Ada Reference Manual (Ada 2022 Draft 34) — Legal Information
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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 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

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abstract_subprogram_declaration ::= 
    [overriding_indicator]
    subprogram_specification is abstract
        [aspect_specification];

Static Semantics

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 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

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Only a tagged type shall have the reserved word abstract in its declaration. 
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A subprogram declared by an abstract_subprogram_declaration or a formal_abstract_subprogram_declaration (see 12.6) 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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If a type has an implicitly declared primitive subprogram that is inherited or is a predefined 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 nonabstract null extension inherits a function with a controlling result, then: 
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If the type is abstract or untagged, the implicitly declared subprogram is abstract. 
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Otherwise, the subprogram shall be overridden with a nonabstract subprogram or, in the case of a private extension inheriting a nonabstract 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 is allowed to be inherited as is, without being overridden for the formal type itself; a nonabstract version will necessarily be provided by the actual type.
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A call on an abstract subprogram shall be a dispatching call; nondispatching calls to an abstract subprogram are not allowed. In addition to the places where Legality Rules normally apply (see 12.3), these rules also apply in the private part of an instance of a generic unit. 
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 If the name or prefix given in an iterator_procedure_call (see 5.5.3) denotes an abstract subprogram, the subprogram shall be a dispatching subprogram.
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The type of an aggregate, of an object created by an object_declaration or an allocator, or of 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 function has an access result type designating an abstract type, then the function shall be abstract. The type denoted by a return_subtype_indication (see 6.5) shall not be abstract. A generic function shall not have an abstract result type or an access result type designating an abstract type. 
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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.
10/3
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 or a controlling access result shall not be declared in the private part, unless it is overriding a function implicitly declared in the visible part. 
11/2
A generic actual subprogram shall not be an abstract subprogram unless the generic formal subprogram is declared by a formal_abstract_subprogram_declaration. The prefix of an attribute_reference for the Access, Unchecked_Access, or Address attributes shall not denote an abstract subprogram. 

Dynamic Semantics

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  The elaboration of an abstract_subprogram_declaration has no effect. 
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NOTE 1   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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NOTE 2   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;
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NOTE 3   Notes on the example: Given the above abstract type, one can derive various (nonabstract) extensions of the type, representing alternative implementations of a set. One possibility is to use a bit vector, but impose an upper bound on the largest element representable, while another possible implementation is a hash table, trading off space for flexibility. 
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