3.4.1 Derivation Classes
In addition to the various language-defined classes
of types, types can be grouped into derivation classes.
Static Semantics
A derived type is
derived
from its parent type
directly; it is derived
indirectly
from any type from which its parent type is derived. A derived type,
interface type, type extension, task type, protected type, or formal
derived type is also derived from every ancestor of each of its progenitor
types, if any.
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).
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 another construct. Class-wide and
universal types are implicitly defined, to act as representatives for
an entire class of types, as follows:
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).
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.
Universal types are defined for (and belong to) the integer, real, fixed
point, and access classes, and are referred to in this standard as respectively,
universal_integer,
universal_real,
universal_fixed,
and
universal_access.
These are analogous to class-wide types for these language-defined elementary
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,
or the literal
null) is “universal” in that it is
acceptable where some particular type in the class is expected (see
8.6).
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.
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.
A class-wide or universal type
is said to
cover all of the types in its class. A specific type
covers only itself.
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 numeric 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.
An
ultimate
ancestor of a type is an ancestor of that type that is not itself
a descendant of any other type. Every untagged type has a unique ultimate
ancestor.
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.
26 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
1 + 4 < 7
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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