3.4 Derived Types and Classes
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.
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
Legality Rules
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.
If the reserved word
limited appears in
a
derived_type_definition,
the parent type shall be a limited type. If the parent type is a tagged
formal type, then in addition to the places where Legality Rules normally
apply (see
12.3), this rule applies also in
the private part of an instance of a generic unit.
Static Semantics
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.
The first subtype of the derived type excludes
null (see
3.10) if and only if the parent
subtype excludes null.
The
characteristics
and implicitly declared primitive subprograms of the derived type are
defined as follows:
If the parent type or a progenitor type belongs
to a class of types, then the derived type also belongs to that class.
The following sets of types, as well as any higher-level sets composed
from them, are classes in this sense, and hence the characteristics defining
these classes are inherited by derived types from their parent or progenitor
types: signed integer, modular integer, ordinary fixed, decimal fixed,
floating point, enumeration, boolean, character, access-to-constant,
general access-to-variable, pool-specific access-to-variable, access-to-subprogram,
array, string, non-array composite, nonlimited, untagged record, tagged,
task, protected, and synchronized tagged.
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.
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:
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;
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;
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.
This paragraph
was deleted.
For each predefined operator of the parent type,
there is a corresponding predefined operator of the derived type.
For each user-defined primitive
subprogram (other than a user-defined equality operator — see below)
of the parent type or of a progenitor 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 and any progenitor
types 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).
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
or progenitor type, after systematic replacement of each subtype of its
profile (see
6.1) that is of the parent or
progenitor type, other than those subtypes found in the designated profile
of an
access_definition,
with a
corresponding subtype of the derived type.
For
a given subtype of the parent or progenitor type, the corresponding subtype
of the derived type is defined as follows:
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.
If the derived type is a record
extension, then the corresponding subtype is the first subtype of the
derived type.
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).
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 or progenitor type by the derived
type is handled as part of the normal type conversion associated with
parameter passing — see
6.4.1.
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.
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
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.
16
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.
17 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.
18 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.
19 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.
20 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).
21 Any discriminants of the parent type
are either all inherited, or completely replaced with a new set of discriminants.
22 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.
23 If the reserved word
abstract
is given in the declaration of a type, the type is abstract (see
3.9.3).
24 An interface type that has a progenitor
type “is derived from” that type. A
derived_type_definition,
however, never defines an interface type.
Examples
Examples of derived
type declarations:
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
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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