6.1 Subprogram Declarations
Syntax
Reason: {
AI95-00395-01}
{
AI05-0090-1}
The “sequence of characters” of the string literal of the
operator is a technical term (see
2.6), and
does not include the surrounding quote characters. As defined in
2.2,
lexical elements are “formed” from a sequence of characters.
Spaces are not allowed, and upper and lower case is not significant.
Discussion: {
AI12-0395-1}
Only implementation-defined aspects are allowed on formal parameters
in Ada 2022. Implementers are cautioned that any aspect allowed on a
formal parameter will need conformance rules. If, for instance, an aspect
changed the representation of a parameter, rules would be needed to ensure
that the representation is the same for the specification and body.
mode ::= [
in] |
in out |
out
Name Resolution Rules
A
formal parameter is
an object [directly visible within a
subprogram_body]
that represents the actual parameter passed to the subprogram in a call;
it is declared by a
parameter_specification.
For a formal parameter, the expected type for its
default_expression,
if any, is that of the formal parameter.
Legality Rules
{
AI05-0143-1}
The
parameter mode of a formal parameter conveys
the direction of information transfer with the actual parameter:
in,
in out, or
out. Mode
in is the default, and is the
mode of a parameter defined by an
access_definition.
Ramification: {
AI95-00348-01}
{
AI05-0177-1}
Abstract subprograms , null procedures, and expression functions are
not declared by
subprogram_declarations,
and so do not require completion (although the latter two can
be
completions). Protected subprograms are declared by
subprogram_declarations,
and so require completion. Note that an abstract subprogram is a subprogram,
a null procedure is a subprogram, an expression function is a subprogram,
and a protected subprogram is a subprogram, but a generic subprogram
is not a subprogram.
Proof: {
AI05-0229-1}
When the Import aspect is True for any entity, no completion is allowed
(see
B.1).
Static Semantics
The
profile of (a view
of) a callable entity is either a
parameter_profile
or
parameter_and_result_profile[;
it embodies information about the interface to that entity — for
example, the profile includes information about parameters passed to
the callable entity. All callable entities have a profile — enumeration
literals, other subprograms, and entries. An access-to-subprogram type
has a designated profile.] Associated with a profile is a calling convention.
A
subprogram_declaration
declares a procedure or a function, as indicated by the initial reserved
word, with name and profile as given by its specification.
{
AI95-00231-01}
{
AI95-00254-01}
{
AI95-00318-02}
An
access parameter is a formal
in
parameter specified by an
access_definition.
An
access result type is a function result
type specified by an
access_definition.
An access parameter or result type is of an anonymous access type (see
3.10). [Access parameters of an access-to-object
type allow dispatching calls to be controlled by access values. Access
parameters of an access-to-subprogram type permit calls to subprograms
passed as parameters irrespective of their accessibility level.]
Discussion: {
AI95-00318-02}
Access result types have normal accessibility and thus don't have any
special properties worth noting here.
The
subtypes of a profile are:
For any non-access parameters, the nominal subtype
of the parameter.
{
AI95-00254-01}
For any access parameters of an access-to-object type, the designated
subtype of the parameter type.
{
AI95-00254-01}
{
AI05-0164-1}
For any access parameters of an access-to-subprogram type, the subtypes
of the designated profile of the parameter type.
{
AI95-00318-02}
For any access result type of an access-to-object type, the designated
subtype of the result type.
{
AI95-00318-02}
{
AI05-0164-1}
For any access result type of an access-to-subprogram type, the subtypes
of the designated profile of the result type.
[
The
types of a profile
are the types of those subtypes.]
Dynamic Semantics
NOTE 3 The evaluation of
default_expressions
is caused by certain calls, as described in
6.4.1.
They are not evaluated during the elaboration of the subprogram declaration.
NOTE 4 Subprograms can be called
recursively and can be called concurrently from multiple tasks.
Examples
Examples of subprogram
declarations:
procedure Traverse_Tree;
procedure Increment(X :
in out Integer);
procedure Right_Indent(Margin :
out Line_Size); --
see 3.5.4
procedure Switch(From, To :
in out Link); --
see 3.10.1
function Random
return Probability; --
see 3.5.7
{
AI12-0056-1}
function Min_Cell(X : Link)
return Cell; --
see 3.10.1
function Next_Frame(K : Positive)
return Frame; --
see 3.10
function Dot_Product(Left, Right : Vector)
return Real; --
see 3.6
function Find(B :
aliased in out Barrel; Key : String)
return Real;
--
see 4.1.5function "*"(Left, Right : Matrix)
return Matrix; --
see 3.6
Examples of in
parameters with default expressions:
procedure Print_Header(Pages :
in Natural;
Header :
in Line := (1 .. Line'Last => ' '); --
see 3.6
Center :
in Boolean := True);
Extensions to Ada 83
Wording Changes from Ada 83
We have incorporated the rules from RM83-6.5,
“Function Subprograms” here and in
6.3,
“
Subprogram Bodies”
We have incorporated the definitions of RM83-6.6,
“Parameter and Result Type Profile - Overloading of Subprograms”
here.
Extensions to Ada 95
{
AI95-00318-02}
The return type of a function can be an anonymous access type.
Wording Changes from Ada 95
{
AI95-00254-01}
A description of the purpose of anonymous access-to-subprogram parameters
and the definition of the profile of subprograms containing them was
added.
Extensions to Ada 2005
{
AI05-0142-4}
Parameters can now be explicitly aliased, allowing
parts of function results to designate parameters and forcing by-reference
parameter passing.
{
AI05-0143-1}
The parameters of a function can now have any mode.
Wording Changes from Ada 2005
Extensions to Ada 2012
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
