3.9.3 Abstract Types and Subprograms
An
abstract type is a tagged type intended for use as an ancestor
of other types, but which is not allowed to have objects of its own.
An
abstract subprogram
is a subprogram that has no body, but is intended to be overridden at
some point when inherited. Because objects of an abstract type cannot
be created, a dispatching call to an abstract subprogram always dispatches
to some overriding body.
Syntax
Static Semantics
Interface
types (see
3.9.4) are abstract types. In
addition, a tagged type that has the reserved word
abstract in
its declaration is an abstract type. The class-wide type (see
3.4.1)
rooted at an abstract type is not itself an abstract type.
Legality Rules
Only a tagged type shall have the reserved word abstract
in its declaration.
If a type has an implicitly
declared primitive subprogram that is inherited or is the predefined
equality operator, and the corresponding primitive subprogram of the
parent or ancestor type is abstract or is a function with a controlling
access result, or if a type other than a null extension inherits a function
with a controlling result, then:
If the type is abstract or untagged, the implicitly
declared subprogram is abstract.
Otherwise, the subprogram shall be overridden with
a nonabstract subprogram or, in the case of a private extension inheriting
a function with a controlling result, have a full type that is a null
extension; for a type declared in the visible part of a package, the
overriding may be either in the visible or the private part. Such a subprogram
is said to
require overriding.
However, if
the type is a generic formal type, the subprogram need not be overridden
for the formal type itself; a nonabstract version will necessarily be
provided by the actual type.
A call on an abstract subprogram shall be a dispatching
call; nondispatching calls to an abstract subprogram are not allowed.
The type of an
aggregate,
or of an object created by an
object_declaration
or an
allocator,
or a generic formal object of mode
in, shall not be abstract.
The type of the target of an assignment operation (see
5.2)
shall not be abstract. The type of a component shall not be abstract.
If the result type of a function is abstract, then the function shall
be abstract.
If a partial view is not abstract, the corresponding
full view shall not be abstract. If a generic formal type is abstract,
then for each primitive subprogram of the formal that is not abstract,
the corresponding primitive subprogram of the actual shall not be abstract.
For an abstract type declared in a visible part,
an abstract primitive subprogram shall not be declared in the private
part, unless it is overriding an abstract subprogram implicitly declared
in the visible part. For a tagged type declared in a visible part, a
primitive function with a controlling result shall not be declared in
the private part, unless it is overriding a function implicitly declared
in the visible part.
Dynamic Semantics
76 Abstractness is not inherited; to declare
an abstract type, the reserved word abstract has to be used in
the declaration of the type extension.
77 A class-wide type is never abstract.
Even if a class is rooted at an abstract type, the class-wide type for
the class is not abstract, and an object of the class-wide type can be
created; the tag of such an object will identify some nonabstract type
in the class.
Examples
Example of an abstract
type representing a set of natural numbers:
package Sets is
subtype Element_Type is Natural;
type Set is abstract tagged null record;
function Empty return Set is abstract;
function Union(Left, Right : Set) return Set is abstract;
function Intersection(Left, Right : Set) return Set is abstract;
function Unit_Set(Element : Element_Type) return Set is abstract;
procedure Take(Element : out Element_Type;
From : in out Set) is abstract;
end Sets;
78 Notes on the example: Given the
above abstract type, one could then derive various (nonabstract) extensions
of the type, representing alternative implementations of a set. One might
use a bit vector, but impose an upper bound on the largest element representable,
while another might use a hash table, trading off space for flexibility.
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