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3.9.3 Abstract Types and Subprograms

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   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.

Legality Rules

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   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.
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   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.
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   For a derived type, if the parent or ancestor type has an abstract primitive subprogram, or a primitive function with a controlling result, then:
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   A call on an abstract subprogram shall be a dispatching call; nondispatching calls to an abstract subprogram are not allowed.
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   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.
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   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.
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    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.
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    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.
NOTES
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74  Abstractness is not inherited; to declare an abstract type, the reserved word abstract has to be used in the declaration of the type extension.
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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.

Examples

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    Example of an abstract type representing a set of natural numbers:
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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;
NOTES
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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.

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