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3.4.1 Derivation Classes
1
In addition to the various language-defined classes
of types, types can be grouped into derivation classes.
Static Semantics
2
A derived type is
derived
from its parent type
directly; it is derived
indirectly
from any type from which its parent type is derived.
The
derivation class of types for a type
T (also called the class
rooted at
T) is the set consisting of
T (the
root
type of the class) and all types derived from
T (directly
or indirectly) plus any associated universal or class-wide types (defined
below).
3
Every type is either a
specific type, a
class-wide type, or a
universal type.
A
specific type is one defined by a
type_declaration,
a
formal_type_declaration, or a
full type definition embedded in a declaration for an object. Class-wide
and universal types are implicitly defined, to act as representatives
for an entire class of types, as follows:
4
- Class-wide types
-
Class-wide types are defined for (and belong to) each derivation class
rooted at a tagged type (see 3.9). Given a
subtype S of a tagged type T, S'Class is the subtype_mark
for a corresponding subtype of the tagged class-wide type T'Class.
Such types are called ``class-wide'' because when a formal parameter
is defined to be of a class-wide type T'Class, an actual parameter
of any type in the derivation class rooted at T is acceptable
(see 8.6).
5
- The set
of values for a class-wide type T'Class is the discriminated union
of the set of values of each specific type in the derivation class rooted
at T (the tag acts as the implicit discriminant -- see 3.9).
Class-wide types have no primitive subprograms of their own. However,
as explained in 3.9.2, operands of a class-wide
type T'Class can be used as part of a dispatching call on a primitive
subprogram of the type T. The only components (including discriminants)
of T'Class that are visible are those of T. If S is a first
subtype, then S'Class is a first subtype.
6
- Universal types
-
Universal types are defined for (and belong to) the integer, real, and
fixed point classes, and are referred to in this standard as respectively,
universal_integer, universal_real, and universal_fixed.
These are analogous to class-wide types for these language-defined numeric
classes. As with class-wide types, if a formal parameter is of a universal
type, then an actual parameter of any type in the corresponding class
is acceptable. In addition, a value of a universal type (including an
integer or real numeric_literal)
is ``universal'' in that it is acceptable where some particular type
in the class is expected (see 8.6).
7
- The set of values of a universal
type is the undiscriminated union of the set of values possible for any
definable type in the associated class. Like class-wide types, universal
types have no primitive subprograms of their own. However, their ``universality''
allows them to be used as operands with the primitive subprograms of
any type in the corresponding class.
8
The integer
and real numeric classes each have a specific root type in addition to
their universal type, named respectively
root_integer and
root_real.
9
A class-wide or universal
type is said to
cover all of the types in its class. A specific
type covers only itself.
10
A specific type
T2
is defined to be a
descendant of a type
T1 if
T2
is the same as
T1, or if
T2 is derived (directly or indirectly)
from
T1. A class-wide type
T2'Class is defined to be a
descendant of type
T1 if
T2 is a descendant of
T1.
Similarly, the universal types are defined to be descendants of the root
types of their classes.
If a type
T2 is a
descendant of a type
T1, then
T1 is called an
ancestor
of
T2.
The
ultimate
ancestor of a type is the ancestor of the type that is not a descendant
of any other type.
11
An inherited component (including
an inherited discriminant) of a derived type is inherited
from
a given ancestor of the type if the corresponding component was inherited
by each derived type in the chain of derivations going back to the given
ancestor.
12
18 Because operands of
a universal type are acceptable to the predefined operators of any type
in their class, ambiguity can result. For universal_integer and
universal_real, this potential ambiguity is resolved by giving
a preference (see 8.6) to the predefined operators
of the corresponding root types (root_integer and root_real,
respectively). Hence, in an apparently ambiguous expression like
13
1 + 4 < 7
14
where each of the literals is of type
universal_integer, the predefined operators of root_integer
will be preferred over those of other specific integer types, thereby
resolving the ambiguity.
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