Derived Types and Classes
Author: JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor

Contents Index Search Previous Next

# 3.4 Derived Types and Classes

1
A derived_type_definition defines a new type (and its first subtype) whose characteristics are derived from those of a parent type.

#### Syntax

2
derived_type_definition ::= [abstract] new parent_subtype_indication [record_extension_part]

#### Legality Rules

3
The parent_subtype_indication defines the parent subtype; its type is the parent type.
4
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.
5
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.

#### Static Semantics

6
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.
7
The characteristics of the derived type are defined as follows:
8

- Each class of types that includes the parent type also includes the derived type.

9

- If the parent type is an elementary type or an array type, then the set of possible values of the derived type is a copy of the set of possible values of the parent type. For a scalar type, the base range of the derived type is the same as that of the parent type.

10

- If the parent type is a composite type other than an array type, then the components, protected subprograms, and entries that are declared for the derived type are as follows:

11

- The discriminants specified by a new known_discriminant_part, if there is one; otherwise, each discriminant of the parent type (implicitly declared in the same order with the same specifications) -- in the latter case, the discriminants are said to be inherited, or if unknown in the parent, are also unknown in the derived type;

12

- Each nondiscriminant component, entry, and protected subprogram of the parent type, implicitly declared in the same order with the same declarations; these components, entries, and protected subprograms are said to be inherited;

13

- Each component declared in a record_extension_part, if any.

14

Declarations of components, protected subprograms, and entries, whether implicit or explicit, occur immediately within the declarative region of the type, in the order indicated above, following the parent subtype_indication.

15

- The derived type is limited if and only if the parent type is limited.

16

- For each predefined operator of the parent type, there is a corresponding predefined operator of the derived type.

17

- For each user-defined primitive subprogram (other than a user-defined equality operator -- see below) of the parent type that already exists at the place of the derived_type_definition, there exists a corresponding inherited primitive subprogram of the derived type with the same defining name. Primitive user-defined equality operators of the parent type are also inherited by the derived type, except when the derived type is a nonlimited record extension, and the inherited operator would have a profile that is type conformant with the profile of the corresponding predefined equality operator; in this case, the user-defined equality operator is not inherited, but is rather incorporated into the implementation of the predefined equality operator of the record extension (see 4.5.2).

18

The profile of an inherited subprogram (including an inherited enumeration literal) is obtained from the profile of the corresponding (user-defined) primitive subprogram of the parent type, after systematic replacement of each subtype of its profile (see 6.1) that is of the parent type with a corresponding subtype of the derived type. For a given subtype of the parent type, the corresponding subtype of the derived type is defined as follows:

19

- If the declaration of the derived type has neither a known_discriminant_part nor a record_extension_part, then the corresponding subtype has a constraint that corresponds (as defined above for the first subtype of the derived type) to that of the given subtype.

20

- If the derived type is a record extension, then the corresponding subtype is the first subtype of the derived type.

21

- If the derived type has a new known_discriminant_part but is not a record extension, then the corresponding subtype is constrained to those values that when converted to the parent type belong to the given subtype (see 4.6).

22

The same formal parameters have default_expressions in the profile of the inherited subprogram. Any type mismatch due to the systematic replacement of the parent type by the derived type is handled as part of the normal type conversion associated with parameter passing -- see 6.4.1.

23
If a primitive subprogram of the parent 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.
24
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.
25
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

26
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.
27
For the execution of a call on an inherited subprogram, a call on the corresponding primitive subprogram of the parent 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 parent's subprogram is converted to the derived type.
NOTES
28
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.
29
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.
30
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.
31
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.
32
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).
33
15 Any discriminants of the parent type are either all inherited, or completely replaced with a new set of discriminants.
34
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.
35
17 If the reserved word abstract is given in the declaration of a type, the type is abstract (see 3.9.3).

#### Examples

36
Examples of derived type declarations:
37
`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`
38
`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;`

Contents Index Search Previous Next Legal