1

In addition to the various language-defined classes
of types, types can be grouped into *derivation classes*.

2/2

{*AI95-00251-01*}
{*AI95-00401-01*}
{*derived from (directly or indirectly)*}
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. {*derivation class
(for a type)*} {*root
type (of a class)*} {*rooted
at a type*} 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).

2.a

2.b

3/2

{*AI95-00230-01*}
Every type is either a *specific* type, a *class-wide* type,
or a *universal* type. {*specific
type*} A specific type is one defined by
a type_declaration,
a formal_type_declaration,
or a full type definition embedded in another construct 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:

3.a

4

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

5

{*first subtype*}
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.

5.a

6/2

Universal types are defined for [(and belong to)] the integer, real, and fixed point, and access classes, and are referred to in this standard as respectively,

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.

7.a

7.b/2

{*AI95-00230-01*}
We considered defining class-wide or perhaps universal types for all
derivation classes, not just tagged classes and these four
elementary three numeric classes.
However, this was felt to overly weaken the strong-typing model in some
situations. Tagged types preserve strong type distinctions thanks to
the run-time tag. Class-wide or universal types for untagged types would
weaken the compile-time type distinctions without providing a compensating
run-time-checkable distinction.

7.c

We considered defining standard names for the
universal numeric types so they could be used in formal parameter specifications.
However, this was felt to impose an undue implementation burden for some
implementations.

7.d

8

{*root_integer*
[partial]} {*root_real*
[partial]} 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

{*cover (a type)*}
A class-wide or universal type is said to *cover*
all of the types in its class. A specific type covers only itself.

10/2

{*AI95-00230-01*}
{*AI95-00251-01*}
{*descendant (of a type)*} 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.
{*ancestor (of a type)*} If
a type *T2* is a descendant of a type *T1*, then *T1*
is called an *ancestor* of *T2*. {*ultimate
ancestor (of a type)*} {*ancestor
(ultimate)*} An The
*ultimate ancestor* of a type is an the
ancestor of that the
type that is not itself a descendant of
any other type. Every untagged type has a unique
ultimate ancestor.

10.a

10.b

A specific type is an ancestor of itself. The
root of a derivation class is an ancestor of all types in the class,
including any class-wide types in the class.

10.c

10.d/2

- {
*AI95-00251-01*} Each type has at most one parent, and one or more ancestor types; each untagged type has exactly one ultimate ancestor. In Ada 83, the term “parent type” was sometimes used more generally to include any ancestor type (e.g. RM83-9.4(14)). In Ada 95, we restrict parent to mean the immediate ancestor.

10.e

- A class of types has at most one root type; a derivation class has exactly one root type.

10.f

- The root of a class is an ancestor of all of the types in the class (including itself).

10.g

- The type
*root_integer*is the root of the integer class, and is the ultimate ancestor of all integer types. A similar statement applies to*root_real*.

10.h/2

10.i/2

11

{*inherited (from
an ancestor type)*} 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.

NOTES

12

20 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.

14.a

14.b/2

{*AI95-00230-01*}
Updated the wording to define the *universal_access*
type. This was defined to make **null** for anonymous access types
sensible.

14.c/2

{*AI95-00251-01*}
{*AI95-00401-01*}
The definitions of ancestors and descendants were
updated to allow multiple ancestors (necessary to support interfaces).

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