- An abstract type is a tagged type intended for use as a parent type for
type extensions, 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.
- An abstract type is a specific type that has the reserved word abstract
in its declaration. Only a tagged type is allowed to be declared abstract.
- A subprogram declared by an abstract_subprogram_declaration (see
6.1) is an abstract subprogram. If it is a primitive subprogram of a tagged
type, then the tagged type shall be abstract.
- For a derived type, if the parent or ancestor type has an abstract
primitive subprogram, or a primitive function with a controlling result,
- If the derived type is abstract or untagged, the inherited
subprogram is abstract.
- Otherwise, the subprogram shall be overridden with a nonabstract
subprogram; for a type declared in the visible part of a package,
the overriding may be either in the visible or the private part.
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.
- A generic actual subprogram shall not be an abstract subprogram. The
prefix of an attribute_reference for the Access, Unchecked_Access, or Address
attributes shall not denote an abstract subprogram.
(74) Abstractness is not inherited; to declare an abstract type, the
reserved word abstract has to be used in the declaration of the type
(75) 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.
- 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;
(76) 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.
-- Email comments, additions, corrections, gripes, kudos, etc. to:
Magnus Kempe -- Magnus.Kempe@di.epfl.ch
Page last generated: 95-03-12