12.5.1 Formal Private and Derived Types
In its most general form, the category determined
for a formal private type is all types, but it can be restricted to only
nonlimited types or to only tagged types. The category determined for
a formal derived type is the derivation class rooted at the ancestor
of a formal derived type is the subtype denoted by the subtype_mark
of the formal_derived_type_definition
For a formal derived type declaration, the reserved words with private
shall appear if and only if the ancestor type is a tagged type; in this
case the formal derived type is a private extension of the ancestor type
and the ancestor shall not be a class-wide type. Similarly, an interface_list
or the optional reserved words abstract
shall appear only if the ancestor type is a tagged type. The reserved
shall appear only if the ancestor
type and any progenitor types are limited types. The reserved word synchronized
shall appear (rather than limited
) if the ancestor type or any
of the progenitor types are synchronized interfaces.
The actual type for a formal derived type shall
be a descendant of the ancestor type and every progenitor of the formal
type. If the reserved word synchronized appears in the declaration
of the formal derived type, the actual type shall be a synchronized tagged
If the formal subtype is definite, then the actual
subtype shall also be definite.
- If the ancestor subtype is constrained,
the actual subtype shall be constrained, and shall be statically compatible
with the ancestor;
- If the ancestor subtype is an unconstrained
access or composite subtype, the actual subtype shall be unconstrained.
- If the ancestor subtype is an unconstrained
discriminated subtype, then the actual shall have the same number of
discriminants, and each discriminant of the actual shall correspond to
a discriminant of the ancestor, in the sense of 3.7.
- If the ancestor subtype is an access
subtype, the actual subtype shall exclude null if and only if the ancestor
subtype excludes null.
- The actual type shall be a type with
the same number of discriminants.
- The actual subtype shall be unconstrained.
- The subtype of each discriminant of
the actual type shall statically match the subtype of the corresponding
discriminant of the formal type.
For a generic formal type with an unknown_discriminant_part
the actual may, but need not, have discriminants, and may be definite
The category determined
for a formal private type is as follows:
Type Definition Determined Category
limited private the category of all types
private the category of all nonlimited types
tagged limited private the category of all tagged types
tagged private the category of all nonlimited tagged types
The presence of the reserved word abstract
determines whether the actual type may be abstract.
A formal private or derived type is a private or
derived type, respectively. A formal derived tagged type is a private
extension. A formal private or derived type is abstract if the reserved
word abstract appears in its declaration.
If the ancestor type is a composite type that is
not an array type, the formal type inherits components from the ancestor
type (including discriminants if a new discriminant_part
is not specified), as for a derived type defined by a derived_type_definition
For a formal derived type, the predefined operators
and inherited user-defined subprograms are determined by the ancestor
type and any progenitor types, and are implicitly declared at the earliest
place, if any, immediately within the declarative region in which the
formal type is declared, where the corresponding primitive subprogram
of the ancestor or progenitor is visible (see 7.3.1
In an instance, the copy of such an implicit declaration declares a view
of the corresponding primitive subprogram of the ancestor or progenitor
of the formal derived type, even if this primitive has been overridden
for the actual type. When the ancestor or progenitor of the formal derived
type is itself a formal type, the copy of the implicit declaration declares
a view of the corresponding copied operation of the ancestor or progenitor.
In the case of a formal private extension, however, the tag of the formal
type is that of the actual type, so if the tag in a call is statically
determined to be that of the formal type, the body executed will be that
corresponding to the actual type.
For a prefix
S that denotes a formal indefinite subtype, the following attribute is
S'Definite yields True if the
actual subtype corresponding to S is definite; otherwise it yields False.
The value of this attribute is of the predefined type Boolean.
In the case where
a formal type is tagged with unknown discriminants, and the actual type
is a class-wide type T'Class:
- For the purposes of defining the primitive
operations of the formal type, each of the primitive operations of the
actual type is considered to be a subprogram (with an intrinsic calling
convention — see 6.3.1) whose body
consists of a dispatching call upon the corresponding operation of T,
with its formal parameters as the actual parameters. If it is a function,
the result of the dispatching call is returned.
- If the corresponding operation of
T has no controlling formal parameters, then the controlling tag
value is determined by the context of the call, according to the rules
for tag-indeterminate calls (see 3.9.2 and
5.2). In the case where the tag would be statically
determined to be that of the formal type, the call raises Program_Error.
If such a function is renamed, any call on the renaming raises Program_Error.
9 In accordance
with the general rule that the actual type shall belong to the category
determined for the formal (see 12.5
- If the formal type is nonlimited,
then so shall be the actual;
- For a formal derived type, the
actual shall be in the class rooted at the ancestor subtype.
10 The actual type can be abstract only
if the formal type is abstract (see 3.9.3
11 If the formal has a discriminant_part
the actual can be either definite or indefinite. Otherwise, the actual
has to be definite.