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6.4 Subprogram Calls
1
{subprogram call}
A
subprogram call is either a
procedure_call_statement
or a
function_call; [it invokes
the execution of the
subprogram_body.
The call specifies the association of the actual parameters, if any,
with formal parameters of the subprogram.]
Syntax
2
procedure_call_statement
::=
procedure_name;
|
procedure_prefix actual_parameter_part;
3
function_call
::=
function_name
|
function_prefix actual_parameter_part
4
actual_parameter_part
::=
(
parameter_association {,
parameter_association})
5
parameter_association
::=
[
formal_parameter_selector_name =>]
explicit_actual_parameter
6
explicit_actual_parameter
::= expression |
variable_name
7
{named association}
{positional association}
A
parameter_association
is
named or
positional according to whether or not the
formal_parameter_selector_name
is specified. Any positional associations shall precede any named associations.
Named associations are not allowed if the
prefix
in a subprogram call is an
attribute_reference.
7.a
Ramification: This means
that the formal parameter names used in describing predefined attributes
are to aid presentation of their semantics, but are not intended for
use in actual calls.
Name Resolution Rules
8
The name
or prefix given in a procedure_call_statement
shall resolve to denote a callable entity that is a procedure, or an
entry renamed as (viewed as) a procedure. The name
or prefix given in a function_call
shall resolve to denote a callable entity that is a function. [When there
is an actual_parameter_part, the
prefix can be an implicit_dereference
of an access-to-subprogram value.]
8.a
Ramification: The function
can be an operator, enumeration literal, attribute that is a function,
etc.
9
A subprogram call shall contain at most one association
for each formal parameter. Each formal parameter without an association
shall have a
default_expression
(in the profile of the view denoted by the
name
or
prefix). This rule is an overloading
rule (see
8.6).
Dynamic Semantics
10
{execution (subprogram call)
[partial]} For the execution of a subprogram
call, the
name or
prefix
of the call is evaluated, and each
parameter_association
is evaluated (see
6.4.1). If a
default_expression
is used, an implicit
parameter_association
is assumed for this rule. These evaluations are done in an arbitrary
order. The
subprogram_body is then
executed. Finally, if the subprogram completes normally, then after it
is left, any necessary assigning back of formal to actual parameters
occurs (see
6.4.1).
10.a
Discussion: The implicit
association for a default is only for this run-time rule. At compile
time, the visibility rules are applied to the default at the place where
it occurs, not at the place of a call.
10.b
10.c
10.d
10.e
Normally, the subprogram_body
that is executed by the above rule is the one for the subprogram being
called. For an enumeration literal, implicitly declared (but noninherited)
subprogram, or an attribute that is a subprogram, an implicit body is
assumed. For a dispatching call, 3.9.2, ``Dispatching
Operations of Tagged Types'' defines which subprogram_body
is executed.
11
{Program_Error (raised by
failure of run-time check)} The exception
Program_Error is raised at the point of a
function_call
if the function completes normally without executing a
return_statement.
11.a
Discussion: We are committing
to raising the exception at the point of call, for uniformity -- see
AI83-00152. This happens after the function is left, of course.
11.b
Note that there is no name for
suppressing this check, since the check imposes no time overhead and
minimal space overhead (since it can usually be statically eliminated
as dead code).
12
A
function_call
denotes a constant, as defined in
6.5; the
nominal subtype of the constant is given by the result subtype of the
function.
{nominal subtype (of the result of a function_call)
[partial]} {constant (result
of a function_call) [partial]} Examples
13
Examples of
procedure calls:
14
Traverse_Tree; -- see 6.1
Print_Header(128, Title, True); -- see 6.1
15
Switch(From => X, To => Next); -- see 6.1
Print_Header(128, Header => Title, Center => True); -- see 6.1
Print_Header(Header => Title, Center => True, Pages => 128); -- see 6.1
16
Examples of function
calls:
17
Dot_Product(U, V) -- see 6.1 and 6.3
Clock -- see 9.6
F.all -- presuming F is of an access-to-subprogram type -- see 3.10
18
Examples of procedures
with default expressions:
19
procedure Activate(Process : in Process_Name;
After : in Process_Name := No_Process;
Wait : in Duration := 0.0;
Prior : in Boolean := False);
20
procedure Pair(Left, Right : in Person_Name := new Person); -- see 3.10.1
21
Examples of their
calls:
22
Activate(X);
Activate(X, After => Y);
Activate(X, Wait => 60.0, Prior => True);
Activate(X, Y, 10.0, False);
23
Pair;
Pair(Left => new Person, Right => new Person);
24
7 If a default_expression
is used for two or more parameters in a multiple parameter_specification,
the default_expression is evaluated
once for each omitted parameter. Hence in the above examples, the two
calls of Pair are equivalent.
Examples
25
Examples of
overloaded subprograms:
26
procedure Put(X : in Integer);
procedure Put(X : in String);
27
procedure Set(Tint : in Color);
procedure Set(Signal : in Light);
28
Examples of their
calls:
29
Put(28);
Put("no possible ambiguity here");
30
Set(Tint => Red);
Set(Signal => Red);
Set(Color'(Red));
31
-- Set(Red) would be ambiguous since Red may
-- denote a value either of type Color or of type Light
Wording Changes from Ada 83
31.a
We have gotten rid of parameters
``of the form of a type conversion'' (see RM83-6.4.1(3)). The new view
semantics of type_conversions allows
us to use normal type_conversions
instead.
31.b
We have moved wording about
run-time semantics of parameter associations to 6.4.1.
31.c
We have moved wording about
raising Program_Error for a function that falls off the end to here from
RM83-6.5.
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