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3.4 Derived Types and Classes

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A derived_type_definition defines a derived type (and its first subtype) whose characteristics are derived from those of a parent type, and possibly from progenitor types.
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  A class of types is a set of types that is closed under derivation; that is, if the parent or a progenitor type of a derived type belongs to a class, then so does the derived type. By saying that a particular group of types forms a class, we are saying that all derivatives of a type in the set inherit the characteristics that define that set. The more general term category of types is used for a set of types whose defining characteristics are not necessarily inherited by derivatives; for example, limited, abstract, and interface are all categories of types, but not classes of types.

Syntax

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derived_type_definition ::= 
    [abstract] [limitednew parent_subtype_indication [[and interface_listrecord_extension_part]

Legality Rules

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The parent_subtype_indication defines the parent subtype; its type is the parent type. The interface_list defines the progenitor types (see 3.9.4). A derived type has one parent type and zero or more progenitor types.
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A type shall be completely defined (see 3.11.1) prior to being specified as the parent type in a derived_type_definition — the full_type_declarations for the parent type and any of its subcomponents have to precede the derived_type_definition.
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If there is a record_extension_part, the derived type is called a record extension of the parent type. A record_extension_part shall be provided if and only if the parent type is a tagged type. An interface_list shall be provided only if the parent type is a tagged type. 
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  If the reserved word limited appears in a derived_type_definition, the parent type shall be a limited type. 

Static Semantics

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The first subtype of the derived type is unconstrained if a known_discriminant_part is provided in the declaration of the derived type, or if the parent subtype is unconstrained. Otherwise, the constraint of the first subtype corresponds to that of the parent subtype in the following sense: it is the same as that of the parent subtype except that for a range constraint (implicit or explicit), the value of each bound of its range is replaced by the corresponding value of the derived type. 
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  The first subtype of the derived type excludes null (see 3.10) if and only if the parent subtype excludes null.
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The characteristics of the derived type are defined as follows: 
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 If a primitive subprogram of the parent or progenitor type is visible at the place of the derived_type_definition, then the corresponding inherited subprogram is implicitly declared immediately after the derived_type_definition. Otherwise, the inherited subprogram is implicitly declared later or not at all, as explained in 7.3.1.
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A derived type can also be defined by a private_extension_declaration (see 7.3) or a formal_derived_type_definition (see 12.5.1). Such a derived type is a partial view of the corresponding full or actual type.
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All numeric types are derived types, in that they are implicitly derived from a corresponding root numeric type (see 3.5.4 and 3.5.6).

Dynamic Semantics

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The elaboration of a derived_type_definition creates the derived type and its first subtype, and consists of the elaboration of the subtype_indication and the record_extension_part, if any. If the subtype_indication depends on a discriminant, then only those expressions that do not depend on a discriminant are evaluated. 
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 For the execution of a call on an inherited subprogram, a call on the corresponding primitive subprogram of the parent or progenitor type is performed; the normal conversion of each actual parameter to the subtype of the corresponding formal parameter (see 6.4.1) performs any necessary type conversion as well. If the result type of the inherited subprogram is the derived type, the result of calling the subprogram of the parent or progenitor is converted to the derived type, or in the case of a null extension, extended to the derived type using the equivalent of an extension_aggregate with the original result as the ancestor_part and null record as the record_component_association_list.
NOTES
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10  Classes are closed under derivation — any class that contains a type also contains its derivatives. Operations available for a given class of types are available for the derived types in that class.
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11  Evaluating an inherited enumeration literal is equivalent to evaluating the corresponding enumeration literal of the parent type, and then converting the result to the derived type. This follows from their equivalence to parameterless functions.
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12  A generic subprogram is not a subprogram, and hence cannot be a primitive subprogram and cannot be inherited by a derived type. On the other hand, an instance of a generic subprogram can be a primitive subprogram, and hence can be inherited.
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13  If the parent type is an access type, then the parent and the derived type share the same storage pool; there is a null access value for the derived type and it is the implicit initial value for the type. See 3.10.
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14  If the parent type is a boolean type, the predefined relational operators of the derived type deliver a result of the predefined type Boolean (see 4.5.2). If the parent type is an integer type, the right operand of the predefined exponentiation operator is of the predefined type Integer (see 4.5.6).
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15  Any discriminants of the parent type are either all inherited, or completely replaced with a new set of discriminants.
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16  For an inherited subprogram, the subtype of a formal parameter of the derived type need not have any value in common with the first subtype of the derived type. 
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17  If the reserved word abstract is given in the declaration of a type, the type is abstract (see 3.9.3).
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18  An interface type that has a progenitor type “is derived from” that type. A derived_type_definition, however, never defines an interface type.
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19  It is illegal for the parent type of a derived_type_definition to be a synchronized tagged type. 

Examples

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Examples of derived type declarations: 
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type Local_Coordinate is new Coordinate;   --  two different types
type Midweek is new Day range Tue .. Thu;  --  see 3.5.1
type Counter is new Positive;              --  same range as Positive 
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type Special_Key is new Key_Manager.Key;   --  see 7.3.1
  -- the inherited subprograms have the following specifications: 
  --         procedure Get_Key(K : out Special_Key);
  --         function "<"(X,Y : Special_Key) return Boolean;

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