Section 5: Statements
1 [A statement defines an action to be performed upon its execution.]
2/2 {AI95-00318-02} [This section describes the general rules applicable to
all statements. Some statements are discussed in later sections: Procedure_-
call_statements and return statements are described in 6, "Subprograms". Entry_-
call_statements, requeue_statements, delay_statements, accept_statements,
select_statements, and abort_statements are described in 9, "
Tasks and Synchronization". Raise_statements are described in 11, "
Exceptions", and code_statements in 13. The remaining forms of statements are
presented in this section.]
Wording Changes from Ada 83
2.a/2 {AI95-00318-02} The description of return statements has been
moved to 6.5, "Return Statements", so that it is closer to the
description of subprograms.
5.1 Simple and Compound Statements - Sequences of Statements
1 [A statement is either simple or compound. A simple_statement encloses no
other statement. A compound_statement can enclose simple_statements and other
compound_statements.]
Syntax
2 sequence_of_statements ::= statement {statement}
3 statement ::=
{label} simple_statement | {label} compound_statement
4/2 {AI95-00318-02} simple_statement ::= null_statement
| assignment_statement | exit_statement
| goto_statement | procedure_call_statement
| simple_return_statement | entry_call_statement
| requeue_statement | delay_statement
| abort_statement | raise_statement
| code_statement
5/2 {AI95-00318-02} compound_statement ::=
if_statement | case_statement
| loop_statement | block_statement
| extended_return_statement
| accept_statement | select_statement
6 null_statement ::= null;
7 label ::= <<label_statement_identifier>>
8 statement_identifier ::= direct_name
9 The direct_name of a statement_identifier shall be an identifier (not
an operator_symbol).
Name Resolution Rules
10 The direct_name of a statement_identifier shall resolve to denote its
corresponding implicit declaration (see below).
Legality Rules
11 Distinct identifiers shall be used for all statement_identifiers that
appear in the same body, including inner block_statements but excluding inner
program units.
Static Semantics
12 For each statement_identifier, there is an implicit declaration (with the
specified identifier) at the end of the declarative_part of the innermost
block_statement or body that encloses the statement_identifier. The implicit
declarations occur in the same order as the statement_identifiers occur in the
source text. If a usage name denotes such an implicit declaration, the entity
it denotes is the label, loop_statement, or block_statement with the given
statement_identifier.
12.a Reason: We talk in terms of individual statement_identifiers here
rather than in terms of the corresponding statements, since a
given statement may have multiple statement_identifiers.
12.b A block_statement that has no explicit declarative_part has an
implicit empty declarative_part, so this rule can safely refer to
the declarative_part of a block_statement.
12.c The scope of a declaration starts at the place of the declaration
itself (see 8.2). In the case of a label, loop, or block name, it
follows from this rule that the scope of the implicit declaration
starts before the first explicit occurrence of the corresponding
name, since this occurrence is either in a statement label, a
loop_statement, a block_statement, or a goto_statement. An
implicit declaration in a block_statement may hide a declaration
given in an outer program unit or block_statement (according to
the usual rules of hiding explained in 8.3).
12.d The syntax rule for label uses statement_identifier which is a
direct_name (not a defining_identifier), because labels are
implicitly declared. The same applies to loop and block names. In
other words, the label itself is not the defining occurrence; the
implicit declaration is.
12.e We cannot consider the label to be a defining occurrence. An
example that can tell the difference is this:
12.f declare
-- Label Foo is implicitly declared here.
begin
for Foo in ... loop
...
<<Foo>> -- Illegal.
...
end loop;
end;
12.g The label in this example is hidden from itself by the loop
parameter with the same name; the example is illegal. We
considered creating a new syntactic category name, separate from
direct_name and selector_name, for use in the case of statement
labels. However, that would confuse the rules in Section 8, so we
didn't do it.
Dynamic Semantics
13 {execution (null_statement) [partial]} The execution of a null_statement
has no effect.
14/2 {AI95-00318-02} {transfer of control} A transfer of control is the
run-time action of an exit_statement, return statement, goto_statement, or
requeue_statement, selection of a terminate_alternative, raising of an
exception, or an abort, which causes the next action performed to be one other
than what would normally be expected from the other rules of the language. [As
explained in 7.6.1, a transfer of control can cause the execution of
constructs to be completed and then left, which may trigger finalization.]
15 {execution (sequence_of_statements) [partial]} The execution of a
sequence_of_statements consists of the execution of the individual statements
in succession until the sequence_ is completed.
15.a Ramification: It could be completed by reaching the end of it, or
by a transfer of control.
NOTES
16 1 A statement_identifier that appears immediately within the
declarative region of a named loop_statement or an accept_statement is
nevertheless implicitly declared immediately within the declarative
region of the innermost enclosing body or block_statement; in other
words, the expanded name for a named statement is not affected by
whether the statement occurs inside or outside a named loop or an
accept_statement - only nesting within block_statements is relevant to
the form of its expanded name.
16.a Discussion: Each comment in the following example gives the
expanded name associated with an entity declared in the task body:
16.b task body Compute is
Sum : Integer := 0; -- Compute.Sum
begin
Outer: -- Compute.Outer
for I in 1..10 loop -- Compute.Outer.I
Blk: -- Compute.Blk
declare
Sum : Integer := 0; -- Compute.Blk.Sum
begin
accept Ent(I : out Integer; J : in Integer) do
-- Compute.Ent.I, Compute.Ent.J
Compute.Ent.I := Compute.Outer.I;
Inner: -- Compute.Blk.Inner
for J in 1..10 loop
-- Compute.Blk.Inner.J
Sum := Sum + Compute.Blk.Inner.J * Compute.Ent.J;
end loop Inner;
end Ent;
Compute.Sum := Compute.Sum + Compute.Blk.Sum;
end Blk;
end loop Outer;
Record_Result(Sum);
end Compute;
Examples
17 Examples of labeled statements:
18 <<Here>> <<Ici>> <<Aqui>> <<Hier>> null;
19 <<After>> X := 1;
Extensions to Ada 83
19.a {extensions to Ada 83} The requeue_statement is new.
Wording Changes from Ada 83
19.b We define the syntactic category statement_identifier to simplify
the description. It is used for labels, loop names, and block
names. We define the entity associated with the implicit
declarations of statement names.
19.c Completion includes completion caused by a transfer of control,
although RM83-5.1(6) did not take this view.
Extensions to Ada 95
19.d/2 {AI95-00318-02} {extensions to Ada 95} The
extended_return_statement is new (simple_return_statement is
merely renamed).
5.2 Assignment Statements
1 [An assignment_statement replaces the current value of a variable with the
result of evaluating an expression.]
Syntax
2 assignment_statement ::=
variable_name := expression;
3 The execution of an assignment_statement includes the evaluation of the
expression and the assignment of the value of the expression into the target.
{assignment operation [distributed]} {assign: See assignment operation} [An
assignment operation (as opposed to an assignment_statement) is performed in
other contexts as well, including object initialization and by-copy parameter
passing.] {target (of an assignment operation)}
{target (of an assignment_statement)} The target of an assignment operation is
the view of the object to which a value is being assigned; the target of an
assignment_statement is the variable denoted by the variable_name.
3.a Discussion: Don't confuse this notion of the "target" of an
assignment with the notion of the "target object" of an entry call
or requeue.
3.b Don't confuse the term "assignment operation" with the
assignment_statement. The assignment operation is just one part of
the execution of an assignment_statement. The assignment operation
is also a part of the execution of various other constructs; see
7.6.1, "Completion and Finalization" for a complete list. Note
that when we say, "such-and-such is assigned to so-and-so", we
mean that the assignment operation is being applied, and that
so-and-so is the target of the assignment operation.
Name Resolution Rules
4/2 {AI95-00287-01} {expected type (assignment_statement variable_name)
[partial]} The variable_name of an assignment_statement is expected to be of
any type. {expected type (assignment_statement expression) [partial]} The
expected type for the expression is the type of the target.
4.a Implementation Note: An assignment_statement as a whole is a
"complete context," so if the variable_name of an
assignment_statement is overloaded, the expression can be used to
help disambiguate it. For example:
4.b type P1 is access R1;
type P2 is access R2;
4.c function F return P1;
function F return P2;
4.d X : R1;
begin
F.all := X; -- Right hand side helps resolve left hand side
Legality Rules
5/2 {AI95-00287-01} The target [denoted by the variable_name] shall be a
variable of a nonlimited type.
6 If the target is of a tagged class-wide type T'Class, then the
expression shall either be dynamically tagged, or of type T and
tag-indeterminate (see 3.9.2).
6.a Reason: This is consistent with the general rule that a single
dispatching operation shall not have both dynamically tagged and
statically tagged operands. Note that for an object initialization
(as opposed to the assignment_statement), a statically tagged
initialization expression is permitted, since there is no chance
for confusion (or Tag_Check failure). Also, in an object
initialization, tag-indeterminate expressions of any type covered
by T'Class would be allowed, but with an assignment_statement,
that might not work if the tag of the target was for a type that
didn't have one of the dispatching operations in the
tag-indeterminate expression.
Dynamic Semantics
7 {execution (assignment_statement) [partial]} For the execution of an
assignment_statement, the variable_name and the expression are first evaluated
in an arbitrary order.
7.a Ramification: Other rules of the language may require that the
bounds of the variable be determined prior to evaluating the
expression, but that does not necessarily require evaluation of
the variable_name, as pointed out by the ACID.
8 When the type of the target is class-wide:
9 * {controlling tag value (for the expression in an assignment_statement)
[partial]} If the expression is tag-indeterminate (see 3.9.2), then
the controlling tag value for the expression is the tag of the target;
9.a Ramification: See 3.9.2, "
Dispatching Operations of Tagged Types".
10 * {Tag_Check [partial]} {check, language-defined (Tag_Check)}
{Constraint_Error (raised by failure of run-time check)} Otherwise
[(the expression is dynamically tagged)], a check is made that the tag
of the value of the expression is the same as that of the target; if
this check fails, Constraint_Error is raised.
11 The value of the expression is converted to the subtype of the target.
[The conversion might raise an exception (see 4.6).]
{implicit subtype conversion (assignment_statement) [partial]}
11.a Ramification: 4.6, "Type Conversions" defines what actions and
checks are associated with subtype conversion. For non-array
subtypes, it is just a constraint check presuming the types match.
For array subtypes, it checks the lengths and slides if the target
is constrained. "Sliding" means the array doesn't have to have the
same bounds, so long as it is the same length.
12 In cases involving controlled types, the target is finalized, and an
anonymous object might be used as an intermediate in the assignment, as
described in 7.6.1, "Completion and Finalization". {assignment operation}
{assignment operation (during execution of an assignment_statement)} In any
case, the converted value of the expression is then assigned to the target,
which consists of the following two steps:
12.a To be honest: 7.6.1 actually says that finalization happens
always, but unless controlled types are involved, this
finalization during an assignment_statement does nothing.
13 * The value of the target becomes the converted value.
14 * If any part of the target is controlled, its value is adjusted as
explained in clause 7.6. {adjustment (as part of assignment)
[partial]}
14.a Ramification: If any parts of the object are controlled, abort is
deferred during the assignment operation itself, but not during
the rest of the execution of an assignment_statement.
NOTES
15 2 The tag of an object never changes; in particular, an
assignment_statement does not change the tag of the target.
16/2 This paragraph was deleted.{AI95-00363-01}
16.a Ramification: The implicit subtype conversion described above for
assignment_statements is performed only for the value of the
right-hand side expression as a whole; it is not performed for
subcomponents of the value.
16.b The determination of the type of the variable of an
assignment_statement may require consideration of the expression
if the variable name can be interpreted as the name of a variable
designated by the access value returned by a function call, and
similarly, as a component or slice of such a variable (see 8.6
, "The Context of Overload Resolution").
Examples
17 Examples of assignment statements:
18 Value := Max_Value - 1;
Shade := Blue;
19 Next_Frame(F)(M, N) := 2.5; -- see 4.1.1
U := Dot_Product(V, W); -- see 6.3
20 Writer := (Status => Open, Unit => Printer, Line_Count => 60); -- see 3.8.1
Next_Car.all := (72074, null); -- see 3.10.1
21 Examples involving scalar subtype conversions:
22 I, J : Integer range 1 .. 10 := 5;
K : Integer range 1 .. 20 := 15;
...
23 I := J; -- identical ranges
K := J; -- compatible ranges
J := K; -- will raise Constraint_Error if K > 10
24 Examples involving array subtype conversions:
25 A : String(1 .. 31);
B : String(3 .. 33);
...
26 A := B; -- same number of components
27 A(1 .. 9) := "tar sauce";
A(4 .. 12) := A(1 .. 9); -- A(1 .. 12) = "tartar sauce"
NOTES
28 3 Notes on the examples: Assignment_statements are allowed even in
the case of overlapping slices of the same array, because the
variable_name and expression are both evaluated before copying the value into
the variable. In the above example, an implementation yielding A(1 ..
12) = "tartartartar" would be incorrect.
Extensions to Ada 83
28.a {extensions to Ada 83} We now allow user-defined finalization and
value adjustment actions as part of assignment_statements (see
7.6, "User-Defined Assignment and Finalization").
Wording Changes from Ada 83
28.b The special case of array assignment is subsumed by the concept of
a subtype conversion, which is applied for all kinds of types, not
just arrays. For arrays it provides "sliding". For numeric types
it provides conversion of a value of a universal type to the
specific type of the target. For other types, it generally has no
run-time effect, other than a constraint check.
28.c We now cover in a general way in 3.7.2 the erroneous execution
possible due to changing the value of a discriminant when the
variable in an assignment_statement is a subcomponent that depends
on discriminants.
Incompatibilities With Ada 95
28.d/2 {AI95-00287-01} {incompatibilities with Ada 95} The change of the
limited check from a resolution rule to a legality rule is not
quite upward compatible. For example.
28.e type AccNonLim is access NonLim;
function Foo (Arg : in Integer) return AccNonLim;
type AccLim is access Lim;
function Foo (Arg : in Integer) return AccLim;
Foo(2).all := Foo(1).all;.
28.f where NonLim is a nonlimited type and Lim is a limited type. The
assignment is legal in Ada 95 (only the first Foo would be
considered), and is ambiguous in Ada 2005. We made the change
because we want limited types to be as similar to nonlimited types
as possible. Limited expressions are now allowed in all other
contexts (with a similar incompatibility), and it would be odd if
assignments had different resolution rules (which would eliminate
ambiguities in some cases). Moreover, examples like this one are
rare, as they depend on assigning into overloaded function calls.
5.3 If Statements
1 [An if_statement selects for execution at most one of the enclosed
sequences_of_statements, depending on the (truth) value of one or more
corresponding conditions.]
Syntax
2 if_statement ::=
if condition then
sequence_of_statements
{elsif condition then
sequence_of_statements}
[else
sequence_of_statements]
end if;
3 condition ::= boolean_expression
Name Resolution Rules
4 {expected type (condition) [partial]} A condition is expected to be of any
boolean type.
Dynamic Semantics
5 {execution (if_statement) [partial]} For the execution of an
if_statement, the condition specified after if, and any conditions specified
after elsif, are evaluated in succession (treating a final else as elsif True
then), until one evaluates to True or all conditions are evaluated and yield
False. If a condition evaluates to True, then the corresponding
sequence_of_statements is executed; otherwise none of them is executed.
5.a Ramification: The part about all evaluating to False can't happen
if there is an else, since that is herein considered equivalent to
elsif True then.
Examples
6 Examples of if statements:
7 if Month = December and Day = 31 then
Month := January;
Day := 1;
Year := Year + 1;
end if;
8 if Line_Too_Short then
raise Layout_Error;
elsif Line_Full then
New_Line;
Put(Item);
else
Put(Item);
end if;
9 if My_Car.Owner.Vehicle /= My_Car then -- see 3.10.1
Report ("Incorrect data");
end if;
5.4 Case Statements
1 [A case_statement selects for execution one of a number of alternative
sequences_of_statements; the chosen alternative is defined by the value of an
expression.]
Syntax
2 case_statement ::=
case expression is
case_statement_alternative
{case_statement_alternative}
end case;
3 case_statement_alternative ::=
when discrete_choice_list =>
sequence_of_statements
Name Resolution Rules
4 {expected type (case expression) [partial]} The expression is expected to
be of any discrete type.
{expected type (case_statement_alternative discrete_choice) [partial]} The
expected type for each discrete_choice is the type of the expression.
Legality Rules
5 The expressions and discrete_ranges given as discrete_choices of a
case_statement shall be static. [A discrete_choice others, if present, shall
appear alone and in the last discrete_choice_list.]
6 The possible values of the expression shall be covered as follows:
7 * If the expression is a name [(including a type_conversion or a
function_call)] having a static and constrained nominal subtype, or is
a qualified_expression whose subtype_mark denotes a static and
constrained scalar subtype, then each non-others discrete_choice shall
cover only values in that subtype, and each value of that subtype
shall be covered by some discrete_choice [(either explicitly or by
others)].
7.a Ramification: Although not official names of objects, a value
conversion still has a defined nominal subtype, namely its target
subtype. See 4.6.
8 * If the type of the expression is root_integer, universal_integer, or a
descendant of a formal scalar type, then the case_statement shall have
an others discrete_choice.
8.a Reason: This is because the base range is implementation defined
for root_integer and universal_integer, and not known statically
in the case of a formal scalar type.
9 * Otherwise, each value of the base range of the type of the
expression shall be covered [(either explicitly or by others)].
10 Two distinct discrete_choices of a case_statement shall not cover the same
value.
10.a Ramification: The goal of these coverage rules is that any
possible value of the expression of a case_statement should be
covered by exactly one discrete_choice of the case_statement, and
that this should be checked at compile time. The goal is achieved
in most cases, but there are two minor loopholes:
10.b * If the expression reads an object with an invalid
representation (e.g. an uninitialized object), then the value
can be outside the covered range. This can happen for static
constrained subtypes, as well as nonstatic or unconstrained
subtypes. It cannot, however, happen if the case_statement has
the discrete_choice others, because others covers all values,
even those outside the subtype.
10.c/2 * {AI95-00114-01} If the compiler chooses to represent the value
of an expression of an unconstrained subtype in a way that
includes values outside the bounds of the subtype, then those
values can be outside the covered range. For example, if X:
Integer := Integer'Last;, and the case expression is X+1, then
the implementation might choose to produce the correct value,
which is outside the bounds of Integer. (It might raise
Constraint_Error instead.) This case can only happen for
non-generic subtypes that are either unconstrained or
nonstatic (or both). It can only happen if there is no others
discrete_choice.
10.d In the uninitialized variable case, the value might be anything;
hence, any alternative can be chosen, or Constraint_Error can be
raised. (We intend to prevent, however, jumping to random memory
locations and the like.) In the out-of-range case, the behavior is
more sensible: if there is an others, then the implementation may
choose to raise Constraint_Error on the evaluation of the
expression (as usual), or it may choose to correctly evaluate the
expression and therefore choose the others alternative. Otherwise
(no others), Constraint_Error is raised either way - on the
expression evaluation, or for the case_statement itself.
10.e For an enumeration type with a discontiguous set of internal codes
(see 13.4), the only way to get values in between the proper
values is via an object with an invalid representation; there is
no "out-of-range" situation that can produce them.
Dynamic Semantics
11 {execution (case_statement) [partial]} For the execution of a
case_statement the expression is first evaluated.
12 If the value of the expression is covered by the discrete_choice_list of
some case_statement_alternative, then the sequence_of_statements of the
_alternative is executed.
13 {Overflow_Check [partial]} {check, language-defined (Overflow_Check)}
{Constraint_Error (raised by failure of run-time check)} Otherwise (the value
is not covered by any discrete_choice_list, perhaps due to being outside the
base range), Constraint_Error is raised.
13.a Ramification: In this case, the value is outside the base range of
its type, or is an invalid representation.
NOTES
14 4 The execution of a case_statement chooses one and only one
alternative. Qualification of the expression of a case_statement by a
static subtype can often be used to limit the number of choices that
need be given explicitly.
Examples
15 Examples of case statements:
16 case Sensor is
when Elevation => Record_Elevation(Sensor_Value);
when Azimuth => Record_Azimuth (Sensor_Value);
when Distance => Record_Distance (Sensor_Value);
when others => null;
end case;
17 case Today is
when Mon => Compute_Initial_Balance;
when Fri => Compute_Closing_Balance;
when Tue .. Thu => Generate_Report(Today);
when Sat .. Sun => null;
end case;
18 case Bin_Number(Count) is
when 1 => Update_Bin(1);
when 2 => Update_Bin(2);
when 3 | 4 =>
Empty_Bin(1);
Empty_Bin(2);
when others => raise Error;
end case;
Incompatibilities With Ada 83
18.a.1/1 {incompatibilities with Ada 83} In Ada 95, function_calls and
type_conversions are names, whereas in Ada 83, they were
expressions. Therefore, if the expression of a case_statement is a
function_call or type_conversion, and the result subtype is
static, it is illegal to specify a choice outside the bounds of
the subtype. For this case in Ada 83 choices only are required to
be in the base range of the type.
18.a.2/1 In addition, the rule about which choices must be covered is
unchanged in Ada 95. Therefore, for a case_statement whose
expression is a function_call or type_conversion, Ada 83 required
covering all choices in the base range, while Ada 95 only requires
covering choices in the bounds of the subtype. If the
case_statement does not include an others discrete_choice, then a
legal Ada 83 case_statement will be illegal in Ada 95 if the
bounds of the subtype are different than the bounds of the base
type.
Extensions to Ada 83
18.a {extensions to Ada 83} In Ada 83, the expression in a
case_statement is not allowed to be of a generic formal type. This
restriction is removed in Ada 95; an others discrete_choice is
required instead.
18.b In Ada 95, a function call is the name of an object; this was not
true in Ada 83 (see 4.1, "Names"). This change makes the following
case_statement legal:
18.c subtype S is Integer range 1..2;
function F return S;
case F is
when 1 => ...;
when 2 => ...;
-- No others needed.
end case;
18.d Note that the result subtype given in a function
renaming_declaration is ignored; for a case_statement whose
expression calls a such a function, the full coverage rules are
checked using the result subtype of the original function. Note
that predefined operators such as "+" have an unconstrained result
subtype (see 4.5.1). Note that generic formal functions do not
have static result subtypes. Note that the result subtype of an
inherited subprogram need not correspond to any namable subtype;
there is still a perfectly good result subtype, though.
Wording Changes from Ada 83
18.e Ada 83 forgot to say what happens for "legally" out-of-bounds
values.
18.f We take advantage of rules and terms (e.g. cover a value) defined
for discrete_choices and discrete_choice_lists in 3.8.1, "
Variant Parts and Discrete Choices".
18.g In the Name Resolution Rule for the case expression, we no longer
need RM83-5.4(3)'s "which must be determinable independently of
the context in which the expression occurs, but using the fact
that the expression must be of a discrete type," because the
expression is now a complete context. See 8.6, "
The Context of Overload Resolution".
18.h Since type_conversions are now defined as names, their coverage
rule is now covered under the general rule for names, rather than
being separated out along with qualified_expressions.
5.5 Loop Statements
1 [A loop_statement includes a sequence_of_statements that is to be executed
repeatedly, zero or more times.]
Syntax
2 loop_statement ::=
[loop_statement_identifier:]
[iteration_scheme] loop
sequence_of_statements
end loop [loop_identifier];
3 iteration_scheme ::= while condition
| for loop_parameter_specification
4 loop_parameter_specification ::=
defining_identifier in [reverse] discrete_subtype_definition
5 If a loop_statement has a loop_statement_identifier, then the
identifier shall be repeated after the end loop; otherwise, there
shall not be an identifier after the end loop.
Static Semantics
6 {loop parameter} A loop_parameter_specification declares a loop parameter,
which is an object whose subtype is that defined by the
discrete_subtype_definition. {parameter: See also loop parameter}
Dynamic Semantics
7 {execution (loop_statement) [partial]} For the execution of a
loop_statement, the sequence_of_statements is executed repeatedly, zero or
more times, until the loop_statement is complete. The loop_statement is
complete when a transfer of control occurs that transfers control out of the
loop, or, in the case of an iteration_scheme, as specified below.
8 {execution (loop_statement with a while iteration_scheme) [partial]} For
the execution of a loop_statement with a while iteration_scheme, the condition
is evaluated before each execution of the sequence_of_statements; if the value
of the condition is True, the sequence_of_statements is executed; if False,
the execution of the loop_statement is complete.
9 {execution (loop_statement with a for iteration_scheme) [partial]}
{elaboration (loop_parameter_specification) [partial]} For the execution of a
loop_statement with a for iteration_scheme, the loop_parameter_specification
is first elaborated. This elaboration creates the loop parameter and
elaborates the discrete_subtype_definition. If the discrete_subtype_definition
defines a subtype with a null range, the execution of the loop_statement is
complete. Otherwise, the sequence_of_statements is executed once for each
value of the discrete subtype defined by the discrete_subtype_definition (or
until the loop is left as a consequence of a transfer of control).
{assignment operation (during execution of a for loop)} Prior to each such
iteration, the corresponding value of the discrete subtype is assigned to the
loop parameter. These values are assigned in increasing order unless the
reserved word reverse is present, in which case the values are assigned in
decreasing order.
9.a Ramification: The order of creating the loop parameter and
evaluating the discrete_subtype_definition doesn't matter, since
the creation of the loop parameter has no side effects (other than
possibly raising Storage_Error, but anything can do that).
NOTES
10 5 A loop parameter is a constant; it cannot be updated within the
sequence_of_statements of the loop (see 3.3).
11 6 An object_declaration should not be given for a loop parameter,
since the loop parameter is automatically declared by the
loop_parameter_specification. The scope of a loop parameter extends
from the loop_parameter_specification to the end of the
loop_statement, and the visibility rules are such that a loop
parameter is only visible within the sequence_of_statements of the
loop.
11.a Implementation Note: An implementation could give a warning if a
variable is hidden by a loop_parameter_specification.
12 7 The discrete_subtype_definition of a for loop is elaborated just
once. Use of the reserved word reverse does not alter the discrete
subtype defined, so that the following iteration_schemes are not
equivalent; the first has a null range.
13 for J in reverse 1 .. 0
for J in 0 .. 1
13.a Ramification: If a loop_parameter_specification has a static
discrete range, the subtype of the loop parameter is static.
Examples
14 Example of a loop statement without an iteration scheme:
15 loop
Get(Current_Character);
exit when Current_Character = '*';
end loop;
16 Example of a loop statement with a while iteration scheme:
17 while Bid(N).Price < Cut_Off.Price loop
Record_Bid(Bid(N).Price);
N := N + 1;
end loop;
18 Example of a loop statement with a for iteration scheme:
19 for J in Buffer'Range loop -- works even with a null range
if Buffer(J) /= Space then
Put(Buffer(J));
end if;
end loop;
20 Example of a loop statement with a name:
21 Summation:
while Next /= Head loop -- see 3.10.1
Sum := Sum + Next.Value;
Next := Next.Succ;
end loop Summation;
Wording Changes from Ada 83
21.a The constant-ness of loop parameters is specified in 3.3, "
Objects and Named Numbers".
5.6 Block Statements
1 [A block_statement encloses a handled_sequence_of_statements optionally
preceded by a declarative_part.]
Syntax
2 block_statement ::=
[block_statement_identifier:]
[declare
declarative_part]
begin
handled_sequence_of_statements
end [block_identifier];
3 If a block_statement has a block_statement_identifier, then the
identifier shall be repeated after the end; otherwise, there shall not
be an identifier after the end.
Static Semantics
4 A block_statement that has no explicit declarative_part has an implicit
empty declarative_part.
4.a Ramification: Thus, other rules can always refer to the
declarative_part of a block_statement.
Dynamic Semantics
5 {execution (block_statement) [partial]} The execution of a
block_statement consists of the elaboration of its declarative_part followed
by the execution of its handled_sequence_of_statements.
Examples
6 Example of a block statement with a local variable:
7 Swap:
declare
Temp : Integer;
begin
Temp := V; V := U; U := Temp;
end Swap;
7.a Ramification: If task objects are declared within a
block_statement whose execution is completed, the
block_statement is not left until all its dependent tasks are
terminated (see 7.6). This rule applies to completion caused by a
transfer of control.
7.b Within a block_statement, the block name can be used in expanded
names denoting local entities such as Swap.Temp in the above
example (see 4.1.3).
Wording Changes from Ada 83
7.c The syntax rule for block_statement now uses the syntactic
category handled_sequence_of_statements.
5.7 Exit Statements
1 [An exit_statement is used to complete the execution of an enclosing
loop_statement; the completion is conditional if the exit_statement includes a
condition.]
Syntax
2 exit_statement ::=
exit [loop_name] [when condition];
Name Resolution Rules
3 The loop_name, if any, in an exit_statement shall resolve to denote a
loop_statement.
Legality Rules
4 {apply (to a loop_statement by an exit_statement)} Each exit_statement
applies to a loop_statement; this is the loop_statement being exited. An exit_-
statement with a name is only allowed within the loop_statement denoted by the
name, and applies to that loop_statement. An exit_statement without a name is
only allowed within a loop_statement, and applies to the innermost enclosing
one. An exit_statement that applies to a given loop_statement shall not appear
within a body or accept_statement, if this construct is itself enclosed by the
given loop_statement.
Dynamic Semantics
5 {execution (exit_statement) [partial]} For the execution of an
exit_statement, the condition, if present, is first evaluated. If the value of
the condition is True, or if there is no condition, a transfer of control is
done to complete the loop_statement. If the value of the condition is False,
no transfer of control takes place.
NOTES
6 8 Several nested loops can be exited by an exit_statement that names
the outer loop.
Examples
7 Examples of loops with exit statements:
8 for N in 1 .. Max_Num_Items loop
Get_New_Item(New_Item);
Merge_Item(New_Item, Storage_File);
exit when New_Item = Terminal_Item;
end loop;
9 Main_Cycle:
loop
-- initial statements
exit Main_Cycle when Found;
-- final statements
end loop Main_Cycle;
5.8 Goto Statements
1 [A goto_statement specifies an explicit transfer of control from this
statement to a target statement with a given label.]
Syntax
2 goto_statement ::= goto label_name;
Name Resolution Rules
3 {target statement (of a goto_statement)} The label_name shall resolve to
denote a label; the statement with that label is the target statement.
Legality Rules
4 The innermost sequence_of_statements that encloses the target statement
shall also enclose the goto_statement. Furthermore, if a goto_statement is
enclosed by an accept_statement or a body, then the target statement shall not
be outside this enclosing construct.
4.a Ramification: The goto_statement can be a statement of an inner
sequence_.
4.b It follows from the second rule that if the target statement is
enclosed by such a construct, then the goto_statement cannot be
outside.
Dynamic Semantics
5 {execution (goto_statement) [partial]} The execution of a goto_statement
transfers control to the target statement, completing the execution of any
compound_statement that encloses the goto_statement but does not enclose the
target.
NOTES
6 9 The above rules allow transfer of control to a statement of an
enclosing sequence_of_statements but not the reverse. Similarly, they
prohibit transfers of control such as between alternatives of a
case_statement, if_statement, or select_statement; between
exception_handlers; or from an exception_handler of a
handled_sequence_of_statements back to its sequence_of_statements.
Examples
7 Example of a loop containing a goto statement:
8 <<Sort>>
for I in 1 .. N-1 loop
if A(I) > A(I+1) then
Exchange(A(I), A(I+1));
goto Sort;
end if;
end loop;
Generated by dwww version 1.15 on Thu Jun 20 14:01:38 CEST 2024.