Section 13: Representation Issues
1/1 This section describes features for querying and controlling certain
aspects of entities and for interfacing to hardware.
13.1 Operational and Representation Items
0.1/1 Representation and operational items can be used to specify aspects of
entities. Two kinds of aspects of entities can be specified: aspects of
representation and operational aspects. Representation items specify how the
types and other entities of the language are to be mapped onto the underlying
machine. Operational items specify other properties of entities.
1/1 There are six kinds of representation items: attribute_definition_clauses
for representation attributes, enumeration_representation_clauses, record_-
representation_clauses, at_clauses, component_clauses, and representation
pragmas. They can be provided to give more efficient representation or to
interface with features that are outside the domain of the language (for
example, peripheral hardware).
1.1/1 An operational item is an attribute_definition_clause for an operational
attribute.
1.2/1 An operational item or a representation item applies to an entity
identified by a local_name, which denotes an entity declared local to the
current declarative region, or a library unit declared immediately preceding a
representation pragma in a compilation.
Syntax
2/1 aspect_clause ::= attribute_definition_clause
| enumeration_representation_clause
| record_representation_clause
| at_clause
3 local_name ::= direct_name
| direct_name'attribute_designator
| library_unit_name
4/1 A representation pragma is allowed only at places where an
aspect_clause or compilation_unit is allowed.
Name Resolution Rules
5/1 In an operational item or representation item, if the local_name is a
direct_name, then it shall resolve to denote a declaration (or, in the case of
a pragma, one or more declarations) that occurs immediately within the same
declarative region as the item. If the local_name has an
attribute_designator, then it shall resolve to denote an
implementation-defined component (see 13.5.1) or a class-wide type implicitly
declared immediately within the same declarative region as the item. A
local_name that is a library_unit_name (only permitted in a representation
pragma) shall resolve to denote the library_item that immediately precedes
(except for other pragmas) the representation pragma.
Legality Rules
6/1 The local_name of an aspect_clause or representation pragma shall
statically denote an entity (or, in the case of a pragma, one or more
entities) declared immediately preceding it in a compilation, or within the
same declarative_part, package_specification, task_definition, protected_-
definition, or record_definition as the representation or operational item. If
a local_name denotes a local callable entity, it may do so through a local
subprogram_renaming_declaration (as a way to resolve ambiguity in the presence
of overloading); otherwise, the local_name shall not denote a renaming_-
declaration.
7/2 The representation of an object consists of a certain number of bits (the
size of the object). For an object of an elementary type, these are the bits
that are normally read or updated by the machine code when loading, storing,
or operating-on the value of the object. For an object of a composite type,
these are the bits reserved for this object, and include bits occupied by
subcomponents of the object. If the size of an object is greater than that of
its subtype, the additional bits are padding bits. For an elementary object,
these padding bits are normally read and updated along with the others. For a
composite object, padding bits might not be read or updated in any given
composite operation, depending on the implementation.
8 A representation item directly specifies an aspect of representation of
the entity denoted by the local_name, except in the case of a type-related
representation item, whose local_name shall denote a first subtype, and which
directly specifies an aspect of the subtype's type. A representation item that
names a subtype is either subtype-specific (Size and Alignment clauses) or
type-related (all others). Subtype-specific aspects may differ for different
subtypes of the same type.
8.1/1 An operational item directly specifies an operational aspect of the type
of the subtype denoted by the local_name. The local_name of an operational
item shall denote a first subtype. An operational item that names a subtype is
type-related.
9 A representation item that directly specifies an aspect of a subtype or
type shall appear after the type is completely defined (see 3.11.1), and
before the subtype or type is frozen (see 13.14). If a representation item is
given that directly specifies an aspect of an entity, then it is illegal to
give another representation item that directly specifies the same aspect of
the entity.
9.1/1 An operational item that directly specifies an aspect of a type shall
appear before the type is frozen (see 13.14). If an operational item is given
that directly specifies an aspect of a type, then it is illegal to give
another operational item that directly specifies the same aspect of the type.
10 For an untagged derived type, no type-related representation items are
allowed if the parent type is a by-reference type, or has any user-defined
primitive subprograms.
11/2 Operational and representation aspects of a generic formal parameter are
the same as those of the actual. Operational and representation aspects are
the same for all views of a type. A type-related representation item is not
allowed for a descendant of a generic formal untagged type.
12 A representation item that specifies the Size for a given subtype, or the
size or storage place for an object (including a component) of a given
subtype, shall allow for enough storage space to accommodate any value of the
subtype.
13/1 A representation or operational item that is not supported by the
implementation is illegal, or raises an exception at run time.
13.1/2 A type_declaration is illegal if it has one or more progenitors, and a
representation item applies to an ancestor, and this representation item
conflicts with the representation of some other ancestor. The cases that cause
conflicts are implementation defined.
Static Semantics
14 If two subtypes statically match, then their subtype-specific aspects
(Size and Alignment) are the same.
15/1 A derived type inherits each type-related aspect of representation of its
parent type that was directly specified before the declaration of the derived
type, or (in the case where the parent is derived) that was inherited by the
parent type from the grandparent type. A derived subtype inherits each
subtype-specific aspect of representation of its parent subtype that was
directly specified before the declaration of the derived type, or (in the case
where the parent is derived) that was inherited by the parent subtype from the
grandparent subtype, but only if the parent subtype statically matches the
first subtype of the parent type. An inherited aspect of representation is
overridden by a subsequent representation item that specifies the same aspect
of the type or subtype.
15.1/2 In contrast, whether operational aspects are inherited by an untagged
derived type depends on each specific aspect. Operational aspects are never
inherited for a tagged type. When operational aspects are inherited by an
untagged derived type, aspects that were directly specified by operational
items that are visible at the point of the derived type declaration, or (in
the case where the parent is derived) that were inherited by the parent type
from the grandparent type are inherited. An inherited operational aspect is
overridden by a subsequent operational item that specifies the same aspect of
the type.
15.2/2 When an aspect that is a subprogram is inherited, the derived type
inherits the aspect in the same way that a derived type inherits a
user-defined primitive subprogram from its parent (see 3.4).
16 Each aspect of representation of an entity is as follows:
17 * If the aspect is specified for the entity, meaning that it is either
directly specified or inherited, then that aspect of the entity is as
specified, except in the case of Storage_Size, which specifies a
minimum.
18 * If an aspect of representation of an entity is not specified, it is
chosen by default in an unspecified manner.
18.1/1 If an operational aspect is specified for an entity (meaning that it is
either directly specified or inherited), then that aspect of the entity is as
specified. Otherwise, the aspect of the entity has the default value for that
aspect.
18.2/2 A representation item that specifies an aspect of representation that
would have been chosen in the absence of the representation item is said to be
confirming.
Dynamic Semantics
19/1 For the elaboration of an aspect_clause, any evaluable constructs within
it are evaluated.
Implementation Permissions
20 An implementation may interpret aspects of representation in an
implementation-defined manner. An implementation may place
implementation-defined restrictions on representation items. A recommended
level of support is specified for representation items and related features in
each subclause. These recommendations are changed to requirements for
implementations that support the Systems Programming Annex (see C.2, "
Required Representation Support").
Implementation Advice
21 The recommended level of support for all representation items is qualified
as follows:
21.1/2 * A confirming representation item should be supported.
22 * An implementation need not support representation items containing
nonstatic expressions, except that an implementation should support a
representation item for a given entity if each nonstatic expression in
the representation item is a name that statically denotes a constant
declared before the entity.
23 * An implementation need not support a specification for the Size for a
given composite subtype, nor the size or storage place for an object
(including a component) of a given composite subtype, unless the
constraints on the subtype and its composite subcomponents (if any)
are all static constraints.
24/2 * An implementation need not support a nonconfirming representation
item if it could cause an aliased object or an object of a
by-reference type to be allocated at a nonaddressable location or,
when the alignment attribute of the subtype of such an object is
nonzero, at an address that is not an integral multiple of that
alignment.
25/2 * An implementation need not support a nonconfirming representation
item if it could cause an aliased object of an elementary type to have
a size other than that which would have been chosen by default.
26/2 * An implementation need not support a nonconfirming representation
item if it could cause an aliased object of a composite type, or an
object whose type is by-reference, to have a size smaller than that
which would have been chosen by default.
27/2 * An implementation need not support a nonconfirming subtype-specific
representation item specifying an aspect of representation of an
indefinite or abstract subtype.
28/2 For purposes of these rules, the determination of whether a
representation item applied to a type could cause an object to have some
property is based solely on the properties of the type itself, not on any
available information about how the type is used. In particular, it presumes
that minimally aligned objects of this type might be declared at some point.
13.2 Pragma Pack
1 A pragma Pack specifies that storage minimization should be the main
criterion when selecting the representation of a composite type.
Syntax
2 The form of a pragma Pack is as follows:
3 pragma Pack(first_subtype_local_name);
Legality Rules
4 The first_subtype_local_name of a pragma Pack shall denote a composite
subtype.
Static Semantics
5 A pragma Pack specifies the packing aspect of representation; the type (or
the extension part) is said to be packed. For a type extension, the parent
part is packed as for the parent type, and a pragma Pack causes packing only
of the extension part.
Implementation Advice
6 If a type is packed, then the implementation should try to minimize
storage allocated to objects of the type, possibly at the expense of speed of
accessing components, subject to reasonable complexity in addressing
calculations.
6.1/2 If a packed type has a component that is not of a by-reference type and
has no aliased part, then such a component need not be aligned according to
the Alignment of its subtype; in particular it need not be allocated on a
storage element boundary.
7 The recommended level of support for pragma Pack is:
8 * For a packed record type, the components should be packed as tightly
as possible subject to the Sizes of the component subtypes, and
subject to any record_representation_clause that applies to the type;
the implementation may, but need not, reorder components or cross
aligned word boundaries to improve the packing. A component whose Size
is greater than the word size may be allocated an integral number of
words.
9 * For a packed array type, if the component subtype's Size is less than
or equal to the word size, and Component_Size is not specified for the
type, Component_Size should be less than or equal to the Size of the
component subtype, rounded up to the nearest factor of the word size.
13.3 Operational and Representation Attributes
1/1 The values of certain implementation-dependent characteristics can be
obtained by interrogating appropriate operational or representation
attributes. Some of these attributes are specifiable via an
attribute_definition_clause.
Syntax
2 attribute_definition_clause ::=
for local_name'attribute_designator use expression;
| for local_name'attribute_designator use name;
Name Resolution Rules
3 For an attribute_definition_clause that specifies an attribute that
denotes a value, the form with an expression shall be used. Otherwise, the
form with a name shall be used.
4 For an attribute_definition_clause that specifies an attribute that
denotes a value or an object, the expected type for the expression or name is
that of the attribute. For an attribute_definition_clause that specifies an
attribute that denotes a subprogram, the expected profile for the name is the
profile required for the attribute. For an attribute_definition_clause that
specifies an attribute that denotes some other kind of entity, the name shall
resolve to denote an entity of the appropriate kind.
Legality Rules
5/1 An attribute_designator is allowed in an attribute_definition_clause only
if this International Standard explicitly allows it, or for an
implementation-defined attribute if the implementation allows it. Each
specifiable attribute constitutes an operational aspect or aspect of
representation.
6 For an attribute_definition_clause that specifies an attribute that
denotes a subprogram, the profile shall be mode conformant with the one
required for the attribute, and the convention shall be Ada. Additional
requirements are defined for particular attributes.
Static Semantics
7/2 A Size clause is an attribute_definition_clause whose
attribute_designator is Size. Similar definitions apply to the other
specifiable attributes.
8 A storage element is an addressable element of storage in the machine. A
word is the largest amount of storage that can be conveniently and efficiently
manipulated by the hardware, given the implementation's run-time model. A word
consists of an integral number of storage elements.
8.1/2 A machine scalar is an amount of storage that can be conveniently and
efficiently loaded, stored, or operated upon by the hardware. Machine scalars
consist of an integral number of storage elements. The set of machine scalars
is implementation defined, but must include at least the storage element and
the word. Machine scalars are used to interpret component_clauses when the
nondefault bit ordering applies.
9/1 The following representation attributes are defined: Address, Alignment,
Size, Storage_Size, and Component_Size.
10/1 For a prefix X that denotes an object, program unit, or label:
11 X'Address Denotes the address of the first of the storage elements
allocated to X. For a program unit or label, this value refers
to the machine code associated with the corresponding body or
statement. The value of this attribute is of type
System.Address.
12 Address may be specified for stand-alone objects and for
program units via an attribute_definition_clause.
Erroneous Execution
13 If an Address is specified, it is the programmer's responsibility to
ensure that the address is valid; otherwise, program execution is erroneous.
Implementation Advice
14 For an array X, X'Address should point at the first component of the
array, and not at the array bounds.
15 The recommended level of support for the Address attribute is:
16 * X'Address should produce a useful result if X is an object that is
aliased or of a by-reference type, or is an entity whose Address has
been specified.
17 * An implementation should support Address clauses for imported
subprograms.
18/2 * This paragraph was deleted.
19 * If the Address of an object is specified, or it is imported or
exported, then the implementation should not perform optimizations
based on assumptions of no aliases.
NOTES
20 1 The specification of a link name in a pragma Export (see B.1) for a
subprogram or object is an alternative to explicit specification of
its link-time address, allowing a link-time directive to place the
subprogram or object within memory.
21 2 The rules for the Size attribute imply, for an aliased object X,
that if X'Size = Storage_Unit, then X'Address points at a storage
element containing all of the bits of X, and only the bits of X.
Static Semantics
22/2 For a prefix X that denotes an object:
23/2 X'Alignment
The value of this attribute is of type universal_integer, and
nonnegative; zero means that the object is not necessarily
aligned on a storage element boundary. If X'Alignment is not
zero, then X is aligned on a storage unit boundary and
X'Address is an integral multiple of X'Alignment (that is, the
Address modulo the Alignment is zero).
24/2 This paragraph was deleted.
25/2 Alignment may be specified for stand-alone objects via an
attribute_definition_clause; the expression of such a clause
shall be static, and its value nonnegative.
26/2 This paragraph was deleted.
26.1/2 For every subtype S:
26.2/2 S'Alignment
The value of this attribute is of type universal_integer, and
nonnegative.
26.3/2 For an object X of subtype S, if S'Alignment is not zero, then
X'Alignment is a nonzero integral multiple of S'Alignment
unless specified otherwise by a representation item.
26.4/2 Alignment may be specified for first subtypes via an attribute_-
definition_clause; the expression of such a clause shall be
static, and its value nonnegative.
Erroneous Execution
27 Program execution is erroneous if an Address clause is given that
conflicts with the Alignment.
28/2 For an object that is not allocated under control of the implementation,
execution is erroneous if the object is not aligned according to its Alignment.
Implementation Advice
29 The recommended level of support for the Alignment attribute for subtypes
is:
30/2 * An implementation should support an Alignment clause for a discrete
type, fixed point type, record type, or array type, specifying an
Alignment value that is zero or a power of two, subject to the
following:
31/2 * An implementation need not support an Alignment clause for a signed
integer type specifying an Alignment greater than the largest
Alignment value that is ever chosen by default by the implementation
for any signed integer type. A corresponding limitation may be imposed
for modular integer types, fixed point types, enumeration types,
record types, and array types.
32/2 * An implementation need not support a nonconfirming Alignment clause
which could enable the creation of an object of an elementary type
which cannot be easily loaded and stored by available machine
instructions.
32.1/2 * An implementation need not support an Alignment specified for a
derived tagged type which is not a multiple of the Alignment of the
parent type. An implementation need not support a nonconfirming
Alignment specified for a derived untagged by-reference type.
33 The recommended level of support for the Alignment attribute for objects
is:
34/2 * This paragraph was deleted.
35 * For stand-alone library-level objects of statically constrained
subtypes, the implementation should support all Alignments supported
by the target linker. For example, page alignment is likely to be
supported for such objects, but not for subtypes.
35.1/2 * For other objects, an implementation should at least support the
alignments supported for their subtype, subject to the following:
35.2/2 * An implementation need not support Alignments specified for objects
of a by-reference type or for objects of types containing aliased
subcomponents if the specified Alignment is not a multiple of the
Alignment of the subtype of the object.
NOTES
36 3 Alignment is a subtype-specific attribute.
37/2 This paragraph was deleted.
38 4 A component_clause, Component_Size clause, or a pragma Pack can
override a specified Alignment.
Static Semantics
39/1 For a prefix X that denotes an object:
40 X'Size Denotes the size in bits of the representation of the object.
The value of this attribute is of the type universal_integer.
41 Size may be specified for stand-alone objects via an
attribute_definition_clause; the expression of such a clause
shall be static and its value nonnegative.
Implementation Advice
41.1/2 The size of an array object should not include its bounds.
42/2 The recommended level of support for the Size attribute of objects is the
same as for subtypes (see below), except that only a confirming Size clause
need be supported for an aliased elementary object.
43/2 * This paragraph was deleted.
Static Semantics
44 For every subtype S:
45 S'Size If S is definite, denotes the size (in bits) that the
implementation would choose for the following objects of
subtype S:
46 * A record component of subtype S when the record type is
packed.
47 * The formal parameter of an instance of
Unchecked_Conversion that converts from subtype S to some
other subtype.
48 If S is indefinite, the meaning is implementation defined. The
value of this attribute is of the type universal_integer. The
Size of an object is at least as large as that of its subtype,
unless the object's Size is determined by a Size clause, a
component_clause, or a Component_Size clause. Size may be
specified for first subtypes via an attribute_definition_-
clause; the expression of such a clause shall be static and
its value nonnegative.
Implementation Requirements
49 In an implementation, Boolean'Size shall be 1.
Implementation Advice
50/2 If the Size of a subtype allows for efficient independent addressability
(see 9.10) on the target architecture, then the Size of the following objects
of the subtype should equal the Size of the subtype:
51 * Aliased objects (including components).
52 * Unaliased components, unless the Size of the component is determined
by a component_clause or Component_Size clause.
53 A Size clause on a composite subtype should not affect the internal layout
of components.
54 The recommended level of support for the Size attribute of subtypes is:
55 * The Size (if not specified) of a static discrete or fixed point
subtype should be the number of bits needed to represent each value
belonging to the subtype using an unbiased representation, leaving
space for a sign bit only if the subtype contains negative values. If
such a subtype is a first subtype, then an implementation should
support a specified Size for it that reflects this representation.
56 * For a subtype implemented with levels of indirection, the Size should
include the size of the pointers, but not the size of what they point
at.
56.1/2 * An implementation should support a Size clause for a discrete type,
fixed point type, record type, or array type, subject to the
following:
56.2/2 * An implementation need not support a Size clause for a signed
integer type specifying a Size greater than that of the largest
signed integer type supported by the implementation in the absence
of a size clause (that is, when the size is chosen by default). A
corresponding limitation may be imposed for modular integer types,
fixed point types, enumeration types, record types, and array
types.
56.3/2 * A nonconfirming size clause for the first subtype of a derived
untagged by-reference type need not be supported.
NOTES
57 5 Size is a subtype-specific attribute.
58 6 A component_clause or Component_Size clause can override a
specified Size. A pragma Pack cannot.
Static Semantics
59/1 For a prefix T that denotes a task object (after any implicit
dereference):
60 T'Storage_Size
Denotes the number of storage elements reserved for the task.
The value of this attribute is of the type universal_integer.
The Storage_Size includes the size of the task's stack, if
any. The language does not specify whether or not it includes
other storage associated with the task (such as the "task
control block" used by some implementations.) If a pragma
Storage_Size is given, the value of the Storage_Size attribute
is at least the value specified in the pragma.
61 A pragma Storage_Size specifies the amount of storage to be reserved for
the execution of a task.
Syntax
62 The form of a pragma Storage_Size is as follows:
63 pragma Storage_Size(expression);
64 A pragma Storage_Size is allowed only immediately within a
task_definition.
Name Resolution Rules
65 The expression of a pragma Storage_Size is expected to be of any integer
type.
Dynamic Semantics
66 A pragma Storage_Size is elaborated when an object of the type defined by
the immediately enclosing task_definition is created. For the elaboration of a
pragma Storage_Size, the expression is evaluated; the Storage_Size attribute
of the newly created task object is at least the value of the expression.
67 At the point of task object creation, or upon task activation,
Storage_Error is raised if there is insufficient free storage to accommodate
the requested Storage_Size.
Static Semantics
68/1 For a prefix X that denotes an array subtype or array object (after any
implicit dereference):
69 X'Component_Size
Denotes the size in bits of components of the type of X. The
value of this attribute is of type universal_integer.
70 Component_Size may be specified for array types via an
attribute_definition_clause; the expression of such a clause
shall be static, and its value nonnegative.
Implementation Advice
71 The recommended level of support for the Component_Size attribute is:
72 * An implementation need not support specified Component_Sizes that are
less than the Size of the component subtype.
73 * An implementation should support specified Component_Sizes that are
factors and multiples of the word size. For such Component_Sizes, the
array should contain no gaps between components. For other
Component_Sizes (if supported), the array should contain no gaps
between components when packing is also specified; the implementation
should forbid this combination in cases where it cannot support a
no-gaps representation.
Static Semantics
73.1/1 The following operational attribute is defined: External_Tag.
74/1 For every subtype S of a tagged type T (specific or class-wide):
75/1 S'External_Tag
S'External_Tag denotes an external string representation for
S'Tag; it is of the predefined type String. External_Tag may
be specified for a specific tagged type via an
attribute_definition_clause; the expression of such a clause
shall be static. The default external tag representation is
implementation defined. See 3.9.2 and 13.13.2. The value of
External_Tag is never inherited; the default value is always
used unless a new value is directly specified for a type.
Implementation Requirements
76 In an implementation, the default external tag for each specific tagged
type declared in a partition shall be distinct, so long as the type is
declared outside an instance of a generic body. If the compilation unit in
which a given tagged type is declared, and all compilation units on which it
semantically depends, are the same in two different partitions, then the
external tag for the type shall be the same in the two partitions. What it
means for a compilation unit to be the same in two different partitions is
implementation defined. At a minimum, if the compilation unit is not
recompiled between building the two different partitions that include it, the
compilation unit is considered the same in the two partitions.
NOTES
77/2 7 The following language-defined attributes are specifiable, at least
for some of the kinds of entities to which they apply: Address,
Alignment, Bit_Order, Component_Size, External_Tag, Input,
Machine_Radix, Output, Read, Size, Small, Storage_Pool, Storage_Size,
Stream_Size, and Write.
78 8 It follows from the general rules in 13.1 that if one writes "for
X'Size use Y;" then the X'Size attribute_reference will return Y
(assuming the implementation allows the Size clause). The same is true
for all of the specifiable attributes except Storage_Size.
Examples
79 Examples of attribute definition clauses:
80 Byte : constant := 8;
Page : constant := 2**12;
81 type Medium is range 0 .. 65_000;
for Medium'Size use 2*Byte;
for Medium'Alignment use 2;
Device_Register : Medium;
for Device_Register'Size use Medium'Size;
for Device_Register'Address use System.Storage_Elements.To_Address(16#FFFF_0020#);
82 type Short is delta 0.01 range -100.0 .. 100.0;
for Short'Size use 15;
83 for Car_Name'Storage_Size use -- specify access type's storage pool size
2000*((Car'Size/System.Storage_Unit) +1); -- approximately 2000 cars
84/2 function My_Input(Stream : not null access Ada.Streams.Root_Stream_Type'Class)
return T;
for T'Input use My_Input; -- see 13.13.2
NOTES
85 9 Notes on the examples: In the Size clause for Short, fifteen bits
is the minimum necessary, since the type definition requires
Short'Small <= 2**(-7).
13.4 Enumeration Representation Clauses
1 An enumeration_representation_clause specifies the internal codes for
enumeration literals.
Syntax
2 enumeration_representation_clause ::=
for first_subtype_local_name use enumeration_aggregate;
3 enumeration_aggregate ::= array_aggregate
Name Resolution Rules
4 The enumeration_aggregate shall be written as a one-dimensional
array_aggregate, for which the index subtype is the unconstrained subtype of
the enumeration type, and each component expression is expected to be of any
integer type.
Legality Rules
5 The first_subtype_local_name of an enumeration_representation_clause shall
denote an enumeration subtype.
6/2 Each component of the array_aggregate shall be given by an expression
rather than a <>. The expressions given in the array_aggregate shall be
static, and shall specify distinct integer codes for each value of the
enumeration type; the associated integer codes shall satisfy the predefined
ordering relation of the type.
Static Semantics
7 An enumeration_representation_clause specifies the coding aspect of
representation. The coding consists of the internal code for each enumeration
literal, that is, the integral value used internally to represent each literal.
Implementation Requirements
8 For nonboolean enumeration types, if the coding is not specified for the
type, then for each value of the type, the internal code shall be equal to its
position number.
Implementation Advice
9 The recommended level of support for enumeration_representation_clauses
is:
10 * An implementation should support at least the internal codes in the
range System.Min_Int..System.Max_Int. An implementation need not
support enumeration_representation_clauses for boolean types.
NOTES
11/1 10 Unchecked_Conversion may be used to query the internal codes used
for an enumeration type. The attributes of the type, such as Succ,
Pred, and Pos, are unaffected by the
enumeration_representation_clause. For example, Pos always returns the
position number, not the internal integer code that might have been
specified in an enumeration_representation_clause}.
Examples
12 Example of an enumeration representation clause:
13 type Mix_Code is (ADD, SUB, MUL, LDA, STA, STZ);
14 for Mix_Code use
(ADD => 1, SUB => 2, MUL => 3, LDA => 8, STA => 24, STZ =>33);
13.5 Record Layout
1 The (record) layout aspect of representation consists of the storage
places for some or all components, that is, storage place attributes of the
components. The layout can be specified with a record_representation_clause.
13.5.1 Record Representation Clauses
1 A record_representation_clause specifies the storage representation of
records and record extensions, that is, the order, position, and size of
components (including discriminants, if any).
Syntax
2 record_representation_clause ::=
for first_subtype_local_name use
record [mod_clause]
{component_clause}
end record;
3 component_clause ::=
component_local_name at position range first_bit .. last_bit;
4 position ::= static_expression
5 first_bit ::= static_simple_expression
6 last_bit ::= static_simple_expression
Name Resolution Rules
7 Each position, first_bit, and last_bit is expected to be of any integer
type.
Legality Rules
8/2 The first_subtype_local_name of a record_representation_clause shall
denote a specific record or record extension subtype.
9 If the component_local_name is a direct_name, the local_name shall denote
a component of the type. For a record extension, the component shall not be
inherited, and shall not be a discriminant that corresponds to a discriminant
of the parent type. If the component_local_name has an attribute_designator,
the direct_name of the local_name shall denote either the declaration of the
type or a component of the type, and the attribute_designator shall denote an
implementation-defined implicit component of the type.
10 The position, first_bit, and last_bit shall be static expressions. The
value of position and first_bit shall be nonnegative. The value of last_bit
shall be no less than first_bit - 1.
10.1/2 If the nondefault bit ordering applies to the type, then either:
10.2/2 * the value of last_bit shall be less than the size of the largest
machine scalar; or
10.3/2 * the value of first_bit shall be zero and the value of last_bit + 1
shall be a multiple of System.Storage_Unit.
11 At most one component_clause is allowed for each component of the type,
including for each discriminant (component_clauses may be given for some, all,
or none of the components). Storage places within a component_list shall not
overlap, unless they are for components in distinct variants of the same
variant_part.
12 A name that denotes a component of a type is not allowed within a
record_representation_clause for the type, except as the
component_local_name of a component_clause.
Static Semantics
13/2 A record_representation_clause (without the mod_clause) specifies the
layout.
13.1/2 If the default bit ordering applies to the type, the position,
first_bit, and last_bit of each component_clause directly specify the position
and size of the corresponding component.
13.2/2 If the nondefault bit ordering applies to the type then the layout is
determined as follows:
13.3/2 * the component_clauses for which the value of last_bit is greater
than or equal to the size of the largest machine scalar directly
specify the position and size of the corresponding component;
13.4/2 * for other component_clauses, all of the components having the same
value of position are considered to be part of a single machine
scalar, located at that position; this machine scalar has a size which
is the smallest machine scalar size larger than the largest last_bit
for all component_clauses at that position; the first_bit and
last_bit of each component_clause are then interpreted as bit offsets
in this machine scalar.
14 A record_representation_clause for a record extension does not override
the layout of the parent part; if the layout was specified for the parent
type, it is inherited by the record extension.
Implementation Permissions
15 An implementation may generate implementation-defined components (for
example, one containing the offset of another component). An implementation
may generate names that denote such implementation-defined components; such
names shall be implementation-defined attribute_references. An implemen-
tation may allow such implementation-defined names to be used in record_-
representation_clauses. An implementation can restrict such component_clauses
in any manner it sees fit.
16 If a record_representation_clause is given for an untagged derived type,
the storage place attributes for all of the components of the derived type may
differ from those of the corresponding components of the parent type, even for
components whose storage place is not specified explicitly in the record_-
representation_clause.
Implementation Advice
17 The recommended level of support for record_representation_clauses is:
17.1/2 * An implementation should support machine scalars that correspond to
all of the integer, floating point, and address formats supported by
the machine.
18 * An implementation should support storage places that can be extracted
with a load, mask, shift sequence of machine code, and set with a
load, shift, mask, store sequence, given the available machine
instructions and run-time model.
19 * A storage place should be supported if its size is equal to the Size
of the component subtype, and it starts and ends on a boundary that
obeys the Alignment of the component subtype.
20/2 * For a component with a subtype whose Size is less than the word size,
any storage place that does not cross an aligned word boundary should
be supported.
21 * An implementation may reserve a storage place for the tag field of a
tagged type, and disallow other components from overlapping that
place.
22 * An implementation need not support a component_clause for a component
of an extension part if the storage place is not after the storage
places of all components of the parent type, whether or not those
storage places had been specified.
NOTES
23 11 If no component_clause is given for a component, then the choice
of the storage place for the component is left to the implementation.
If component_clauses are given for all components, the
record_representation_clause completely specifies the representation
of the type and will be obeyed exactly by the implementation.
Examples
24 Example of specifying the layout of a record type:
25 Word : constant := 4; -- storage element is byte, 4 bytes per word
26 type State is (A,M,W,P);
type Mode is (Fix, Dec, Exp, Signif);
27 type Byte_Mask is array (0..7) of Boolean;
type State_Mask is array (State) of Boolean;
type Mode_Mask is array (Mode) of Boolean;
28 type Program_Status_Word is
record
System_Mask : Byte_Mask;
Protection_Key : Integer range 0 .. 3;
Machine_State : State_Mask;
Interrupt_Cause : Interruption_Code;
Ilc : Integer range 0 .. 3;
Cc : Integer range 0 .. 3;
Program_Mask : Mode_Mask;
Inst_Address : Address;
end record;
29 for Program_Status_Word use
record
System_Mask at 0*Word range 0 .. 7;
Protection_Key at 0*Word range 10 .. 11; -- bits 8,9 unused
Machine_State at 0*Word range 12 .. 15;
Interrupt_Cause at 0*Word range 16 .. 31;
Ilc at 1*Word range 0 .. 1; -- second word
Cc at 1*Word range 2 .. 3;
Program_Mask at 1*Word range 4 .. 7;
Inst_Address at 1*Word range 8 .. 31;
end record;
30 for Program_Status_Word'Size use 8*System.Storage_Unit;
for Program_Status_Word'Alignment use 8;
NOTES
31 12 Note on the example: The record_representation_clause defines the
record layout. The Size clause guarantees that (at least) eight
storage elements are used for objects of the type. The Alignment
clause guarantees that aliased, imported, or exported objects of the
type will have addresses divisible by eight.
13.5.2 Storage Place Attributes
Static Semantics
1 For a component C of a composite, non-array object R, the storage place
attributes are defined:
2/2 R.C'Position
If the nondefault bit ordering applies to the composite type,
and if a component_clause specifies the placement of C,
denotes the value given for the position of the
component_clause; otherwise, denotes the same value as
R.C'Address - R'Address. The value of this attribute is of the
type universal_integer.
3/2 R.C'First_Bit
If the nondefault bit ordering applies to the composite type,
and if a component_clause specifies the placement of C,
denotes the value given for the first_bit of the
component_clause; otherwise, denotes the offset, from the
start of the first of the storage elements occupied by C, of
the first bit occupied by C. This offset is measured in bits.
The first bit of a storage element is numbered zero. The value
of this attribute is of the type universal_integer.
4/2 R.C'Last_Bit
If the nondefault bit ordering applies to the composite type,
and if a component_clause specifies the placement of C,
denotes the value given for the last_bit of the
component_clause; otherwise, denotes the offset, from the
start of the first of the storage elements occupied by C, of
the last bit occupied by C. This offset is measured in bits.
The value of this attribute is of the type universal_integer.
Implementation Advice
5 If a component is represented using some form of pointer (such as an
offset) to the actual data of the component, and this data is contiguous with
the rest of the object, then the storage place attributes should reflect the
place of the actual data, not the pointer. If a component is allocated
discontiguously from the rest of the object, then a warning should be
generated upon reference to one of its storage place attributes.
13.5.3 Bit Ordering
1 The Bit_Order attribute specifies the interpretation of the storage place
attributes.
Static Semantics
2 A bit ordering is a method of interpreting the meaning of the storage
place attributes. High_Order_First (known in the vernacular as "big endian")
means that the first bit of a storage element (bit 0) is the most significant
bit (interpreting the sequence of bits that represent a component as an
unsigned integer value). Low_Order_First (known in the vernacular as "little
endian") means the opposite: the first bit is the least significant.
3 For every specific record subtype S, the following attribute is defined:
4 S'Bit_Order Denotes the bit ordering for the type of S. The value of this
attribute is of type System.Bit_Order. Bit_Order may be
specified for specific record types via an
attribute_definition_clause; the expression of such a clause
shall be static.
5 If Word_Size = Storage_Unit, the default bit ordering is implementation
defined. If Word_Size > Storage_Unit, the default bit ordering is the same as
the ordering of storage elements in a word, when interpreted as an integer.
6 The storage place attributes of a component of a type are interpreted
according to the bit ordering of the type.
Implementation Advice
7 The recommended level of support for the nondefault bit ordering is:
8/2 * The implementation should support the nondefault bit ordering in
addition to the default bit ordering.
NOTES
9/2 13 Bit_Order clauses make it possible to write
record_representation_clauses that can be ported between machines
having different bit ordering. They do not guarantee transparent
exchange of data between such machines.
13.6 Change of Representation
1 A type_conversion (see 4.6) can be used to convert between two different
representations of the same array or record. To convert an array from one
representation to another, two array types need to be declared with matching
component subtypes, and convertible index types. If one type has packing
specified and the other does not, then explicit conversion can be used to pack
or unpack an array.
2 To convert a record from one representation to another, two record types
with a common ancestor type need to be declared, with no inherited
subprograms. Distinct representations can then be specified for the record
types, and explicit conversion between the types can be used to effect a
change in representation.
Examples
3 Example of change of representation:
4 -- Packed_Descriptor and Descriptor are two different types
-- with identical characteristics, apart from their
-- representation
5 type Descriptor is
record
-- components of a descriptor
end record;
6 type Packed_Descriptor is new Descriptor;
7 for Packed_Descriptor use
record
-- component clauses for some or for all components
end record;
8 -- Change of representation can now be accomplished by explicit type conversions:
9 D : Descriptor;
P : Packed_Descriptor;
10 P := Packed_Descriptor(D); -- pack D
D := Descriptor(P); -- unpack P
13.7 The Package System
1 For each implementation there is a library package called System which
includes the definitions of certain configuration-dependent characteristics.
Static Semantics
2 The following language-defined library package exists:
3/2 package System is
pragma Pure(System);
4 type Name is implementation-defined-enumeration-type;
System_Name : constant Name := implementation-defined;
5 -- System-Dependent Named Numbers:
6 Min_Int : constant := root_integer'First;
Max_Int : constant := root_integer'Last;
7 Max_Binary_Modulus : constant := implementation-defined;
Max_Nonbinary_Modulus : constant := implementation-defined;
8 Max_Base_Digits : constant := root_real'Digits;
Max_Digits : constant := implementation-defined;
9 Max_Mantissa : constant := implementation-defined;
Fine_Delta : constant := implementation-defined;
10 Tick : constant := implementation-defined;
11 -- Storage-related Declarations:
12 type Address is implementation-defined;
Null_Address : constant Address;
13 Storage_Unit : constant := implementation-defined;
Word_Size : constant := implementation-defined * Storage_Unit;
Memory_Size : constant := implementation-defined;
14 -- Address Comparison:
function "<" (Left, Right : Address) return Boolean;
function "<="(Left, Right : Address) return Boolean;
function ">" (Left, Right : Address) return Boolean;
function ">="(Left, Right : Address) return Boolean;
function "=" (Left, Right : Address) return Boolean;
-- function "/=" (Left, Right : Address) return Boolean;
-- "/=" is implicitly defined
pragma Convention(Intrinsic, "<");
... -- and so on for all language-defined subprograms in this package
15/2 -- Other System-Dependent Declarations:
type Bit_Order is (High_Order_First, Low_Order_First);
Default_Bit_Order : constant Bit_Order := implementation-defined;
16 -- Priority-related declarations (see D.1):
subtype Any_Priority is Integer range implementation-defined;
subtype Priority is Any_Priority range Any_Priority'First ..
implementation-defined;
subtype Interrupt_Priority is Any_Priority range Priority'Last+1 ..
Any_Priority'Last;
17 Default_Priority : constant Priority :=
(Priority'First + Priority'Last)/2;
18 private
... -- not specified by the language
end System;
19 Name is an enumeration subtype. Values of type Name are the names of
alternative machine configurations handled by the implementation. System_Name
represents the current machine configuration.
20 The named numbers Fine_Delta and Tick are of the type universal_real; the
others are of the type universal_integer.
21 The meanings of the named numbers are:
22 Min_Int The smallest (most negative) value allowed for the expressions
of a signed_integer_type_definition.
23 Max_Int The largest (most positive) value allowed for the expressions
of a signed_integer_type_definition.
24 Max_Binary_Modulus
A power of two such that it, and all lesser positive powers of
two, are allowed as the modulus of a modular_type_definition.
25 Max_Nonbinary_Modulus
A value such that it, and all lesser positive integers, are
allowed as the modulus of a modular_type_definition.
26 Max_Base_Digits
The largest value allowed for the requested decimal precision
in a floating_point_definition.
27 Max_Digits The largest value allowed for the requested decimal precision
in a floating_point_definition that has no
real_range_specification. Max_Digits is less than or equal to
Max_Base_Digits.
28 Max_Mantissa
The largest possible number of binary digits in the mantissa
of machine numbers of a user-defined ordinary fixed point
type. (The mantissa is defined in Annex G.)
29 Fine_Delta The smallest delta allowed in an
ordinary_fixed_point_definition that has the real_range_-
specification range -1.0 .. 1.0.
30 Tick A period in seconds approximating the real time interval
during which the value of Calendar.Clock remains constant.
31 Storage_Unit
The number of bits per storage element.
32 Word_Size The number of bits per word.
33 Memory_Size An implementation-defined value that is intended to reflect
the memory size of the configuration in storage elements.
34/2 Address is a definite, nonlimited type with preelaborable initialization
(see 10.2.1). Address represents machine addresses capable of addressing
individual storage elements. Null_Address is an address that is distinct from
the address of any object or program unit.
35/2 Default_Bit_Order shall be a static constant. See 13.5.3 for an
explanation of Bit_Order and Default_Bit_Order.
Implementation Permissions
36/2 An implementation may add additional implementation-defined declarations
to package System and its children. However, it is usually better for the
implementation to provide additional functionality via implementation-defined
children of System.
Implementation Advice
37 Address should be a private type.
NOTES
38 14 There are also some language-defined child packages of System
defined elsewhere.
13.7.1 The Package System.Storage_Elements
Static Semantics
1 The following language-defined library package exists:
2/2 package System.Storage_Elements is
pragma Pure(Storage_Elements);
3 type Storage_Offset is range implementation-defined;
4 subtype Storage_Count
is Storage_Offset range 0..Storage_Offset'Last;
5 type Storage_Element is mod implementation-defined;
for Storage_Element'Size use Storage_Unit;
type Storage_Array is array
(Storage_Offset range <>) of aliased Storage_Element;
for Storage_Array'Component_Size use Storage_Unit;
6 -- Address Arithmetic:
7 function "+"(Left : Address; Right : Storage_Offset)
return Address;
function "+"(Left : Storage_Offset; Right : Address)
return Address;
function "-"(Left : Address; Right : Storage_Offset)
return Address;
function "-"(Left, Right : Address)
return Storage_Offset;
8 function "mod"(Left : Address; Right : Storage_Offset)
return Storage_Offset;
9 -- Conversion to/from integers:
10 type Integer_Address is implementation-defined;
function To_Address(Value : Integer_Address) return Address;
function To_Integer(Value : Address) return Integer_Address;
11 pragma Convention(Intrinsic, "+");
-- ...and so on for all language-defined subprograms declared in this package.
end System.Storage_Elements;
12 Storage_Element represents a storage element. Storage_Offset represents an
offset in storage elements. Storage_Count represents a number of storage
elements. Storage_Array represents a contiguous sequence of storage elements.
13 Integer_Address is a (signed or modular) integer subtype. To_Address and
To_Integer convert back and forth between this type and Address.
Implementation Requirements
14 Storage_Offset'Last shall be greater than or equal to Integer'Last or the
largest possible storage offset, whichever is smaller. Storage_Offset'First
shall be <= (-Storage_Offset'Last).
Implementation Permissions
15/2 This paragraph was deleted.
Implementation Advice
16 Operations in System and its children should reflect the target
environment semantics as closely as is reasonable. For example, on most
machines, it makes sense for address arithmetic to "wrap around." Operations
that do not make sense should raise Program_Error.
13.7.2 The Package System.Address_To_Access_Conversions
Static Semantics
1 The following language-defined generic library package exists:
2 generic
type Object(<>) is limited private;
package System.Address_To_Access_Conversions is
pragma Preelaborate(Address_To_Access_Conversions);
3 type Object_Pointer is access all Object;
function To_Pointer(Value : Address) return Object_Pointer;
function To_Address(Value : Object_Pointer) return Address;
4 pragma Convention(Intrinsic, To_Pointer);
pragma Convention(Intrinsic, To_Address);
end System.Address_To_Access_Conversions;
5/2 The To_Pointer and To_Address subprograms convert back and forth between
values of types Object_Pointer and Address. To_Pointer(X'Address) is equal to
X'Unchecked_Access for any X that allows Unchecked_Access.
To_Pointer(Null_Address) returns null. For other addresses, the behavior is
unspecified. To_Address(null) returns Null_Address. To_Address(Y), where Y /=
null, returns Y.all'Address.
Implementation Permissions
6 An implementation may place restrictions on instantiations of
Address_To_Access_Conversions.
13.8 Machine Code Insertions
1 A machine code insertion can be achieved by a call to a subprogram whose
sequence_of_statements contains code_statements.
Syntax
2 code_statement ::= qualified_expression;
3 A code_statement is only allowed in the handled_sequence_of_statements
of a subprogram_body. If a subprogram_body contains any
code_statements, then within this subprogram_body the only allowed
form of statement is a code_statement (labeled or not), the only
allowed declarative_items are use_clauses, and no exception_handler is
allowed (comments and pragmas are allowed as usual).
Name Resolution Rules
4 The qualified_expression is expected to be of any type.
Legality Rules
5 The qualified_expression shall be of a type declared in package
System.Machine_Code.
6 A code_statement shall appear only within the scope of a with_clause that
mentions package System.Machine_Code.
Static Semantics
7 The contents of the library package System.Machine_Code (if provided) are
implementation defined. The meaning of code_statements is implementation
defined. Typically, each qualified_expression represents a machine instruction
or assembly directive.
Implementation Permissions
8 An implementation may place restrictions on code_statements. An
implementation is not required to provide package System.Machine_Code.
NOTES
9 15 An implementation may provide implementation-defined pragmas
specifying register conventions and calling conventions.
10/2 16 Machine code functions are exempt from the rule that a return
statement is required. In fact, return statements are forbidden, since
only code_statements are allowed.
11 17 Intrinsic subprograms (see 6.3.1, "Conformance Rules") can also be
used to achieve machine code insertions. Interface to assembly
language can be achieved using the features in Annex B, "
Interface to Other Languages".
Examples
12 Example of a code statement:
13 M : Mask;
procedure Set_Mask; pragma Inline(Set_Mask);
14 procedure Set_Mask is
use System.Machine_Code; -- assume "with System.Machine_Code;"
appears somewhere above
begin
SI_Format'(Code => SSM, B => M'Base_Reg, D => M'Disp);
-- Base_Reg and Disp are implementation-defined attributes
end Set_Mask;
13.9 Unchecked Type Conversions
1 An unchecked type conversion can be achieved by a call to an instance of
the generic function Unchecked_Conversion.
Static Semantics
2 The following language-defined generic library function exists:
3 generic
type Source(<>) is limited private;
type Target(<>) is limited private;
function Ada.Unchecked_Conversion(S : Source) return Target;
pragma Convention(Intrinsic, Ada.Unchecked_Conversion);
pragma Pure(Ada.Unchecked_Conversion);
Dynamic Semantics
4 The size of the formal parameter S in an instance of Unchecked_Conversion
is that of its subtype. This is the actual subtype passed to Source, except
when the actual is an unconstrained composite subtype, in which case the
subtype is constrained by the bounds or discriminants of the value of the
actual expression passed to S.
5 If all of the following are true, the effect of an unchecked conversion is
to return the value of an object of the target subtype whose representation is
the same as that of the source object S:
6 * S'Size = Target'Size.
7 * S'Alignment = Target'Alignment.
8 * The target subtype is not an unconstrained composite subtype.
9 * S and the target subtype both have a contiguous representation.
10 * The representation of S is a representation of an object of the target
subtype.
11/2 Otherwise, if the result type is scalar, the result of the function is
implementation defined, and can have an invalid representation (see 13.9.1).
If the result type is nonscalar, the effect is implementation defined; in
particular, the result can be abnormal (see 13.9.1).
Implementation Permissions
12 An implementation may return the result of an unchecked conversion by
reference, if the Source type is not a by-copy type. In this case, the result
of the unchecked conversion represents simply a different (read-only) view of
the operand of the conversion.
13 An implementation may place restrictions on Unchecked_Conversion.
Implementation Advice
14/2 Since the Size of an array object generally does not include its bounds,
the bounds should not be part of the converted data.
15 The implementation should not generate unnecessary run-time checks to
ensure that the representation of S is a representation of the target type. It
should take advantage of the permission to return by reference when possible.
Restrictions on unchecked conversions should be avoided unless required by the
target environment.
16 The recommended level of support for unchecked conversions is:
17 * Unchecked conversions should be supported and should be reversible in
the cases where this clause defines the result. To enable meaningful
use of unchecked conversion, a contiguous representation should be
used for elementary subtypes, for statically constrained array
subtypes whose component subtype is one of the subtypes described in
this paragraph, and for record subtypes without discriminants whose
component subtypes are described in this paragraph.
13.9.1 Data Validity
1 Certain actions that can potentially lead to erroneous execution are not
directly erroneous, but instead can cause objects to become abnormal.
Subsequent uses of abnormal objects can be erroneous.
2 A scalar object can have an invalid representation, which means that the
object's representation does not represent any value of the object's subtype.
The primary cause of invalid representations is uninitialized variables.
3 Abnormal objects and invalid representations are explained in this
subclause.
Dynamic Semantics
4 When an object is first created, and any explicit or default
initializations have been performed, the object and all of its parts are in
the normal state. Subsequent operations generally leave them normal. However,
an object or part of an object can become abnormal in the following ways:
5 * An assignment to the object is disrupted due to an abort (see 9.8) or
due to the failure of a language-defined check (see 11.6).
6/2 * The object is not scalar, and is passed to an in out or out parameter
of an imported procedure, the Read procedure of an instance of
Sequential_IO, Direct_IO, or Storage_IO, or the stream attribute
T'Read, if after return from the procedure the representation of the
parameter does not represent a value of the parameter's subtype.
6.1/2 * The object is the return object of a function call of a nonscalar
type, and the function is an imported function, an instance of
Unchecked_Conversion, or the stream attribute T'Input, if after return
from the function the representation of the return object does not
represent a value of the function's subtype.
6.2/2 For an imported object, it is the programmer's responsibility to ensure
that the object remains in a normal state.
7 Whether or not an object actually becomes abnormal in these cases is not
specified. An abnormal object becomes normal again upon successful completion
of an assignment to the object as a whole.
Erroneous Execution
8 It is erroneous to evaluate a primary that is a name denoting an abnormal
object, or to evaluate a prefix that denotes an abnormal object.
Bounded (Run-Time) Errors
9 If the representation of a scalar object does not represent a value of the
object's subtype (perhaps because the object was not initialized), the object
is said to have an invalid representation. It is a bounded error to evaluate
the value of such an object. If the error is detected, either Constraint_Error
or Program_Error is raised. Otherwise, execution continues using the invalid
representation. The rules of the language outside this subclause assume that
all objects have valid representations. The semantics of operations on invalid
representations are as follows:
10 * If the representation of the object represents a value of the object's
type, the value of the type is used.
11 * If the representation of the object does not represent a value of the
object's type, the semantics of operations on such representations is
implementation-defined, but does not by itself lead to erroneous or
unpredictable execution, or to other objects becoming abnormal.
Erroneous Execution
12/2 A call to an imported function or an instance of Unchecked_Conversion is
erroneous if the result is scalar, the result object has an invalid
representation, and the result is used other than as the expression of an
assignment_statement or an object_declaration, or as the prefix of a Valid
attribute. If such a result object is used as the source of an assignment, and
the assigned value is an invalid representation for the target of the
assignment, then any use of the target object prior to a further assignment to
the target object, other than as the prefix of a Valid attribute reference, is
erroneous.
13 The dereference of an access value is erroneous if it does not designate
an object of an appropriate type or a subprogram with an appropriate profile,
if it designates a nonexistent object, or if it is an access-to-variable value
that designates a constant object. Such an access value can exist, for
example, because of Unchecked_Deallocation, Unchecked_Access, or
Unchecked_Conversion.
NOTES
14 18 Objects can become abnormal due to other kinds of actions that
directly update the object's representation; such actions are
generally considered directly erroneous, however.
13.9.2 The Valid Attribute
1 The Valid attribute can be used to check the validity of data produced by
unchecked conversion, input, interface to foreign languages, and the like.
Static Semantics
2 For a prefix X that denotes a scalar object (after any implicit
dereference), the following attribute is defined:
3 X'Valid Yields True if and only if the object denoted by X is normal
and has a valid representation. The value of this attribute is
of the predefined type Boolean.
NOTES
4 19 Invalid data can be created in the following cases (not counting
erroneous or unpredictable execution):
5 * an uninitialized scalar object,
6 * the result of an unchecked conversion,
7 * input,
8 * interface to another language (including machine code),
9 * aborting an assignment,
10 * disrupting an assignment due to the failure of a language-defined
check (see 11.6), and
11 * use of an object whose Address has been specified.
12 20 X'Valid is not considered to be a read of X; hence, it is not an
error to check the validity of invalid data.
13/2 21 The Valid attribute may be used to check the result of calling an
instance of Unchecked_Conversion (or any other operation that can
return invalid values). However, an exception handler should also be
provided because implementations are permitted to raise
Constraint_Error or Program_Error if they detect the use of an invalid
representation (see 13.9.1).
13.10 Unchecked Access Value Creation
1 The attribute Unchecked_Access is used to create access values in an
unsafe manner - the programmer is responsible for preventing "dangling
references."
Static Semantics
2 The following attribute is defined for a prefix X that denotes an aliased
view of an object:
3 X'Unchecked_Access
All rules and semantics that apply to X'Access (see 3.10.2)
apply also to X'Unchecked_Access, except that, for the
purposes of accessibility rules and checks, it is as if X were
declared immediately within a library package.
NOTES
4 22 This attribute is provided to support the situation where a local
object is to be inserted into a global linked data structure, when the
programmer knows that it will always be removed from the data
structure prior to exiting the object's scope. The Access attribute
would be illegal in this case (see 3.10.2, "
Operations of Access Types").
5 23 There is no Unchecked_Access attribute for subprograms.
13.11 Storage Management
1 Each access-to-object type has an associated storage pool. The storage
allocated by an allocator comes from the pool; instances of
Unchecked_Deallocation return storage to the pool. Several access types can
share the same pool.
2/2 A storage pool is a variable of a type in the class rooted at
Root_Storage_Pool, which is an abstract limited controlled type. By default,
the implementation chooses a standard storage pool for each access-to-object
type. The user may define new pool types, and may override the choice of pool
for an access-to-object type by specifying Storage_Pool for the type.
Legality Rules
3 If Storage_Pool is specified for a given access type, Storage_Size shall
not be specified for it.
Static Semantics
4 The following language-defined library package exists:
5 with Ada.Finalization;
with System.Storage_Elements;
package System.Storage_Pools is
pragma Preelaborate(System.Storage_Pools);
6/2 type Root_Storage_Pool is
abstract new Ada.Finalization.Limited_Controlled with private;
pragma Preelaborable_Initialization(Root_Storage_Pool);
7 procedure Allocate(
Pool : in out Root_Storage_Pool;
Storage_Address : out Address;
Size_In_Storage_Elements : in Storage_Elements.Storage_Count;
Alignment : in Storage_Elements.Storage_Count) is abstract;
8 procedure Deallocate(
Pool : in out Root_Storage_Pool;
Storage_Address : in Address;
Size_In_Storage_Elements : in Storage_Elements.Storage_Count;
Alignment : in Storage_Elements.Storage_Count) is abstract;
9 function Storage_Size(Pool : Root_Storage_Pool)
return Storage_Elements.Storage_Count is abstract;
10 private
... -- not specified by the language
end System.Storage_Pools;
11 A storage pool type (or pool type) is a descendant of Root_Storage_Pool.
The elements of a storage pool are the objects allocated in the pool by
allocators.
12/2 For every access-to-object subtype S, the following representation
attributes are defined:
13 S'Storage_Pool
Denotes the storage pool of the type of S. The type of this
attribute is Root_Storage_Pool'Class.
14 S'Storage_Size
Yields the result of calling Storage_Size(S'Storage_Pool),
which is intended to be a measure of the number of storage
elements reserved for the pool. The type of this attribute is
universal_integer.
15 Storage_Size or Storage_Pool may be specified for a non-derived
access-to-object type via an attribute_definition_clause; the name in a
Storage_Pool clause shall denote a variable.
16 An allocator of type T allocates storage from T's storage pool. If the
storage pool is a user-defined object, then the storage is allocated by
calling Allocate, passing T'Storage_Pool as the Pool parameter. The
Size_In_Storage_Elements parameter indicates the number of storage elements to
be allocated, and is no more than D'Max_Size_In_Storage_Elements, where D is
the designated subtype. The Alignment parameter is D'Alignment. The result
returned in the Storage_Address parameter is used by the allocator as the
address of the allocated storage, which is a contiguous block of memory of
Size_In_Storage_Elements storage elements. Any exception propagated by
Allocate is propagated by the allocator.
17 If Storage_Pool is not specified for a type defined by an
access_to_object_definition, then the implementation chooses a standard
storage pool for it in an implementation-defined manner. In this case, the
exception Storage_Error is raised by an allocator if there is not enough
storage. It is implementation defined whether or not the implementation
provides user-accessible names for the standard pool type(s).
18 If Storage_Size is specified for an access type, then the Storage_Size of
this pool is at least that requested, and the storage for the pool is
reclaimed when the master containing the declaration of the access type is
left. If the implementation cannot satisfy the request, Storage_Error is
raised at the point of the attribute_definition_clause. If neither
Storage_Pool nor Storage_Size are specified, then the meaning of Storage_Size
is implementation defined.
19 If Storage_Pool is specified for an access type, then the specified pool
is used.
20 The effect of calling Allocate and Deallocate for a standard storage pool
directly (rather than implicitly via an allocator or an instance of
Unchecked_Deallocation) is unspecified.
Erroneous Execution
21 If Storage_Pool is specified for an access type, then if Allocate can
satisfy the request, it should allocate a contiguous block of memory, and
return the address of the first storage element in Storage_Address. The block
should contain Size_In_Storage_Elements storage elements, and should be
aligned according to Alignment. The allocated storage should not be used for
any other purpose while the pool element remains in existence. If the request
cannot be satisfied, then Allocate should propagate an exception (such as
Storage_Error). If Allocate behaves in any other manner, then the program
execution is erroneous.
Documentation Requirements
22 An implementation shall document the set of values that a user-defined
Allocate procedure needs to accept for the Alignment parameter. An
implementation shall document how the standard storage pool is chosen, and how
storage is allocated by standard storage pools.
Implementation Advice
23 An implementation should document any cases in which it dynamically
allocates heap storage for a purpose other than the evaluation of an
allocator.
24 A default (implementation-provided) storage pool for an access-to-constant
type should not have overhead to support deallocation of individual objects.
25/2 The storage pool used for an allocator of an anonymous access type should
be determined as follows:
25.1/2 * If the allocator is defining a coextension (see 3.10.2) of an
object being created by an outer allocator, then the storage pool used
for the outer allocator should also be used for the coextension;
25.2/2 * For other access discriminants and access parameters, the storage
pool should be created at the point of the allocator, and be reclaimed
when the allocated object becomes inaccessible;
25.3/2 * Otherwise, a default storage pool should be created at the point
where the anonymous access type is elaborated; such a storage pool
need not support deallocation of individual objects.
NOTES
26 24 A user-defined storage pool type can be obtained by extending the
Root_Storage_Pool type, and overriding the primitive subprograms
Allocate, Deallocate, and Storage_Size. A user-defined storage pool
can then be obtained by declaring an object of the type extension. The
user can override Initialize and Finalize if there is any need for
non-trivial initialization and finalization for a user-defined pool
type. For example, Finalize might reclaim blocks of storage that are
allocated separately from the pool object itself.
27 25 The writer of the user-defined allocation and deallocation
procedures, and users of allocators for the associated access type,
are responsible for dealing with any interactions with tasking. In
particular:
28 * If the allocators are used in different tasks, they require mutual
exclusion.
29 * If they are used inside protected objects, they cannot block.
30 * If they are used by interrupt handlers (see C.3, "
Interrupt Support"), the mutual exclusion mechanism has to work
properly in that context.
31 26 The primitives Allocate, Deallocate, and Storage_Size are declared
as abstract (see 3.9.3), and therefore they have to be overridden when
a new (non-abstract) storage pool type is declared.
Examples
32 To associate an access type with a storage pool object, the user first
declares a pool object of some type derived from Root_Storage_Pool. Then, the
user defines its Storage_Pool attribute, as follows:
33 Pool_Object : Some_Storage_Pool_Type;
34 type T is access Designated;
for T'Storage_Pool use Pool_Object;
35 Another access type may be added to an existing storage pool, via:
36 for T2'Storage_Pool use T'Storage_Pool;
37 The semantics of this is implementation defined for a standard storage
pool.
38 As usual, a derivative of Root_Storage_Pool may define additional
operations. For example, presuming that Mark_Release_Pool_Type has two
additional operations, Mark and Release, the following is a possible use:
39/1 type Mark_Release_Pool_Type
(Pool_Size : Storage_Elements.Storage_Count;
Block_Size : Storage_Elements.Storage_Count)
is new Root_Storage_Pool with private;
40 ...
41 MR_Pool : Mark_Release_Pool_Type (Pool_Size => 2000,
Block_Size => 100);
42 type Acc is access ...;
for Acc'Storage_Pool use MR_Pool;
...
43 Mark(MR_Pool);
... -- Allocate objects using "new Designated(...)".
Release(MR_Pool); -- Reclaim the storage.
13.11.1 The Max_Size_In_Storage_Elements Attribute
1 The Max_Size_In_Storage_Elements attribute is useful in writing
user-defined pool types.
Static Semantics
2 For every subtype S, the following attribute is defined:
3/2 S'Max_Size_In_Storage_Elements
Denotes the maximum value for Size_In_Storage_Elements that
could be requested by the implementation via Allocate for an
access type whose designated subtype is S. For a type with
access discriminants, if the implementation allocates space
for a coextension in the same pool as that of the object
having the access discriminant, then this accounts for any
calls on Allocate that could be performed to provide space for
such coextensions. The value of this attribute is of type
universal_integer.
13.11.2 Unchecked Storage Deallocation
1 Unchecked storage deallocation of an object designated by a value of an
access type is achieved by a call to an instance of the generic procedure
Unchecked_Deallocation.
Static Semantics
2 The following language-defined generic library procedure exists:
3 generic
type Object(<>) is limited private;
type Name is access Object;
procedure Ada.Unchecked_Deallocation(X : in out Name);
pragma Convention(Intrinsic, Ada.Unchecked_Deallocation);
pragma Preelaborate(Ada.Unchecked_Deallocation);
Dynamic Semantics
4 Given an instance of Unchecked_Deallocation declared as follows:
5 procedure Free is
new Ada.Unchecked_Deallocation(
object_subtype_name, access_to_variable_subtype_name);
6 Procedure Free has the following effect:
7 1. After executing Free(X), the value of X is null.
8 2. Free(X), when X is already equal to null, has no effect.
9/2 3. Free(X), when X is not equal to null first performs finalization of
the object designated by X (and any coextensions of the object - see
3.10.2), as described in 7.6.1. It then deallocates the storage
occupied by the object designated by X (and any coextensions). If the
storage pool is a user-defined object, then the storage is deallocated
by calling Deallocate, passing
access_to_variable_subtype_name'Storage_Pool as the Pool parameter.
Storage_Address is the value returned in the Storage_Address parameter
of the corresponding Allocate call. Size_In_Storage_Elements and
Alignment are the same values passed to the corresponding Allocate
call. There is one exception: if the object being freed contains
tasks, the object might not be deallocated.
10/2 After Free(X), the object designated by X, and any subcomponents (and
coextensions) thereof, no longer exist; their storage can be reused for other
purposes.
Bounded (Run-Time) Errors
11 It is a bounded error to free a discriminated, unterminated task object.
The possible consequences are:
12 * No exception is raised.
13 * Program_Error or Tasking_Error is raised at the point of the
deallocation.
14 * Program_Error or Tasking_Error is raised in the task the next time it
references any of the discriminants.
15 In the first two cases, the storage for the discriminants (and for any
enclosing object if it is designated by an access discriminant of the task) is
not reclaimed prior to task termination.
Erroneous Execution
16 Evaluating a name that denotes a nonexistent object is erroneous. The
execution of a call to an instance of Unchecked_Deallocation is erroneous if
the object was created other than by an allocator for an access type whose
pool is Name'Storage_Pool.
Implementation Advice
17 For a standard storage pool, Free should actually reclaim the storage.
NOTES
18 27 The rules here that refer to Free apply to any instance of
Unchecked_Deallocation.
19 28 Unchecked_Deallocation cannot be instantiated for an
access-to-constant type. This is implied by the rules of 12.5.4.
13.11.3 Pragma Controlled
1 Pragma Controlled is used to prevent any automatic reclamation of storage
(garbage collection) for the objects created by allocators of a given access
type.
Syntax
2 The form of a pragma Controlled is as follows:
3 pragma Controlled(first_subtype_local_name);
Legality Rules
4 The first_subtype_local_name of a pragma Controlled shall denote a
non-derived access subtype.
Static Semantics
5 A pragma Controlled is a representation pragma that specifies the
controlled aspect of representation.
6 Garbage collection is a process that automatically reclaims storage, or
moves objects to a different address, while the objects still exist.
7 If a pragma Controlled is specified for an access type with a standard
storage pool, then garbage collection is not performed for objects in that
pool.
Implementation Permissions
8 An implementation need not support garbage collection, in which case, a
pragma Controlled has no effect.
13.12 Pragma Restrictions
1 A pragma Restrictions expresses the user's intent to abide by certain
restrictions. This may facilitate the construction of simpler run-time
environments.
Syntax
2 The form of a pragma Restrictions is as follows:
3 pragma Restrictions(restriction{, restriction});
4/2 restriction ::= restriction_identifier
| restriction_parameter_identifier
=> restriction_parameter_argument
4.1/2 restriction_parameter_argument ::= name | expression
Name Resolution Rules
5 Unless otherwise specified for a particular restriction, the expression is
expected to be of any integer type.
Legality Rules
6 Unless otherwise specified for a particular restriction, the expression
shall be static, and its value shall be nonnegative.
Static Semantics
7/2 The set of restrictions is implementation defined.
Post-Compilation Rules
8 A pragma Restrictions is a configuration pragma; unless otherwise
specified for a particular restriction, a partition shall obey the restriction
if a pragma Restrictions applies to any compilation unit included in the
partition.
8.1/1 For the purpose of checking whether a partition contains constructs that
violate any restriction (unless specified otherwise for a particular
restriction):
8.2/1 * Generic instances are logically expanded at the point of
instantiation;
8.3/1 * If an object of a type is declared or allocated and not explicitly
initialized, then all expressions appearing in the definition for the
type and any of its ancestors are presumed to be used;
8.4/1 * A default_expression for a formal parameter or a generic formal
object is considered to be used if and only if the corresponding
actual parameter is not provided in a given call or instantiation.
Implementation Permissions
9 An implementation may place limitations on the values of the expression
that are supported, and limitations on the supported combinations of
restrictions. The consequences of violating such limitations are
implementation defined.
9.1/1 An implementation is permitted to omit restriction checks for code that
is recognized at compile time to be unreachable and for which no code is
generated.
9.2/1 Whenever enforcement of a restriction is not required prior to
execution, an implementation may nevertheless enforce the restriction prior to
execution of a partition to which the restriction applies, provided that every
execution of the partition would violate the restriction.
NOTES
10/2 29 Restrictions intended to facilitate the construction of efficient
tasking run-time systems are defined in D.7. Restrictions intended for
use when constructing high integrity systems are defined in H.4.
11 30 An implementation has to enforce the restrictions in cases where
enforcement is required, even if it chooses not to take advantage of
the restrictions in terms of efficiency.
13.12.1 Language-Defined Restrictions
Static Semantics
1/2 The following restriction_identifiers are language-defined (additional
restrictions are defined in the Specialized Needs Annexes):
2/2 No_Implementation_Attributes
There are no implementation-defined attributes. This
restriction applies only to the current compilation or
environment, not the entire partition.
3/2 No_Implementation_Pragmas
There are no implementation-defined pragmas or pragma
arguments. This restriction applies only to the current
compilation or environment, not the entire partition.
4/2 No_Obsolescent_Features
There is no use of language features defined in Annex J. It is
implementation-defined if uses of the renamings of J.1 are
detected by this restriction. This restriction applies only to
the current compilation or environment, not the entire
partition.
5/2 The following restriction_parameter_identifier is language defined:
6/2 No_Dependence
Specifies a library unit on which there are no semantic
dependences.
Legality Rules
7/2 The restriction_parameter_argument of a No_Dependence restriction shall be
a name; the name shall have the form of a full expanded name of a library
unit, but need not denote a unit present in the environment.
Post-Compilation Rules
8/2 No compilation unit included in the partition shall depend semantically on
the library unit identified by the name.
13.13 Streams
1 A stream is a sequence of elements comprising values from possibly
different types and allowing sequential access to these values. A stream type
is a type in the class whose root type is Streams.Root_Stream_Type. A stream
type may be implemented in various ways, such as an external sequential file,
an internal buffer, or a network channel.
13.13.1 The Package Streams
Static Semantics
1 The abstract type Root_Stream_Type is the root type of the class of stream
types. The types in this class represent different kinds of streams. A new
stream type is defined by extending the root type (or some other stream type),
overriding the Read and Write operations, and optionally defining additional
primitive subprograms, according to the requirements of the particular kind of
stream. The predefined stream-oriented attributes like T'Read and T'Write make
dispatching calls on the Read and Write procedures of the Root_Stream_Type.
(User-defined T'Read and T'Write attributes can also make such calls, or can
call the Read and Write attributes of other types.)
2 package Ada.Streams is
pragma Pure(Streams);
3/2 type Root_Stream_Type is abstract tagged limited private;
pragma Preelaborable_Initialization(Root_Stream_Type);
4/1 type Stream_Element is mod implementation-defined;
type Stream_Element_Offset is range implementation-defined;
subtype Stream_Element_Count is
Stream_Element_Offset range 0..Stream_Element_Offset'Last;
type Stream_Element_Array is
array(Stream_Element_Offset range <>) of aliased Stream_Element;
5 procedure Read(
Stream : in out Root_Stream_Type;
Item : out Stream_Element_Array;
Last : out Stream_Element_Offset) is abstract;
6 procedure Write(
Stream : in out Root_Stream_Type;
Item : in Stream_Element_Array) is abstract;
7 private
... -- not specified by the language
end Ada.Streams;
8/2 The Read operation transfers stream elements from the specified stream to
fill the array Item. Elements are transferred until Item'Length elements have
been transferred, or until the end of the stream is reached. If any elements
are transferred, the index of the last stream element transferred is returned
in Last. Otherwise, Item'First - 1 is returned in Last. Last is less than
Item'Last only if the end of the stream is reached.
9 The Write operation appends Item to the specified stream.
Implementation Permissions
9.1/1 If Stream_Element'Size is not a multiple of System.Storage_Unit, then
the components of Stream_Element_Array need not be aliased.
NOTES
10 31 See A.12.1, "The Package Streams.Stream_IO" for an example of
extending type Root_Stream_Type.
11/2 32 If the end of stream has been reached, and Item'First is
Stream_Element_Offset'First, Read will raise Constraint_Error.
13.13.2 Stream-Oriented Attributes
1/1 The operational attributes Write, Read, Output, and Input convert values
to a stream of elements and reconstruct values from a stream.
Static Semantics
1.1/2 For every subtype S of an elementary type T, the following
representation attribute is defined:
1.2/2 S'Stream_Size
Denotes the number of bits occupied in a stream by items of
subtype S. Hence, the number of stream elements required per
item of elementary type T is:
1.3/2 T'Stream_Size / Ada.Streams.Stream_Element'Size
1.4/2 The value of this attribute is of type universal_integer and
is a multiple of Stream_Element'Size.
1.5/2 Stream_Size may be specified for first subtypes via an
attribute_definition_clause; the expression of such a clause
shall be static, nonnegative, and a multiple of
Stream_Element'Size.
Implementation Advice
1.6/2 If not specified, the value of Stream_Size for an elementary type should
be the number of bits that corresponds to the minimum number of stream
elements required by the first subtype of the type, rounded up to the nearest
factor or multiple of the word size that is also a multiple of the stream
element size.
1.7/2 The recommended level of support for the Stream_Size attribute is:
1.8/2 * A Stream_Size clause should be supported for a discrete or fixed
point type T if the specified Stream_Size is a multiple of
Stream_Element'Size and is no less than the size of the first subtype
of T, and no greater than the size of the largest type of the same
elementary class (signed integer, modular integer, enumeration,
ordinary fixed point, or decimal fixed point).
Static Semantics
2 For every subtype S of a specific type T, the following attributes are
defined.
3 S'Write S'Write denotes a procedure with the following specification:
4/2 procedure S'Write(
Stream : not null access Ada.Streams.Root_Stream_Type'Class;
Item : in T)
5 S'Write writes the value of Item to Stream.
6 S'Read S'Read denotes a procedure with the following specification:
7/2 procedure S'Read(
Stream : not null access Ada.Streams.Root_Stream_Type'Class;
Item : out T)
8 S'Read reads the value of Item from Stream.
8.1/2 For an untagged derived type, the Write (resp. Read) attribute is
inherited according to the rules given in 13.1 if the attribute is available
for the parent type at the point where T is declared. For a tagged derived
type, these attributes are not inherited, but rather the default
implementations are used.
8.2/2 The default implementations of the Write and Read attributes, where
available, execute as follows:
9/2 For elementary types, Read reads (and Write writes) the number of stream
elements implied by the Stream_Size for the type T; the representation of
those stream elements is implementation defined. For composite types, the
Write or Read attribute for each component is called in canonical order, which
is last dimension varying fastest for an array, and positional aggregate order
for a record. Bounds are not included in the stream if T is an array type. If
T is a discriminated type, discriminants are included only if they have
defaults. If T is a tagged type, the tag is not included. For type extensions,
the Write or Read attribute for the parent type is called, followed by the
Write or Read attribute of each component of the extension part, in canonical
order. For a limited type extension, if the attribute of the parent type or
any progenitor type of T is available anywhere within the immediate scope of
T, and the attribute of the parent type or the type of any of the extension
components is not available at the freezing point of T, then the attribute of
T shall be directly specified.
9.1/2 Constraint_Error is raised by the predefined Write attribute if the
value of the elementary item is outside the range of values representable
using Stream_Size bits. For a signed integer type, an enumeration type, or a
fixed point type, the range is unsigned only if the integer code for the lower
bound of the first subtype is nonnegative, and a (symmetric) signed range that
covers all values of the first subtype would require more than Stream_Size
bits; otherwise the range is signed.
10 For every subtype S'Class of a class-wide type T'Class:
11 S'Class'Write
S'Class'Write denotes a procedure with the following
specification:
12/2 procedure S'Class'Write(
Stream : not null access Ada.Streams.Root_Stream_Type'Class;
Item : in T'Class)
13 Dispatches to the subprogram denoted by the Write attribute of
the specific type identified by the tag of Item.
14 S'Class'Read
S'Class'Read denotes a procedure with the following
specification:
15/2 procedure S'Class'Read(
Stream : not null access Ada.Streams.Root_Stream_Type'Class;
Item : out T'Class)
16 Dispatches to the subprogram denoted by the Read attribute of
the specific type identified by the tag of Item.
Implementation Advice
17/2 This paragraph was deleted.
Static Semantics
18 For every subtype S of a specific type T, the following attributes are
defined.
19 S'Output S'Output denotes a procedure with the following specification:
20/2 procedure S'Output(
Stream : not null access Ada.Streams.Root_Stream_Type'Class;
Item : in T)
21 S'Output writes the value of Item to Stream, including any
bounds or discriminants.
22 S'Input S'Input denotes a function with the following specification:
23/2 function S'Input(
Stream : not null access Ada.Streams.Root_Stream_Type'Class)
return T
24 S'Input reads and returns one value from Stream, using any
bounds or discriminants written by a corresponding S'Output to
determine how much to read.
25/2 For an untagged derived type, the Output (resp. Input) attribute is
inherited according to the rules given in 13.1 if the attribute is available
for the parent type at the point where T is declared. For a tagged derived
type, these attributes are not inherited, but rather the default
implementations are used.
25.1/2 The default implementations of the Output and Input attributes, where
available, execute as follows:
26 * If T is an array type, S'Output first writes the bounds, and S'Input
first reads the bounds. If T has discriminants without defaults,
S'Output first writes the discriminants (using S'Write for each), and
S'Input first reads the discriminants (using S'Read for each).
27/2 * S'Output then calls S'Write to write the value of Item to the stream.
S'Input then creates an object (with the bounds or discriminants, if
any, taken from the stream), passes it to S'Read, and returns the
value of the object. Normal default initialization and finalization
take place for this object (see 3.3.1, 7.6, and 7.6.1).
27.1/2 If T is an abstract type, then S'Input is an abstract function.
28 For every subtype S'Class of a class-wide type T'Class:
29 S'Class'Output
S'Class'Output denotes a procedure with the following
specification:
30/2 procedure S'Class'Output(
Stream : not null access Ada.Streams.Root_Stream_Type'Class;
Item : in T'Class)
31/2 First writes the external tag of Item to Stream (by calling
String'Output(Stream, Tags.External_Tag(Item'Tag)) - see 3.9)
and then dispatches to the subprogram denoted by the Output
attribute of the specific type identified by the tag.
Tag_Error is raised if the tag of Item identifies a type
declared at an accessibility level deeper than that of S.
32 S'Class'Input
S'Class'Input denotes a function with the following
specification:
33/2 function S'Class'Input(
Stream : not null access Ada.Streams.Root_Stream_Type'Class)
return T'Class
34/2 First reads the external tag from Stream and determines the
corresponding internal tag (by calling
Tags.Descendant_Tag(String'Input(Stream), S'Tag) which might
raise Tag_Error - see 3.9) and then dispatches to the
subprogram denoted by the Input attribute of the specific type
identified by the internal tag; returns that result. If the
specific type identified by the internal tag is not covered by
T'Class or is abstract, Constraint_Error is raised.
35/2 In the default implementation of Read and Input for a composite type, for
each scalar component that is a discriminant or whose component_declaration
includes a default_expression, a check is made that the value returned by Read
for the component belongs to its subtype. Constraint_Error is raised if this
check fails. For other scalar components, no check is made. For each component
that is of an access type, if the implementation can detect that the value
returned by Read for the component is not a value of its subtype,
Constraint_Error is raised. If the value is not a value of its subtype and
this error is not detected, the component has an abnormal value, and erroneous
execution can result (see 13.9.1). In the default implementation of Read for a
composite type with defaulted discriminants, if the actual parameter of Read
is constrained, a check is made that the discriminants read from the stream
are equal to those of the actual parameter. Constraint_Error is raised if this
check fails.
36/2 It is unspecified at which point and in which order these checks are
performed. In particular, if Constraint_Error is raised due to the failure of
one of these checks, it is unspecified how many stream elements have been read
from the stream.
37/1 In the default implementation of Read and Input for a type, End_Error is
raised if the end of the stream is reached before the reading of a value of
the type is completed.
38/2 The stream-oriented attributes may be specified for any type via an
attribute_definition_clause. The subprogram name given in such a clause shall
not denote an abstract subprogram. Furthermore, if a stream-oriented attribute
is specified for an interface type by an attribute_definition_clause, the
subprogram name given in the clause shall statically denote a null procedure.
39/2 A stream-oriented attribute for a subtype of a specific type T is
available at places where one of the following conditions is true:
40/2 * T is nonlimited.
41/2 * The attribute_designator is Read (resp. Write) and T is a limited
record extension, and the attribute Read (resp. Write) is available
for the parent type of T and for the types of all of the extension
components.
42/2 * T is a limited untagged derived type, and the attribute was inherited
for the type.
43/2 * The attribute_designator is Input (resp. Output), and T is a limited
type, and the attribute Read (resp. Write) is available for T.
44/2 * The attribute has been specified via an attribute_definition_clause,
and the attribute_definition_clause is visible.
45/2 A stream-oriented attribute for a subtype of a class-wide type T'Class is
available at places where one of the following conditions is true:
46/2 * T is nonlimited;
47/2 * the attribute has been specified via an attribute_definition_clause,
and the attribute_definition_clause is visible; or
48/2 * the corresponding attribute of T is available, provided that if T has
a partial view, the corresponding attribute is available at the end of
the visible part where T is declared.
49/2 An attribute_reference for one of the stream-oriented attributes is
illegal unless the attribute is available at the place of the
attribute_reference. Furthermore, an attribute_reference for T'Input is
illegal if T is an abstract type.
50/2 In the parameter_and_result_profiles for the stream-oriented attributes,
the subtype of the Item parameter is the base subtype of T if T is a scalar
type, and the first subtype otherwise. The same rule applies to the result of
the Input attribute.
51/2 For an attribute_definition_clause specifying one of these attributes,
the subtype of the Item parameter shall be the base subtype if scalar, and the
first subtype otherwise. The same rule applies to the result of the Input
function.
52/2 A type is said to support external streaming if Read and Write attributes
are provided for sending values of such a type between active partitions, with
Write marshalling the representation, and Read unmarshalling the
representation. A limited type supports external streaming only if it has
available Read and Write attributes. A type with a part that is of an access
type supports external streaming only if that access type or the type of some
part that includes the access type component, has Read and Write attributes
that have been specified via an attribute_definition_clause, and that
attribute_definition_clause is visible. An anonymous access type does not
support external streaming. All other types support external streaming.
Erroneous Execution
53/2 If the internal tag returned by Descendant_Tag to T'Class'Input
identifies a type that is not library-level and whose tag has not been
created, or does not exist in the partition at the time of the call, execution
is erroneous.
Implementation Requirements
54/1 For every subtype S of a language-defined nonlimited specific type T, the
output generated by S'Output or S'Write shall be readable by S'Input or
S'Read, respectively. This rule applies across partitions if the
implementation conforms to the Distributed Systems Annex.
55/2 If Constraint_Error is raised during a call to Read because of failure of
one the above checks, the implementation must ensure that the discriminants of
the actual parameter of Read are not modified.
Implementation Permissions
56/2 The number of calls performed by the predefined implementation of the
stream-oriented attributes on the Read and Write operations of the stream type
is unspecified. An implementation may take advantage of this permission to
perform internal buffering. However, all the calls on the Read and Write
operations of the stream type needed to implement an explicit invocation of a
stream-oriented attribute must take place before this invocation returns. An
explicit invocation is one appearing explicitly in the program text, possibly
through a generic instantiation (see 12.3).
NOTES
57 33 For a definite subtype S of a type T, only T'Write and T'Read are
needed to pass an arbitrary value of the subtype through a stream. For
an indefinite subtype S of a type T, T'Output and T'Input will
normally be needed, since T'Write and T'Read do not pass bounds,
discriminants, or tags.
58 34 User-specified attributes of S'Class are not inherited by other
class-wide types descended from S.
Examples
59 Example of user-defined Write attribute:
60/2 procedure My_Write(
Stream : not null access Ada.Streams.Root_Stream_Type'Class;
Item : My_Integer'Base);
for My_Integer'Write use My_Write;
13.14 Freezing Rules
1 This clause defines a place in the program text where each declared entity
becomes "frozen." A use of an entity, such as a reference to it by name, or
(for a type) an expression of the type, causes freezing of the entity in some
contexts, as described below. The Legality Rules forbid certain kinds of uses
of an entity in the region of text where it is frozen.
2 The freezing of an entity occurs at one or more places (freezing points)
in the program text where the representation for the entity has to be fully
determined. Each entity is frozen from its first freezing point to the end of
the program text (given the ordering of compilation units defined in 10.1.4).
3/1 The end of a declarative_part, protected_body, or a declaration of a
library package or generic library package, causes freezing of each entity
declared within it, except for incomplete types. A noninstance body other than
a renames-as-body causes freezing of each entity declared before it within the
same declarative_part.
4/1 A construct that (explicitly or implicitly) references an entity can cause
the freezing of the entity, as defined by subsequent paragraphs. At the place
where a construct causes freezing, each name, expression,
implicit_dereference, or range within the construct causes freezing:
5 * The occurrence of a generic_instantiation causes freezing; also, if a
parameter of the instantiation is defaulted, the default_expression or
default_name for that parameter causes freezing.
6 * The occurrence of an object_declaration that has no corresponding
completion causes freezing.
7 * The declaration of a record extension causes freezing of the parent
subtype.
7.1/2 * The declaration of a record extension, interface type, task unit, or
protected unit causes freezing of any progenitor types specified in
the declaration.
8/1 A static expression causes freezing where it occurs. An object name or
nonstatic expression causes freezing where it occurs, unless the name or
expression is part of a default_expression, a default_name, or a per-object
expression of a component's constraint, in which case, the freezing occurs
later as part of another construct.
8.1/1 An implicit call freezes the same entities that would be frozen by an
explicit call. This is true even if the implicit call is removed via
implementation permissions.
8.2/1 If an expression is implicitly converted to a type or subtype T, then at
the place where the expression causes freezing, T is frozen.
9 The following rules define which entities are frozen at the place where a
construct causes freezing:
10 * At the place where an expression causes freezing, the type of the
expression is frozen, unless the expression is an enumeration literal
used as a discrete_choice of the array_aggregate of an enumeration_-
representation_clause.
11 * At the place where a name causes freezing, the entity denoted by the
name is frozen, unless the name is a prefix of an expanded name; at
the place where an object name causes freezing, the nominal subtype
associated with the name is frozen.
11.1/1 * At the place where an implicit_dereference causes freezing, the
nominal subtype associated with the implicit_dereference is frozen.
12 * At the place where a range causes freezing, the type of the range is
frozen.
13 * At the place where an allocator causes freezing, the designated
subtype of its type is frozen. If the type of the allocator is a
derived type, then all ancestor types are also frozen.
14 * At the place where a callable entity is frozen, each subtype of its
profile is frozen. If the callable entity is a member of an entry
family, the index subtype of the family is frozen. At the place where
a function call causes freezing, if a parameter of the call is
defaulted, the default_expression for that parameter causes freezing.
15 * At the place where a subtype is frozen, its type is frozen. At the
place where a type is frozen, any expressions or names within the full
type definition cause freezing; the first subtype, and any component
subtypes, index subtypes, and parent subtype of the type are frozen as
well. For a specific tagged type, the corresponding class-wide type is
frozen as well. For a class-wide type, the corresponding specific type
is frozen as well.
15.1/2 * At the place where a specific tagged type is frozen, the primitive
subprograms of the type are frozen.
Legality Rules
16 The explicit declaration of a primitive subprogram of a tagged type shall
occur before the type is frozen (see 3.9.2).
17 A type shall be completely defined before it is frozen (see 3.11.1 and
7.3).
18 The completion of a deferred constant declaration shall occur before the
constant is frozen (see 7.4).
19/1 An operational or representation item that directly specifies an aspect
of an entity shall appear before the entity is frozen (see 13.1).
Dynamic Semantics
20/2 The tag (see 3.9) of a tagged type T is created at the point where T is
frozen.
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