12.4 Formal Objects

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

The expected type for the default_expression, if any, of a formal object is the type of the formal object.

For a generic formal object of mode in, the expected type for the actual is the type of the formal.

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.

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. 

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).

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:

 For a formal_object_declaration of mode in out with a null_exclusion or an access_definition that has a null_exclusion, the subtype of the actual matching the formal_object_declaration shall exclude null. In addition, if the actual matching the formal_object_declaration statically denotes the generic formal object of mode in out 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. 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.

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. 

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. 

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.