12.5 Formal Types
[A generic formal subtype can be used to pass to a generic unit a subtype
whose type is in a certain category class
We considered having intermediate
syntactic categories formal_integer_type_definition
, and formal_fixed_point_definition
to be more uniform with the syntax rules for non-generic-formal types.
However, that would make the rules for formal types slightly more complicated,
and it would cause confusion, since formal_discrete_type_definition
would not fit into the scheme very well.
a generic formal subtype, the actual shall be a subtype_mark
it denotes the (generic) actual subtype
When we say simply “formal”
or “actual” (for a generic formal that denotes a subtype)
we're talking about the subtype, not the type, since a name that denotes
denotes a subtype, and the corresponding actual also denotes a subtype.
Ramification: A subtype (other than the
first subtype) of a generic formal type is not a generic formal subtype.
This rule is clearer with the
flat syntax rule for formal_type_definition
given above. Adding formal_integer_type_definition
and others would make this rule harder to state clearly.
We use “category’ rather than “class”
above, because the requirement that classes are closed under derivation
is not important here. Moreover, there are interesting categories that
are not closed under derivation. For instance, limited and interface
are categories that do not form classes.
The actual type shall be in the category class
determined for the formal.
For example, if the category class
determined for the formal is the category class
of all discrete types, then the actual has to be discrete.
Note that this rule does not require the actual to belong to every category class
to which the formal belongs. For example, formal private types are in
the category class
of composite types, but the actual need not be composite. Furthermore,
one can imagine an infinite number of categories classes
that are just arbitrary sets of types that obey
the closed-under-derivation rule, and are therefore technically classes
(even though we don't give them names, since they are uninteresting).
We don't want this rule to apply to those categories classes
“Limited” is not an a
“interesting” category class
but “nonlimited” is; it is legal to pass a nonlimited type
to a limited formal type, but not the other way around. The reserved
word limited limited
really represents a category class
containing both limited and nonlimited types. “Private” is
not a category for this purpose class
a generic formal private type accepts both private and nonprivate actual
It is legal to pass a class-wide subtype as the actual if it is in the
right category class
so long as the formal has unknown discriminants.
[The formal type also belongs to each category class
that contains the determined category class
The primitive subprograms of the type are as for any type in the determined
For a formal type other than a formal derived type, these are the predefined
operators of the type. For an elementary formal
type, the predefined operators are implicitly declared immediately after
the declaration of the formal type. For a composite formal type, the
predefined operators are implicitly declared either immediately after
the declaration of the formal type, or later immediately
within the declarative region in which the type is declared in
its immediate scope according to
the rules of 7.3.1.;
they are implicitly declared immediately after the declaration of the
In an instance, the copy of such an implicit declaration
declares a view of the predefined operator of the actual type, even if
this operator has been overridden for the actual type and even if it is never declared for the actual type
. [The rules
specific to formal derived types are given in 12.5.1
All properties of the type are as for any type in the category class
Some examples: The primitive operations available are as defined by the
language for each category class
The form of constraint
applicable to a formal type in a subtype_indication
depends on the category class
of the type as for a nonformal type. The formal type is tagged if and
only if it is declared as a tagged private type, or as a type derived
from a (visibly) tagged type. (Note that the actual type might be tagged
even if the formal type is not.)
The somewhat cryptic phrase “even if it is
never declared” is intended to deal with the following oddity:
package Q is
type T is limited private;
type T is range 1 .. 10;
type A is array (Positive range <>) of T;
package Q.G is
A1, A2 : A (1 .. 1);
B : Boolean := A1 = A2;
package R is
type C is array (Positive range <>) of Q.T;
package I is new Q.G (C); -- Where is the predefined "=" for C?
An "=" is available
for the formal type A in the private part of Q.G. However, no "="
operator is ever declared for type C, because its component type Q.T
is limited. Still, in the instance I the name "=" declares
a view of the "=" for C which exists-but-is-never-declared.
7 Generic formal types, like all types,
are not named. Instead, a name
can denote a generic formal subtype. Within a generic unit, a generic
formal type is considered as being distinct from all other (formal or
8 A discriminant_part
is allowed only for certain kinds of types, and therefore only for certain
kinds of generic formal types. See 3.7
Examples of generic
type Item is private;
type Buffer(Length : Natural) is limited private;
type Enum is (<>);
type Int is range <>;
type Angle is delta <>;
type Mass is digits <>;
type Table is array (Enum) of Item;
Example of a
generic formal part declaring a formal integer type:
type Rank is range <>;
First : Rank := Rank'First;
Second : Rank := First + 1; -- the operator "+" of the type Rank
Wording Changes from Ada 83
RM83 has separate sections “Generic Formal
Xs” and “Matching Rules for Formal Xs” (for various
X's) with most of the text redundant between the two. We have combined
the two in order to reduce the redundancy. In RM83, there is no “Matching
Rules for Formal Types” section; nor is there a “Generic
Formal Y Types” section (for Y = Private, Scalar, Array, and Access).
This causes, for example, the duplication across all the “Matching
Rules for Y Types” sections of the rule that the actual passed
to a formal type shall be a subtype; the new organization avoids that
The matching rules are stated more concisely.
We no longer consider the multiplying operators
that deliver a result of type universal_fixed to be predefined
for the various types; there is only one of each in package Standard.
Therefore, we need not mention them here as RM83 had to.
Wording Changes from Ada 95
Corrigendum 1 corrected the wording to properly
define the location where operators are defined for formal array types.
The wording here was inconsistent with that in 7.3.1,
“Private Operations”. For the
Amendment, this wording was corrected again, because it didn't reflect
the Corrigendum 1 revisions in 7.3.1.
We use “determines a category” rather
than class, since not all interesting properties form a class.
Extensions to Ada 2005
Wording Changes from Ada 2005
Correction: Updated the wording to acknowledge
the possibility of operations that are never declared for an actual type
but still can be used inside of a generic unit.
Formal incomplete types are added; these are documented
as an extension in the next clause.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe