12.6 Formal Subprograms
1
Formal subprograms
can be used to pass callable entities to a generic unit.
Syntax
2/2
formal_subprogram_declaration ::= formal_concrete_subprogram_declaration
|
formal_abstract_subprogram_declaration
2.1/2
formal_concrete_subprogram_declaration ::=
with subprogram_specification [
is subprogram_default];
2.2/2
formal_abstract_subprogram_declaration ::=
with subprogram_specification is abstract [
subprogram_default];
3/2
subprogram_default ::= default_name | <> |
null
4
4.1/2
A subprogram_default
of null shall not be specified for a formal function or for a
formal_abstract_subprogram_declaration.
Name Resolution Rules
5
The expected profile for the
default_name, if any, is that of the formal
subprogram.
6
For a generic formal subprogram,
the expected profile for the actual is that of the formal subprogram.
Legality Rules
7
The profiles of the formal and any named default
shall be mode-conformant.
8
The profiles of the formal and actual shall be mode-conformant.
8.1/2
For a parameter or
result subtype of a formal_subprogram_declaration
that has an explicit null_exclusion:
8.2/2
- if the actual matching the formal_subprogram_declaration
denotes a generic formal object of another generic unit G, and
the instantiation containing the actual that occurs within the body of
a generic unit G or within the body of a generic unit declared
within the declarative region of the generic unit G, then the
corresponding parameter or result type of the formal subprogram of G
shall have a null_exclusion;
8.3/2
- otherwise, the subtype of the corresponding
parameter or result type of the actual matching the formal_subprogram_declaration
shall exclude null. In addition to the places where
Legality Rules normally apply (see 12.3),
this rule applies also in the private part of an instance of a generic
unit.
8.4/2
If a formal parameter of a
formal_abstract_subprogram_declaration
is of a specific tagged type
T or of an anonymous access type
designating a specific tagged type
T,
T is called a
controlling
type of the
formal_abstract_subprogram_declaration.
Similarly, if the result of a
formal_abstract_subprogram_declaration
for a function is of a specific tagged type
T or of an anonymous
access type designating a specific tagged type
T,
T is
called a controlling type of the
formal_abstract_subprogram_declaration.
A
formal_abstract_subprogram_declaration shall
have exactly one controlling type.
8.5/2
The actual subprogram for a formal_abstract_subprogram_declaration
shall be a dispatching operation of the controlling type or of the actual
type corresponding to the controlling type.
Static Semantics
9
A formal_subprogram_declaration
declares a generic formal subprogram. The types of the formal parameters
and result, if any, of the formal subprogram are those determined by
the subtype_marks given in the formal_subprogram_declaration;
however, independent of the particular subtypes that are denoted by the
subtype_marks, the nominal subtypes of the
formal parameters and result, if any, are defined to be nonstatic, and
unconstrained if of an array type (no applicable index constraint is
provided in a call on a formal subprogram). In an instance, a formal_subprogram_declaration
declares a view of the actual. The profile of this view takes its subtypes
and calling convention from the original profile of the actual entity,
while taking the formal parameter names and
default_expressions from the profile given
in the formal_subprogram_declaration. The
view is a function or procedure, never an entry.
10
If a generic unit has a subprogram_default
specified by a box, and the corresponding actual parameter is omitted,
then it is equivalent to an explicit actual parameter that is a usage
name identical to the defining name of the formal.
10.1/2
If a generic unit has a subprogram_default
specified by the reserved word null, and the corresponding actual
parameter is omitted, then it is equivalent to an explicit actual parameter
that is a null procedure having the profile given in the formal_subprogram_declaration.
10.2/2
The subprogram declared by a formal_abstract_subprogram_declaration
with a controlling type T is a dispatching operation of type T.
11
13 The matching rules for formal subprograms
state requirements that are similar to those applying to
subprogram_renaming_declarations
(see
8.5.4). In particular, the name of a
parameter of the formal subprogram need not be the same as that of the
corresponding parameter of the actual subprogram; similarly, for these
parameters,
default_expressions need not correspond.
12
14 The constraints that apply to a parameter
of a formal subprogram are those of the corresponding formal parameter
of the matching actual subprogram (not those implied by the corresponding
subtype_mark in the _specification
of the formal subprogram). A similar remark applies to the result of
a function. Therefore, to avoid confusion, it is recommended that the
name of a first subtype be used in any declaration
of a formal subprogram.
13
15 The subtype specified for a formal parameter
of a generic formal subprogram can be any visible subtype, including
a generic formal subtype of the same generic_formal_part.
14
16 A formal subprogram is matched by an
attribute of a type if the attribute is a function with a matching specification.
An enumeration literal of a given type matches a parameterless formal
function whose result type is the given type.
15
17 A default_name
denotes an entity that is visible or directly visible at the place of
the generic_declaration; a box used as a default
is equivalent to a name that denotes an entity that is directly visible
at the place of the _instantiation.
16/2
18 The actual subprogram cannot be abstract
unless the formal subprogram is a
formal_abstract_subprogram_declaration
(see
3.9.3).
16.1/2
19 The subprogram declared by a
formal_abstract_subprogram_declaration
is an abstract subprogram. All calls on a subprogram declared by a
formal_abstract_subprogram_declaration
must be dispatching calls. See
3.9.3.
16.2/2
20 A null procedure as a subprogram default
has convention Intrinsic (see
6.3.1).
Examples
17
Examples of generic
formal subprograms:
18/2
with function "+"(X, Y : Item)
return Item
is <>;
with function Image(X : Enum)
return String
is Enum'Image;
with procedure Update
is Default_Update;
with procedure Pre_Action(X :
in Item)
is null; --
defaults to no action
with procedure Write(S :
not null access Root_Stream_Type'Class;
Desc : Descriptor)
is abstract Descriptor'Write; --
see 13.13.2
--
Dispatching operation on Descriptor with default
19
-- given the generic procedure declaration
20
generic
with procedure Action (X : in Item);
procedure Iterate(Seq : in Item_Sequence);
21
-- and the procedure
22
procedure Put_Item(X : in Item);
23
-- the following instantiation is possible
24
procedure Put_List is new Iterate(Action => Put_Item);
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