Formal Objects
Author: JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor

Ada Reference Manual — Legal Information

# 12.4 Formal Objects

1
A generic formal object can be used to pass a value or variable to a generic unit.

#### Syntax

2/3
formal_object_declaration ::=
defining_identifier_list : mode [null_exclusion] subtype_mark [:= default_expression]
[aspect_specification];
| defining_identifier_list : mode access_definition [:= default_expression]
[aspect_specification];

#### Name Resolution Rules

3
The expected type for the default_expression, if any, of a formal object is the type of the formal object.
4
For a generic formal object of mode in, the expected type for the actual is the type of the formal.
5/2
For a generic formal object of mode in out, the type of the actual shall resolve to the type determined by the subtype_mark, or for a formal_object_declaration with an access_definition, to a specific anonymous access type. If the anonymous access type is an access-to-object type, the type of the actual shall have the same designated type as that of the access_definition. If the anonymous access type is an access-to-subprogram type, the type of the actual shall have a designated profile which is type conformant with that of the access_definition.

#### Legality Rules

6
If a generic formal object has a default_expression, then the mode shall be in (either explicitly or by default); otherwise, its mode shall be either in or in out.
7
For a generic formal object of mode in, the actual shall be an expression. For a generic formal object of mode in out, the actual shall be a name that denotes a variable for which renaming is allowed (see 8.5.1).
8/2
In the case where the type of the formal is defined by an access_definition, the type of the actual and the type of the formal:
8.1/2
shall both be access-to-object types with statically matching designated subtypes and with both or neither being access-to-constant types; or
8.2/2
shall both be access-to-subprogram types with subtype conformant designated profiles.
8.3/2
For a formal_object_declaration with a null_exclusion or an access_definition that has a null_exclusion:
8.4/2
if the actual matching the formal_object_declaration denotes the generic formal object of another generic unit G, and the instantiation containing the actual occurs within the body of G or within the body of a generic unit declared within the declarative region of G, then the declaration of the formal object of G shall have a null_exclusion;
8.5/2
otherwise, the subtype of the actual matching the formal_object_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.

#### Static Semantics

9/2
A formal_object_declaration declares a generic formal object. The default mode is in. For a formal object of mode in, the nominal subtype is the one denoted by the subtype_mark or access_definition in the declaration of the formal. For a formal object of mode in out, its type is determined by the subtype_mark or access_definition in the declaration; its nominal subtype is nonstatic, even if the subtype_mark denotes a static subtype; for a composite type, its nominal subtype is unconstrained if the first subtype of the type is unconstrained, even if the subtype_mark denotes a constrained subtype.
10/2
In an instance, a formal_object_declaration of mode in is a full constant declaration and declares a new stand-alone constant object whose initialization expression is the actual, whereas a formal_object_declaration of mode in out declares a view whose properties are identical to those of the actual.

#### Dynamic Semantics

11
For the evaluation of a generic_association for a formal object of mode in, a constant object is created, the value of the actual parameter is converted to the nominal subtype of the formal object, and assigned to the object, including any value adjustment — see 7.6.
NOTES
12
6 The constraints that apply to a generic formal object of mode in out are those of the corresponding generic actual parameter (not those implied by the subtype_mark that appears in the formal_object_declaration). Therefore, to avoid confusion, it is recommended that the name of a first subtype be used for the declaration of such a formal object.

Ada 2005 and 2012 Editions sponsored in part by Ada-Europe