6.1 Subprogram Declarations
The sequence of characters in an operator_symbol
shall form a reserved word, a delimiter, or compound
delimiter that corresponds correspond
to an operator belonging to one of the six categories classes
of operators defined in clause 4.5(spaces
are not allowed and the case of letters is not significant)
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. See 2.2 and 2.9 for
rules related to the use of other_format characters
in delimiters and reserved words.
] | in out
Name Resolution Rules
A formal parameter
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
if any, is that of the formal parameter.
The parameter mode
of a formal parameter conveys
the direction of information transfer with the actual parameter: in
, or out
. Mode in
is the default, and is the
mode of a parameter defined by an access_definition
. The formal parameters of a function, if any, shall have the mode in.
Access parameters are permitted. This restriction
to in parameters is primarily a methodological restriction, though
it also simplifies implementation for some compiler technologies.
Abstract subprograms , and null procedures, and expression functions
are not declared by subprogram_declaration
and so do not require completion (although the
latter two can be completions)
. Protected subprograms are
declared by subprogram_declaration
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.
When the Import aspect is True for any entity,
no completion is allowed (see B.1).
of (a view
of) a callable entity is either a parameter_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.
declares a procedure or a function, as indicated by the initial reserved
word, with name and profile as given by its specification.
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
an anonymous access general
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.
Access result types have normal accessibility and
thus don't have any special properties worth noting here.
subtypes of a profile
For any non-access parameters, the nominal subtype
of the parameter.
For any access parameters of an access-to-object
, the designated subtype of the parameter type.
For any access parameters of an access-to-subprogram
type, the subtypes of the designated profile
of the parameter type.
For any non-access result, the nominal subtype
of the function result. For any result,
the result subtype.
For any access result type of an access-to-object
type, the designated subtype of the result type.
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.]
3 The evaluation of default_expression
is caused by certain calls, as described in 6.4.1
They are not evaluated during the elaboration of the subprogram declaration.
4 Subprograms can be called recursively
and can be called concurrently from multiple tasks.
Examples of subprogram
Increment(X : in out
Right_Indent(Margin : out
Line_Size); -- see 3.5.4
Switch(From, To : in out
Link); -- see 3.10.1
Probability; -- see 3.5.7
Min_Cell(X : Link) return
Cell; -- see 3.10.1
Next_Frame(K : Positive) return
Frame; -- see 3.10
Dot_Product(Left, Right : Vector) return
Real; -- see 3.6
"*"(Left, Right : Matrix) return
Matrix; -- see 3.6
Examples of in
parameters with default expressions:
Print_Header(Pages : in
Header : in
Line := (1 .. Line'Last => ' '); -- see 3.6
Center : in
Boolean := True);
Extensions to Ada 83
Program units that are library units may have
to indicate the parent of a child (see Section 10).
Wording Changes from Ada 83
We have incorporated the rules from RM83-6.5,
“Function Subprograms” here and in 6.3
We have incorporated the definitions of RM83-6.6,
“Parameter and Result Type Profile - Overloading of Subprograms”
Extensions to Ada 95
The return type of a function can be an anonymous
Wording Changes from Ada 95
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
Parameters can now be explicitly
aliased, allowing parts of function results to designate parameters and
forcing by-reference parameter passing.
The parameters of a function can now have any mode.
Wording Changes from Ada 2005
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe