initialized allocator (see 4.8)
2.8/2 * the expression of a return statement (see 6.5)
2.9/2 * the default_expression or actual parameter for a formal object of
mode in (see 12.4)
2.e/2 Discussion: All of these contexts normally require copying; by
restricting the uses as above, we can require the new object to be
built-in-place.
Static Semantics
3/2 {AI95-00419-01} {limited type} A type is limited if it is one of the
following:
4/2 * {AI95-00411-01} {AI95-00419-01} a type with the reserved word limited,
synchronized, task, or protected in its definition;
4.a Ramification: Note that there is always a "definition,"
conceptually, even if there is no syntactic category called "
..._definition".
4.b/2 {AI95-00419-01} This includes interfaces of the above kinds,
derived types with the reserved word limited, as well as task and
protected types.
5/2 * This paragraph was deleted.{AI95-00419-01}
6/2 * {AI95-00419-01} a composite type with a limited component;
6.1/2 * {AI95-00419-01} a derived type whose parent is limited and is not an
interface.
6.a/2 Ramification: {AI95-00419-01} Limitedness is not inherited from
interfaces; it must be explicitly specified when the parent is an
interface.
6.b/2 To be honest: {AI95-00419-01} A derived type can become nonlimited
if limited does not appear and the derivation takes place in the
visible part of a child package, and the parent type is nonlimited
as viewed from the private part or body of the child package.
6.c/2 Reason: {AI95-00419-01} We considered a rule where limitedness was
always inherited from the parent for derived types, but in the
case of a type whose parent is an interface, this meant that the
first interface is treated differently than other interfaces. It
also would have forced users to declare dummy nonlimited
interfaces just to get the limitedness right. We also considered a
syntax like not limited to specify nonlimitedness when the parent
was limited, but that was unsavory. The rule given is more uniform
and simpler to understand.
6.d/2 {AI95-00419-01} The rules for interfaces are asymmetrical, but the
language is not: if the parent interface is limited, the presence
of the word limited determines the limitedness, and nonlimited
progenitors are illegal by the rules in 3.9.4. If the parent
interface is nonlimited, the word limited is illegal by the rules
in 3.4. The net effect is that the order of the interfaces doesn't
matter.
7 {nonlimited type} Otherwise, the type is nonlimited.
8 [There are no predefined equality operators for a limited type.]
Implementation Requirements
8.1/2 {AI95-00287-01} {AI95-00318-02} For an aggregate of a limited type used
to initialize an object as allowed above, the implementation shall not create
a separate anonymous object for the aggregate. For a function_call of a type
with a part that is of a task, protected, or explicitly limited record type
that is used to initialize an object as allowed above, the implementation
shall not create a separate return object (see 6.5) for the function_call. The
aggregate or function_call shall be constructed directly in the new object.
8.a/2 Discussion: {AI95-00318-02} For a function_call, we only require
build-in-place{build-in-place [partial]} for a limited type that
would have been a return-by-reference type in Ada 95. We do this
because we want to minimize disruption to Ada 95 implementations
and users.
NOTES
9/2 14 {AI95-00287-01} {AI95-00318-02} While it is allowed to write
initializations of limited objects, such initializations never copy a
limited object. The source of such an assignment operation must be an
aggregate or function_call, and such aggregates and function_calls
must be built directly in the target object.
9.a/2 To be honest: This isn't quite true if the type can become
nonlimited (see below); function_calls only are required to be
build-in-place for "really" limited types.
Paragraphs 10 through 15 were deleted.
16 15 {become nonlimited} {nonlimited type (becoming nonlimited)}
{limited type (becoming nonlimited)} As illustrated in 7.3.1, an
untagged limited type can become nonlimited under certain
circumstances.
16.a Ramification: Limited private types do not become nonlimited;
instead, their full view can be nonlimited, which has a similar
effect.
16.b It is important to remember that a single nonprivate type can be
both limited and nonlimited in different parts of its scope. In
other words, "limited" is a property that depends on where you are
in the scope of the type. We don't call this a "view property"
because there is no particular declaration to declare the
nonlimited view.
16.c Tagged types never become nonlimited.
Examples
17 Example of a package with a limited type:
18 package IO_Package is
type File_Name is limited private;
19 procedure Open (F : in out File_Name);
procedure Close(F : in out File_Name);
procedure Read (F : in File_Name; Item : out Integer);
procedure Write(F : in File_Name; Item : in Integer);
private
type File_Name is
limited record
Internal_Name : Integer := 0;
end record;
end IO_Package;
20 package body IO_Package is
Limit : constant := 200;
type File_Descriptor is record ... end record;
Directory : array (1 .. Limit) of File_Descriptor;
...
procedure Open (F : in out File_Name) is ... end;
procedure Close(F : in out File_Name) is ... end;
procedure Read (F : in File_Name; Item : out Integer) is ... end;
procedure Write(F : in File_Name; Item : in Integer) is ... end;
begin
...
end IO_Package;
NOTES
21 16 Notes on the example: In the example above, an outside subprogram
making use of IO_Package may obtain a file name by calling Open and
later use it in calls to Read and Write. Thus, outside the package, a
file name obtained from Open acts as a kind of password; its internal
properties (such as containing a numeric value) are not known and no
other operations (such as addition or comparison of internal names)
can be performed on a file name. Most importantly, clients of the
package cannot make copies of objects of type File_Name.
22 This example is characteristic of any case where complete control over
the operations of a type is desired. Such packages serve a dual
purpose. They prevent a user from making use of the internal structure
of the type. They also implement the notion of an encapsulated data
type where the only operations on the type are those given in the
package specification.
23/2 {AI95-00318-02} The fact that the full view of File_Name is explicitly
declared limited means that parameter passing will always be by
reference and function results will always be built directly in the
result object (see 6.2 and 6.5).
Extensions to Ada 83
23.a {extensions to Ada 83} The restrictions in RM83-7.4.4(4), which
disallowed out parameters of limited types in certain cases, are
removed.
Wording Changes from Ada 83
23.b Since limitedness and privateness are orthogonal in Ada 95 (and to
some extent in Ada 83), this is now its own clause rather than
being a subclause of 7.3, "Private Types and Private Extensions
".
Extensions to Ada 95
23.c/2 {AI95-00287-01} {AI95-00318-02} {extensions to Ada 95} Limited
types now have an assignment operation, but its use is restricted
such that all uses are build-in-place. This is accomplished by
restricting uses to aggregates and function_calls. Aggregates were
not allowed to have a limited type in Ada 95, which causes a
compatibility issue discussed in 4.3, "Aggregates". Compatibility
issues with return statements for limited function_calls are
discussed in 6.5, "Return Statements".
Wording Changes from Ada 95
23.d/2 {AI95-00411-01} {AI95-00419-01} Rewrote the definition of limited
to ensure that interfaces are covered, but that limitedness is not
inherited from interfaces. Derived types that explicitly include
limited are now also covered.
7.6 User-Defined Assignment and Finalization
1 [{user-defined assignment} {assignment (user-defined)} Three kinds of
actions are fundamental to the manipulation of objects: initialization,
finalization, and assignment. Every object is initialized, either explicitly
or by default, after being created (for example, by an object_declaration or
allocator). Every object is finalized before being destroyed (for example, by
leaving a subprogram_body containing an object_declaration, or by a call to an
instance of Unchecked_Deallocation). An assignment operation is used as part
of assignment_statements, explicit initialization, parameter passing, and
other operations. {constructor: See initialization}
{constructor: See Initialize} {destructor: See finalization}
2 Default definitions for these three fundamental operations are provided by
the language, but {controlled type} a controlled type gives the user
additional control over parts of these operations. {Initialize} {Finalize}
{Adjust} In particular, the user can define, for a controlled type, an
Initialize procedure which is invoked immediately after the normal default
initialization of a controlled object, a Finalize procedure which is invoked
immediately before finalization of any of the components of a controlled
object, and an Adjust procedure which is invoked as the last step of an
assignment to a (nonlimited) controlled object.]
2.a Glossary entry: {Controlled type} A controlled type supports
user-defined assignment and finalization. Objects are always
finalized before being destroyed.
2.b/2 Ramification: {AI95-00114-01} {AI95-00287-01} Here's the basic
idea of initialization, value adjustment, and finalization,
whether or not user defined: When an object is created, if it is
explicitly assigned an initial value, the object is either
built-in-place from an aggregate or function call (in which case
neither Adjust nor Initialize is applied), or the assignment
copies and adjusts the initial value. Otherwise, Initialize is
applied to it (except in the case of an aggregate as a whole). An
assignment_statement finalizes the target before copying in and
adjusting the new value. Whenever an object goes away, it is
finalized. Calls on Initialize and Adjust happen bottom-up; that
is, components first, followed by the containing object. Calls on
Finalize happen top-down; that is, first the containing object,
and then its components. These ordering rules ensure that any
components will be in a well-defined state when Initialize,
Adjust, or Finalize is applied to the containing object.
Static Semantics
3 The following language-defined library package exists:
4/1 {8652/0020} {AI95-00126-01} package Ada.Finalization is
pragma Preelaborate(Finalization);
pragma Remote_Types(Finalization);
5/2 {AI95-00161-01} type Controlled is abstract tagged private;
pragma Preelaborable_Initialization(Controlled);
6/2 {AI95-00348-01} procedure Initialize
(Object : in out Controlled) is null;
procedure Adjust (Object : in out Controlled) is null;
procedure Finalize (Object : in out Controlled) is null;
7/2 {AI95-00161-01} type Limited_Controlled
is abstract tagged limited private;
pragma Preelaborable_Initialization(Limited_Controlled);
8/2 {AI95-00348-01} procedure Initialize
(Object : in out Limited_Controlled) is null;
procedure Finalize (Object : in out Limited_Controlled) is null;
private
... -- not specified by the language
end Ada.Finalization;
9/2 {AI95-00348-01} {controlled type} A controlled type is a descendant of
Controlled or Limited_Controlled. The predefined "=" operator of type
Controlled always returns True, [since this operator is incorporated into the
implementation of the predefined equality operator of types derived from
Controlled, as explained in 4.5.2.] The type Limited_Controlled is like
Controlled, except that it is limited and it lacks the primitive subprogram
Adjust.
9.a Discussion: We say "nonlimited controlled type" (rather than just
"controlled type"; when we want to talk about descendants of
Controlled only.
9.b Reason: We considered making Adjust and Finalize abstract.
However, a reasonable coding convention is e.g. for Finalize to
always call the parent's Finalize after doing whatever work is
needed for the extension part. (Unlike CLOS, we have no way to do
that automatically in Ada 95.) For this to work, Finalize cannot
be abstract. In a generic unit, for a generic formal abstract
derived type whose ancestor is Controlled or Limited_Controlled,
calling the ancestor's Finalize would be illegal if it were
abstract, even though the actual type might have a concrete
version.
9.c Types Controlled and Limited_Controlled are abstract, even though
they have no abstract primitive subprograms. It is not clear that
they need to be abstract, but there seems to be no harm in it, and
it might make an implementation's life easier to know that there
are no objects of these types - in case the implementation wishes
to make them "magic" in some way.
9.d/2 {AI95-00251-01} For Ada 2005, we considered making these types
interfaces. That would have the advantage of allowing them to be
added to existing trees. But that was rejected both because it
would cause massive disruption to existing implementations, and
because it would be very incompatible due to the "no hidden
interfaces" rule. The latter rule would prevent a tagged private
type from being completed with a derivation from Controlled or
Limited_Controlled - a very common idiom.
9.1/2 {AI95-00360-01} A type is said to need finalization
if:{needs finalization} {type (needs finalization)}
9.2/2 * it is a controlled type, a task type or a protected type; or
9.3/2 * it has a component that needs finalization; or
9.4/2 * it is a limited type that has an access discriminant whose
designated type needs finalization; or
9.5/2 * it is one of a number of language-defined types that are explicitly
defined to need finalization.
9.e/2 Ramification: The fact that a type needs finalization does not
require it to be implemented with a controlled type. It just has
to be recognized by the No_Nested_Finalization restriction.
9.f/2 This property is defined for the type, not for a particular view.
That's necessary as restrictions look in private parts to enforce
their restrictions; the point is to eliminate all controlled
parts, not just ones that are visible.
Dynamic Semantics
10/2 {AI95-00373-01} During the elaboration or evaluation of a construct that
causes an object to be initialized by default, for every controlled
subcomponent of the object that is not assigned an initial value (as defined
in 3.3.1), Initialize is called on that subcomponent. Similarly, if the object
that is initialized by default as a whole is controlled, Initialize is called
on the object.
11/2 {8652/0021} {AI95-00182-01} {AI95-00373-01} For an extension_aggregate
whose ancestor_part is a subtype_mark denoting a controlled subtype, the
Initialize procedure of the ancestor type is called, unless that Initialize
procedure is abstract.
11.a Discussion: Example:
11.b type T1 is new Controlled with
record
... -- some components might have defaults
end record;
11.c type T2 is new Controlled with
record
X : T1; -- no default
Y : T1 := ...; -- default
end record;
11.d A : T2;
B : T2 := ...;
11.e As part of the elaboration of A's declaration, A.Y is assigned a
value; therefore Initialize is not applied to A.Y. Instead, Adjust
is applied to A.Y as part of the assignment operation. Initialize
is applied to A.X and to A, since those objects are not assigned
an initial value. The assignment to A.Y is not considered an
assignment to A.
11.f For the elaboration of B's declaration, Initialize is not called
at all. Instead the assignment adjusts B's value; that is, it
applies Adjust to B.X, B.Y, and B.
11.f.1/2 {8652/0021} {AI95-00182-01} {AI95-00373-01} The ancestor_part of
an extension_aggregate, <> in aggregates, and the return object of
an extended_return_statement are handled similarly.
12 Initialize and other initialization operations are done in an arbitrary
order, except as follows. Initialize is applied to an object after
initialization of its subcomponents, if any [(including both implicit
initialization and Initialize calls)]. If an object has a component with an
access discriminant constrained by a per-object expression, Initialize is
applied to this component after any components that do not have such
discriminants. For an object with several components with such a discriminant,
Initialize is applied to them in order of their component_declarations. For an
allocator, any task activations follow all calls on Initialize.
12.a Reason: The fact that Initialize is done for subcomponents first
allows Initialize for a composite object to refer to its
subcomponents knowing they have been properly initialized.
12.b The fact that Initialize is done for components with access
discriminants after other components allows the Initialize
operation for a component with a self-referential access
discriminant to assume that other components of the enclosing
object have already been properly initialized. For multiple such
components, it allows some predictability.
13 {assignment operation} When a target object with any controlled parts is
assigned a value, [either when created or in a subsequent
assignment_statement,] the assignment operation proceeds as follows:
14 * The value of the target becomes the assigned value.
15 * {adjusting the value of an object} {adjustment} The value of the
target is adjusted.
15.a Ramification: If any parts of the object are controlled, abort is
deferred during the assignment operation.
16 {adjusting the value of an object} {adjustment} To adjust the value of a
[(nonlimited)] composite object, the values of the components of the object
are first adjusted in an arbitrary order, and then, if the object is
controlled, Adjust is called. Adjusting the value of an elementary object has
no effect[, nor does adjusting the value of a composite object with no
controlled parts.]
16.a Ramification: Adjustment is never performed for values of a
by-reference limited type, since these types do not support
copying.
16.b Reason: The verbiage in the Initialize rule about access
discriminants constrained by per-object expressions is not
necessary here, since such types are limited, and therefore are
never adjusted.
17 {execution (assignment_statement) [partial]} For an assignment_statement,
[ after the name and expression have been evaluated, and any conversion
(including constraint checking) has been done,] an anonymous object is
created, and the value is assigned into it; [that is, the assignment operation
is applied]. [(Assignment includes value adjustment.)] The target of the
assignment_statement is then finalized. The value of the anonymous object is
then assigned into the target of the assignment_statement. Finally, the
anonymous object is finalized. [As explained below, the implementation may
eliminate the intermediate anonymous object, so this description subsumes the
one given in 5.2, "Assignment Statements".]
17.a Reason: An alternative design for user-defined assignment might
involve an Assign operation instead of Adjust:
17.b procedure Assign(Target : in out Controlled; Source : in out Controlled);
17.c Or perhaps even a syntax like this:
17.d procedure ":="(Target : in out Controlled; Source : in out Controlled);
17.e Assign (or ":=") would have the responsibility of doing the copy,
as well as whatever else is necessary. This would have the
advantage that the Assign operation knows about both the target
and the source at the same time - it would be possible to do
things like reuse storage belonging to the target, for example,
which Adjust cannot do. However, this sort of design would not
work in the case of unconstrained discriminated variables, because
there is no way to change the discriminants individually. For
example:
17.f type Mutable(D : Integer := 0) is
record
X : Array_Of_Controlled_Things(1..D);
case D is
when 17 => Y : Controlled_Thing;
when others => null;
end D;
end record;
17.g An assignment to an unconstrained variable of type Mutable can
cause some of the components of X, and the component Y, to appear
and/or disappear. There is no way to write the Assign operation to
handle this sort of case.
17.h Forbidding such cases is not an option - it would cause generic
contract model violations.
Implementation Requirements
17.1/2 {8652/0022} {AI95-00083-01} {AI95-00318-02} For an aggregate of a
controlled type whose value is assigned, other than by an
assignment_statement, the implementation shall not create a separate anonymous
object for the aggregate. The aggregate value shall be constructed directly in
the target of the assignment operation and Adjust is not called on the target
object.
17.h.1/2 Reason: {AI95-00318-02} {build-in-place [partial]} This
build-in-place requirement is necessary to prevent elaboration
problems with deferred constants of controlled types. Consider:
17.h.2/1 package P is
type Dyn_String is private;
Null_String : constant Dyn_String;
...
private
type Dyn_String is new Ada.Finalization.Controlled with ...
procedure Finalize(X : in out Dyn_String);
procedure Adjust(X : in out Dyn_String);
Null_String : constant Dyn_String :=
(Ada.Finalization.Controlled with ...);
...
end P;
17.h.3/1 When Null_String is elaborated, the bodies of Finalize and Adjust
clearly have not been elaborated. Without this rule, this
declaration would necessarily raise Program_Error (unless the
permissions given below are used by the implementation).
17.i/2 Ramification: An aggregate used in the return expression of a
simple_return_statement has to be built-in-place in the anonymous
return object, as this is similar to an object declaration. (This
is a change from Ada 95, but it is not an inconsistency as it only
serves to restrict implementation choices.) But this only covers
the aggregate; a separate anonymous return object can still be
used unless it too is required to be built-in-place (see 7.5).
Implementation Permissions
18 An implementation is allowed to relax the above rules [(for nonlimited
controlled types)] in the following ways:
18.a Proof: The phrase "for nonlimited controlled types" follows from
the fact that all of the following permissions apply to cases
involving assignment. It is important because the programmer can
count on a stricter semantics for limited controlled types.
19 * For an assignment_statement that assigns to an object the value of
that same object, the implementation need not do anything.
19.a Ramification: In other words, even if an object is controlled and
a combination of Finalize and Adjust on the object might have a
net side effect, they need not be performed.
20 * For an assignment_statement for a noncontrolled type, the
implementation may finalize and assign each component of the variable
separately (rather than finalizing the entire variable and assigning
the entire new value) unless a discriminant of the variable is changed
by the assignment.
20.a Reason: For example, in a slice assignment, an anonymous object is
not necessary if the slice is copied component-by-component in the
right direction, since array types are not controlled (although
their components may be). Note that the direction, and even the
fact that it's a slice assignment, can in general be determined
only at run time.
21/2 * {AI95-00147-01} For an aggregate or function call whose value is
assigned into a target object, the implementation need not create a
separate anonymous object if it can safely create the value of the
aggregate or function call directly in the target object. Similarly,
for an assignment_statement, the implementation need not create an
anonymous object if the value being assigned is the result of
evaluating a name denoting an object (the source object) whose storage
cannot overlap with the target. If the source object might overlap
with the target object, then the implementation can avoid the need for
an intermediary anonymous object by exercising one of the above
permissions and perform the assignment one component at a time (for an
overlapping array assignment), or not at all (for an assignment where
the target and the source of the assignment are the same object).
21.a Ramification: In the aggregate case, only one value adjustment is
necessary, and there is no anonymous object to be finalized.
21.b/2 {AI95-00147-01} Similarly, in the function call case, the
anonymous object can be eliminated. Note, however, that Adjust
must be called directly on the target object as the last step of
the assignment, since some of the subcomponents may be
self-referential or otherwise position-dependent. This Adjust can
be eliminated only by using one of the following permissions.
22/2 {AI95-00147-01} Furthermore, an implementation is permitted to omit
implicit Initialize, Adjust, and Finalize calls and associated assignment
operations on an object of a nonlimited controlled type provided that:
23/2 * any omitted Initialize call is not a call on a user-defined
Initialize procedure, and
23.a/2 To be honest: This does not apply to any calls to a user-defined
Initialize routine that happen to occur in an Adjust or Finalize
routine. It is intended that it is never necessary to look inside
of an Adjust or Finalize routine to determine if the call can be
omitted.
23.b/2 Reason: We don't want to eliminate objects for which the
Initialize might have side effects (such as locking a resource).
24/2 * any usage of the value of the object after the implicit Initialize or
Adjust call and before any subsequent Finalize call on the object does
not change the external effect of the program, and
25/2 * after the omission of such calls and operations, any execution of the
program that executes an Initialize or Adjust call on an object or
initializes an object by an aggregate will also later execute a
Finalize call on the object and will always do so prior to assigning a
new value to the object, and
26/2 * the assignment operations associated with omitted Adjust calls are
also omitted.
27/2 This permission applies to Adjust and Finalize calls even if the implicit
calls have additional external effects.
27.a/2 Reason: The goal of the above permissions is to allow typical dead
assignment and dead variable removal algorithms to work for
nonlimited controlled types. We require that "pairs" of
Initialize/Adjust/Finalize operations are removed. (These aren't
always pairs, which is why we talk about "any execution of the
program".)
Extensions to Ada 83
27.b {extensions to Ada 83} Controlled types and user-defined
finalization are new to Ada 95. (Ada 83 had finalization semantics
only for masters of tasks.)
Extensions to Ada 95
27.c/2 {AI95-00161-01} {extensions to Ada 95} Amendment Correction: Types
Controlled and Limited_Controlled now have
Preelaborable_Initialization, so that objects of types derived
from these types can be used in preelaborated packages.
Wording Changes from Ada 95
27.d/2 {8652/0020} {AI95-00126-01} Corrigendum: Clarified that
Ada.Finalization is a remote types package.
27.e/2 {8652/0021} {AI95-00182-01} Corrigendum: Added wording to clarify
that the default initialization (whatever it is) of an ancestor
part is used.
27.f/2 {8652/0022} {AI95-00083-01} Corrigendum: Clarified that Adjust is
never called on an aggregate used for the initialization of an
object or subaggregate, or passed as a parameter.
27.g/2 {AI95-00147-01} Additional optimizations are allowed for
nonlimited controlled types. These allow traditional dead variable
elimination to be applied to such types.
27.h/2 {AI95-00318-02} Corrected the build-in-place requirement for
controlled aggregates to be consistent with the requirements for
limited types.
27.i/2 {AI95-00348-01} The operations of types Controlled and
Limited_Controlled are now declared as null procedures (see 6.7)
to make the semantics clear (and to provide a good example of what
null procedures can be used for).
27.j/2 {AI95-00360-01} Types that need finalization are defined; this is
used by the No_Nested_Finalization restriction (see D.7, "
Tasking Restrictions").
27.k/2 {AI95-00373-01} Generalized the description of objects that have
Initialize called for them to say that it is done for all objects
that are initialized by default. This is needed so that all of the
new cases are covered.
7.6.1 Completion and Finalization
1 [This subclause defines completion and leaving of the execution of
constructs and entities. A master is the execution of a construct that
includes finalization of local objects after it is complete (and after waiting
for any local tasks - see 9.3), but before leaving. Other constructs and
entities are left immediately upon completion. {cleanup: See finalization}
{destructor: See finalization} ]
Dynamic Semantics
2/2 {AI95-00318-02} {completion and leaving (completed and left)}
{completion (run-time concept)} The execution of a construct or entity is
complete when the end of that execution has been reached, or when a transfer
of control (see 5.1) causes it to be abandoned. {normal completion}
{completion (normal)} {abnormal completion} {completion (abnormal)} Completion
due to reaching the end of execution, or due to the transfer of control of an
exit_statement, return statement, goto_statement, or requeue_statement or of
the selection of a terminate_alternative is normal completion. Completion is
abnormal otherwise [- when control is transferred out of a construct due to
abort or the raising of an exception].
2.a Discussion: Don't confuse the run-time concept of completion with
the compile-time concept of completion defined in 3.11.1.
3/2 {AI95-00162-01} {AI95-00416-01} {leaving} {left} After execution of a
construct or entity is complete, it is left, meaning that execution continues
with the next action, as defined for the execution that is taking place.
{master} Leaving an execution happens immediately after its completion, except
in the case of a master: the execution of a body other than a package_body;
the execution of a statement; or the evaluation of an expression,
function_call, or range that is not part of an enclosing expression,
function_call, range, or simple_statement other than a
simple_return_statement. A master is finalized after it is complete, and
before it is left.
3.a/2 Reason: {AI95-00162-01} {AI95-00416-01} Expressions and
statements are masters so that objects created by subprogram calls
(in aggregates, allocators for anonymous access-to-object types,
and so on) are finalized and have their tasks awaited before the
expressions or statements are left. Note that expressions like the
condition of an if_statement are masters, because they are not
enclosed by a simple_statement. Similarly, a function_call which
is renamed is a master, as it is not in a simple_statement.
3.b/2 {AI95-00416-01} We have to include function_calls in the contexts
that do not cause masters to occur so that expressions contained
in a function_call (that is not part of an expression or
simple_statement) do not individually become masters. We certainly
do not want the parameter expressions of a function_call to be
separate masters, as they would then be finalized before the
function is called.
3.c/2 Ramification: {AI95-00416-01} The fact that a function_call is a
master does not change the accessibility of the return object
denoted by the function_call; that depends on the use of the
function_call. The function_call is the master of any short-lived
entities (such as aggregates used as parameters of types with task
or controlled parts).
4 {finalization (of a master)} For the finalization of a master, dependent
tasks are first awaited, as explained in 9.3. Then each object whose
accessibility level is the same as that of the master is finalized if the
object was successfully initialized and still exists. [These actions are
performed whether the master is left by reaching the last statement or via a
transfer of control.] When a transfer of control causes completion of an
execution, each included master is finalized in order, from innermost outward.
4.a Ramification: As explained in 3.10.2, the set of objects with the
same accessibility level as that of the master includes objects
declared immediately within the master, objects declared in nested
packages, objects created by allocators (if the ultimate ancestor
access type is declared in one of those places) and subcomponents
of all of these things. If an object was already finalized by
Unchecked_Deallocation, then it is not finalized again when the
master is left.
4.b Note that any object whose accessibility level is deeper than that
of the master would no longer exist; those objects would have been
finalized by some inner master. Thus, after leaving a master, the
only objects yet to be finalized are those whose accessibility
level is less deep than that of the master.
4.c To be honest: Subcomponents of objects due to be finalized are not
finalized by the finalization of the master; they are finalized by
the finalization of the containing object.
4.d Reason: We need to finalize subcomponents of objects even if the
containing object is not going to get finalized because it was not
fully initialized. But if the containing object is finalized, we
don't want to require repeated finalization of the subcomponents,
as might normally be implied by the recursion in finalization of a
master and the recursion in finalization of an object.
4.e To be honest: Formally, completion and leaving refer to executions
of constructs or entities. However, the standard sometimes
(informally) refers to the constructs or entities whose executions
are being completed. Thus, for example, "the subprogram call or
task is complete" really means "the execution of the subprogram
call or task is complete."
5 {finalization (of an object) [distributed]} For the finalization of an
object:
6 * If the object is of an elementary type, finalization has no effect;
7 * If the object is of a controlled type, the Finalize procedure is
called;
8 * If the object is of a protected type, the actions defined in 9.4 are
performed;
9/2 * {AI95-00416-01} If the object is of a composite type, then after
performing the above actions, if any, every component of the object is
finalized in an arbitrary order, except as follows: if the object has
a component with an access discriminant constrained by a per-object
expression, this component is finalized before any components that do
not have such discriminants; for an object with several components
with such a discriminant, they are finalized in the reverse of the
order of their component_declarations;
9.a Reason: This allows the finalization of a component with an access
discriminant to refer to other components of the enclosing object
prior to their being finalized.
9.1/2 * {AI95-00416-01} If the object has coextensions (see 3.10.2), each
coextension is finalized after the object whose access discriminant
designates it.
10 {execution (instance of Unchecked_Deallocation) [partial]} Immediately
before an instance of Unchecked_Deallocation reclaims the storage of an
object, the object is finalized. [If an instance of Unchecked_Deallocation is
never applied to an object created by an allocator, the object will still
exist when the corresponding master completes, and it will be finalized then.]
11/2 {AI95-00280-01} The order in which the finalization of a master performs
finalization of objects is as follows: Objects created by declarations in the
master are finalized in the reverse order of their creation. For objects that
were created by allocators for an access type whose ultimate ancestor is
declared in the master, this rule is applied as though each such object that
still exists had been created in an arbitrary order at the first freezing
point (see 13.14) of the ultimate ancestor type; the finalization of these
objects is called the finalization of the
collection{finalization of the collection} {collection (finalization of)} . After the
finalization of a master is complete, the objects finalized as part of its
finalization cease to exist, as do any types and subtypes defined and created
within the master.{exist (cease to) [partial]} {cease to exist (object)
[partial]} {cease to exist (type)}
11.a Reason: Note that we talk about the type of the allocator here.
There may be access values of a (general) access type pointing at
objects created by allocators for some other type; these are not
finalized at this point.
11.b The freezing point of the ultimate ancestor access type is chosen
because before that point, pool elements cannot be created, and
after that point, access values designating (parts of) the pool
elements can be created. This is also the point after which the
pool object cannot have been declared. We don't want to finalize
the pool elements until after anything finalizing objects that
contain access values designating them. Nor do we want to finalize
pool elements after finalizing the pool object itself.
11.c Ramification: Finalization of allocated objects is done according
to the (ultimate ancestor) allocator type, not according to the
storage pool in which they are allocated. Pool finalization might
reclaim storage (see 13.11, "Storage Management"), but has nothing
(directly) to do with finalization of the pool elements.
11.d Note that finalization is done only for objects that still exist;
if an instance of Unchecked_Deallocation has already gotten rid of
a given pool element, that pool element will not be finalized when
the master is left.
11.e Note that a deferred constant declaration does not create the
constant; the full constant declaration creates it. Therefore, the
order of finalization depends on where the full constant
declaration occurs, not the deferred constant declaration.
11.f An imported object is not created by its declaration. It is
neither initialized nor finalized.
11.g Implementation Note: An implementation has to ensure that the
storage for an object is not reclaimed when references to the
object are still possible (unless, of course, the user explicitly
requests reclamation via an instance of Unchecked_Deallocation).
This implies, in general, that objects cannot be deallocated one
by one as they are finalized; a subsequent finalization might
reference an object that has been finalized, and that object had
better be in its (well-defined) finalized state.
12/2 {AI95-00256-01} {execution (assignment_statement) [partial]} The target
of an assignment_statement is finalized before copying in the new value, as
explained in 7.6.
13/2 {8652/0021} {AI95-00182-01} {AI95-00162-01} The master of an object is
the master enclosing its creation whose accessibility level (see 3.10.2) is
equal to that of the object.
13.a/2 This paragraph was deleted.{AI95-00162-01}
13.b/2 This paragraph was deleted.
13.c/2 This paragraph was deleted.
13.d/2 Reason: {AI95-00162-01} This effectively imports all of the
special rules for the accessibility level of renames, allocators,
and so on, and applies them to determine where objects created in
them are finalized. For instance, the master of a rename of a
subprogram is that of the renamed subprogram.
13.1/2 {8652/0023} {AI95-00169-01} {AI95-00162-01} In the case of an
expression that is a master, finalization of any (anonymous) objects occurs as
the final part of evaluation of the expression.
Bounded (Run-Time) Errors
14/1 {8652/0023} {AI95-00169-01} {bounded error (cause) [partial]} It is a
bounded error for a call on Finalize or Adjust that occurs as part of object
finalization or assignment to propagate an exception. The possible
consequences depend on what action invoked the Finalize or Adjust operation:
14.a Ramification: It is not a bounded error for Initialize to
propagate an exception. If Initialize propagates an exception,
then no further calls on Initialize are performed, and those
components that have already been initialized (either explicitly
or by default) are finalized in the usual way.
14.a.1/1 {8652/0023} {AI95-00169-01} It also is not a bounded error for an
explicit call to Finalize or Adjust to propagate an exception. We
do not want implementations to have to treat explicit calls to
these routines specially.
15 * {Program_Error (raised by failure of run-time check)} For a Finalize
invoked as part of an assignment_statement, Program_Error is raised at
that point.
16/2 * {8652/0024} {AI95-00193-01} {AI95-00256-01} For an Adjust invoked as
part of assignment operations other than those invoked as part of an
assignment_statement, other adjustments due to be performed might or
might not be performed, and then Program_Error is raised. During its
propagation, finalization might or might not be applied to objects
whose Adjust failed.
{Program_Error (raised by failure of run-time check)} For an Adjust
invoked as part of an assignment_statement, any other adjustments due
to be performed are performed, and then Program_Error is raised.
16.a/2 Reason: {8652/0024} {AI95-00193-01} {AI95-00256-01} In the case of
assignments that are part of initialization, there is no need to
complete all adjustments if one propagates an exception, as the
object will immediately be finalized. So long as a subcomponent is
not going to be finalized, it need not be adjusted, even if it is
initialized as part of an enclosing composite assignment operation
for which some adjustments are performed. However, there is no
harm in an implementation making additional Adjust calls (as long
as any additional components that are adjusted are also
finalized), so we allow the implementation flexibility here. On
the other hand, for an assignment_statement, it is important that
all adjustments be performed, even if one fails, because all
controlled subcomponents are going to be finalized. Other kinds of
assignment are more like initialization than
assignment_statements, so we include them as well in the
permission.
16.a.1/1 Ramification: {8652/0024} {AI95-00193-01} Even if an Adjust
invoked as part of the initialization of a controlled object
propagates an exception, objects whose initialization (including
any Adjust or Initialize calls) successfully completed will be
finalized. The permission above only applies to objects whose
Adjust failed. Objects for which Adjust was never even invoked
must not be finalized.
17 * {Program_Error (raised by failure of run-time check)} For a Finalize
invoked as part of a call on an instance of Unchecked_Deallocation,
any other finalizations due to be performed are performed, and then
Program_Error is raised.
17.a.1/1 Discussion: {8652/0104} {AI95-00179-01} The standard does not
specify if storage is recovered in this case. If storage is not
recovered (and the object continues to exist), Finalize may be
called on the object again (when the allocator's master is
finalized).
17.1/1 * {8652/0023} {AI95-00169-01}
{Program_Error (raised by failure of run-time check)} For a Finalize
invoked as part of the finalization of the anonymous object created by
a function call or aggregate, any other finalizations due to be
performed are performed, and then Program_Error is raised.
17.2/1 * {8652/0023} {AI95-00169-01}
{Program_Error (raised by failure of run-time check)} For a Finalize
invoked due to reaching the end of the execution of a master, any
other finalizations associated with the master are performed, and
Program_Error is raised immediately after leaving the master.
18/2 * {AI95-00318-02}
{Program_Error (raised by failure of run-time check)} For a Finalize
invoked by the transfer of control of an exit_statement, return
statement, goto_statement, or requeue_statement, Program_Error is
raised no earlier than after the finalization of the master being
finalized when the exception occurred, and no later than the point
where normal execution would have continued. Any other finalizations
due to be performed up to that point are performed before raising
Program_Error.
18.a Ramification: For example, upon leaving a block_statement due to a
goto_statement, the Program_Error would be raised at the point of
the target statement denoted by the label, or else in some more
dynamically nested place, but not so nested as to allow an
exception_handler that has visibility upon the finalized object to
handle it. For example,
18.b procedure Main is
begin
<<The_Label>>
Outer_Block_Statement : declare
X : Some_Controlled_Type;
begin
Inner_Block_Statement : declare
Y : Some_Controlled_Type;
Z : Some_Controlled_Type;
begin
goto The_Label;
exception
when Program_Error => ... -- Handler number 1.
end;
exception
when Program_Error => ... -- Handler number 2.
end;
exception
when Program_Error => ... -- Handler number 3.
end Main;
18.c The goto_statement will first cause Finalize(Y) to be called.
Suppose that Finalize(Y) propagates an exception. Program_Error
will be raised after leaving Inner_Block_Statement, but before
leaving Main. Thus, handler number 1 cannot handle this
Program_Error; it will be handled either by handler number 2 or
handler number 3. If it is handled by handler number 2, then
Finalize(Z) will be done before executing the handler. If it is
handled by handler number 3, then Finalize(Z) and Finalize(X) will
both be done before executing the handler.
19 * For a Finalize invoked by a transfer of control that is due to raising
an exception, any other finalizations due to be performed for the same
master are performed; Program_Error is raised immediately after
leaving the master.
19.a Ramification: If, in the above example, the goto_statement were
replaced by a raise_statement, then the Program_Error would be
handled by handler number 2, and Finalize(Z) would be done before
executing the handler.
19.b Reason: We considered treating this case in the same way as the
others, but that would render certain exception_handlers useless.
For example, suppose the only exception_handler is one for others
in the main subprogram. If some deeply nested call raises an
exception, causing some Finalize operation to be called, which
then raises an exception, then normal execution "would have
continued" at the beginning of the exception_handler. Raising
Program_Error at that point would cause that handler's code to be
skipped. One would need two nested exception_handlers to be sure
of catching such cases!
19.c On the other hand, the exception_handler for a given master should
not be allowed to handle exceptions raised during finalization of
that master.
20 * For a Finalize invoked by a transfer of control due to an abort or
selection of a terminate alternative, the exception is ignored; any
other finalizations due to be performed are performed.
20.a Ramification: This case includes an asynchronous transfer of
control.
20.b To be honest:
{Program_Error (raised by failure of run-time check)} This violates
the general principle that it is always possible for a bounded
error to raise Program_Error (see 1.1.5, "
Classification of Errors").
NOTES
21 17 The rules of Section 10 imply that immediately prior to partition
termination, Finalize operations are applied to library-level
controlled objects (including those created by allocators of
library-level access types, except those already finalized). This
occurs after waiting for library-level tasks to terminate.
21.a Discussion: We considered defining a pragma that would apply to a
controlled type that would suppress Finalize operations for
library-level objects of the type upon partition termination. This
would be useful for types whose finalization actions consist of
simply reclaiming global heap storage, when this is already
provided automatically by the environment upon program
termination.
22 18 A constant is only constant between its initialization and
finalization. Both initialization and finalization are allowed to
change the value of a constant.
23 19 Abort is deferred during certain operations related to controlled
types, as explained in 9.8. Those rules prevent an abort from causing
a controlled object to be left in an ill-defined state.
24 20 The Finalize procedure is called upon finalization of a controlled
object, even if Finalize was called earlier, either explicitly or as
part of an assignment; hence, if a controlled type is visibly
controlled (implying that its Finalize primitive is directly
callable), or is nonlimited (implying that assignment is allowed), its
Finalize procedure should be designed to have no ill effect if it is
applied a second time to the same object.
24.a Discussion: Or equivalently, a Finalize procedure should be "
idempotent"; applying it twice to the same object should be
equivalent to applying it once.
24.b Reason: A user-written Finalize procedure should be idempotent
since it can be called explicitly by a client (at least if the
type is "visibly" controlled). Also, Finalize is used implicitly
as part of the assignment_statement if the type is nonlimited, and
an abort is permitted to disrupt an assignment_statement between
finalizing the left-hand side and assigning the new value to it
(an abort is not permitted to disrupt an assignment operation
between copying in the new value and adjusting it).
24.c/2 Discussion: {AI95-00287-01} Either Initialize or Adjust, but not
both, is applied to (almost) every controlled object when it is
created: Initialize is done when no initial value is assigned to
the object, whereas Adjust is done as part of assigning the
initial value. The one exception is the object initialized by an
aggregate (both the anonymous object created for an aggregate, or
an object initialized by an aggregate that is built-in-place);
Initialize is not applied to the aggregate as a whole, nor is the
value of the aggregate or object adjusted.
24.d {assignment operation (list of uses)} All of the following use the
assignment operation, and thus perform value adjustment:
24.e * the assignment_statement (see 5.2);
24.f * explicit initialization of a stand-alone object (see 3.3.1) or
of a pool element (see 4.8);
24.g * default initialization of a component of a stand-alone object
or pool element (in this case, the value of each component is
assigned, and therefore adjusted, but the value of the object
as a whole is not adjusted);
24.h/2 * {AI95-00318-02} function return, when the result is not
built-in-place (adjustment of the result happens before
finalization of the function);
24.i * predefined operators (although the only one that matters is
concatenation; see 4.5.3);
24.j * generic formal objects of mode in (see 12.4); these are
defined in terms of constant declarations; and
24.k/2 * {AI95-00287-01} aggregates (see 4.3), when the result is not
built-in-place (in this case, the value of each component, and
the parent part, for an extension_aggregate, is assigned, and
therefore adjusted, but the value of the aggregate as a whole
is not adjusted; neither is Initialize called);
24.l The following also use the assignment operation, but adjustment
never does anything interesting in these cases:
24.m * By-copy parameter passing uses the assignment operation (see
6.4.1), but controlled objects are always passed by reference,
so the assignment operation never does anything interesting in
this case. If we were to allow by-copy parameter passing for
controlled objects, we would need to make sure that the actual
is finalized before doing the copy back for [in] out
parameters. The finalization of the parameter itself needs to
happen after the copy back (if any), similar to the
finalization of an anonymous function return object or
aggregate object.
24.n * For loops use the assignment operation (see 5.5), but since
the type of the loop parameter is never controlled, nothing
interesting happens there, either.
24.n.1/2 * {AI95-00318-02} Objects initialized by function results and
aggregates that are built-in-place. In this case, the
assignment operation is never executed, and no adjustment
takes place. While built-in-place is always allowed, it is
required for some types - see 7.5 and 7.6 - and that's
important since limited types have no Adjust to call.
24.o/2 This paragraph was deleted.{AI95-00287-01}
24.p Finalization of the parts of a protected object are not done as
protected actions. It is possible (in pathological cases) to
create tasks during finalization that access these parts in
parallel with the finalization itself. This is an erroneous use of
shared variables.
24.q Implementation Note: One implementation technique for finalization
is to chain the controlled objects together on a per-task list.
When leaving a master, the list can be walked up to a marked
place. The links needed to implement the list can be declared
(privately) in types Controlled and Limited_Controlled, so they
will be inherited by all controlled types.
24.r Another implementation technique, which we refer to as the "
PC-map" approach essentially implies inserting exception handlers at
various places, and finalizing objects based on where the
exception was raised.
24.s {PC-map approach to finalization}
{program-counter-map approach to finalization} The PC-map approach
is for the compiler/linker to create a map of code addresses; when
an exception is raised, or abort occurs, the map can be consulted
to see where the task was executing, and what finalization needs
to be performed. This approach was given in the Ada 83 Rationale
as a possible implementation strategy for exception handling - the
map is consulted to determine which exception handler applies.
24.t If the PC-map approach is used, the implementation must take care
in the case of arrays. The generated code will generally contain a
loop to initialize an array. If an exception is raised part way
through the array, the components that have been initialized must
be finalized, and the others must not be finalized.
24.u It is our intention that both of these implementation methods
should be possible.
Wording Changes from Ada 83
24.v Finalization depends on the concepts of completion and leaving,
and on the concept of a master. Therefore, we have moved the
definitions of these concepts here, from where they used to be in
Section 9. These concepts also needed to be generalized somewhat.
Task waiting is closely related to user-defined finalization; the
rules here refer to the task-waiting rules of Section 9.
Wording Changes from Ada 95
24.w/2 {8652/0021} {AI95-00182-01} Corrigendum: Fixed the wording to say
that anonymous objects aren't finalized until the object can't be
used anymore.
24.x/2 {8652/0023} {AI95-00169-01} Corrigendum: Added wording to clarify
what happens when Adjust or Finalize raises an exception; some
cases had been omitted.
24.y/2 {8652/0024} {AI95-00193-01} {AI95-00256-01} Corrigendum: Stated
that if Adjust raises an exception during initialization, nothing
further is required. This is corrected in Ada 2005 to include all
kinds of assignment other than assignment_statements.
24.z/2 {AI95-00162-01} {AI95-00416-01} Revised the definition of master
to include expressions and statements, in order to cleanly define
what happens for tasks and controlled objects created as part of a
subprogram call. Having done that, all of the special wording to
cover those cases is eliminated (at least until the Ada comments
start rolling in).
24.aa/2 {AI95-00280-01} We define finalization of the collection here, so
as to be able to conveniently refer to it in other rules
(especially in 4.8, "Allocators").
24.bb/2 {AI95-00416-01} Clarified that a coextension is finalized at the
same time as the outer object. (This was intended for Ada 95, but
since the concept did not have a name, it was overlooked.)
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