Object-oriented Software in Ada 95

194 Child libraries
14.2 Specification of generic component
The formal type specification constrains the actual type passed as a parameter to belong to a particular category of
types. Examples of these categories are listed in the table below:
Formal type specification In Actual parameter can belong to Note
type T
Ada 83 the following types
is private √ Any non limited type. 1
is limited private √ Any type. 2
is tagged r Any non limited tagged type. 1
is limited tagged r Any tagged type. 2
is () √ Any discrete type, constrained type. 1,5
() is private r Any discrete or indefinite non 2,3
limited type.
() is limited r Any discrete or indefinite type. 2,3
private
is mod r Any modular type. 1
is range √ Any integer type. 1
is digits √ Any float type. 1
is delta √ Any fixed ordinary type. 1
is delta digits r Any fixed decimal type. 1
is access √ Any access type. 4
with procedure ... √ procedure matching the signature. 6
with package ... r package matching the signature. 6
Note 1
Note 2
Note 3
The formal parameter in the generic unit is restricted to a use compatible with the actual parameter.
The formal parameter is restricted to operations which are compatible with a limited type. Thus,
assignment of, and the default comparison for equality and not equality are prohibited.
Cannot be used to declare an indefinite type without declaring its range. For example, the indefinite
type:
type String is array (Positive range ) of Character;
cannot be declared without specifying the range.
Note 4 Access types are covered in chapter 14.
May also be is access all or is access constant
Note 5
Note 6
Ada 83 has the well-known problem that an indefinite type may be used as a formal parameter. If the
formal parameter is used to declare an object in the body of the generic unit, then on the instantiation
of the unit an error message will be generated from the body of the generic unit.
Used to specify a procedure, function or package that is used in the body of the generic unit.
© M A Smith - May not be reproduced without permission