3.7.1 Discriminant Constraints
Language Design Principles
The rules in this clause are intentionally parallel
to those given in Record Aggregates.
Syntax
{named discriminant
association} A
discriminant_association
is said to be
named if it has one or more
discriminant_selector_names;
{positional discriminant association}
it is otherwise said to be
positional. In
a
discriminant_constraint,
any positional associations shall precede any named associations.
Name Resolution Rules
Each
selector_name
of a named
discriminant_association
shall resolve to denote a discriminant of the subtype being constrained;
{associated discriminants (of a named
discriminant_association)} the discriminants
so named are the
associated discriminants of the named association.
{associated discriminants (of a positional
discriminant_association)} For a positional
association, the
associated discriminant is the one whose
discriminant_specification
occurred in the corresponding position in the
known_discriminant_part
that defined the discriminants of the subtype being constrained.
{expected type (discriminant_association
expression) [partial]} The expected type
for the
expression
in a
discriminant_association
is that of the associated discriminant(s).
Legality Rules
{
8652/0008}
{
AI95-00168-01}
{
AI95-00363-01}
A
discriminant_constraint
is only allowed in a
subtype_indication
whose
subtype_mark
denotes either an unconstrained discriminated subtype, or an unconstrained
access subtype whose designated subtype is an unconstrained discriminated
subtype. However, in the case of an access subtype, a
discriminant_constraint
is illegal if the designated type has a partial view that is constrained
or, for a general access subtype, has
default_expressions
for its discriminants. In addition to the places where Legality Rules
normally apply (see
12.3), these rules apply
also in the private part of an instance of a generic unit. In a generic
body, this rule is checked presuming all formal access types of the generic
might be general access types, and all untagged discriminated formal
types of the generic might have
default_expressions
for their discriminants.
{generic contract
issue [partial]} Reason: {
AI95-00363-01}
The second rule is necessary to prevent objects from changing so that
they no longer match their constraint. In Ada 95, we attempted to prevent
this by banning every case where an aliased object could be unconstrained
or be changed by an enclosing assignment. New ways to cause this problem
were being discovered frequently, meaning that new rules had to be dreamed
up to cover them. Meanwhile, aliased objects and components were getting
more and more limited. In Ada 2005, we sweep away all of that cruft and
replace it by a simple rule “thou shalt not create an access subtype
that can point to an item whose discriminants can be changed by assignment”.
The
expression
associated with an access discriminant shall be of a type convertible
to the anonymous access type.
{convertible
(required) [partial]} Ramification: This implies both convertibility
of designated types, and static accessibility. This implies that if an
object of type T with an access discriminant is created by an allocator
for an access type A, then it requires that the type of the
expression
associated with the access discriminant have an accessibility level that
is not statically deeper than that of A. This is to avoid dangling references.
Dynamic Semantics
{compatibility (discriminant
constraint with a subtype) [partial]} A
discriminant_constraint
is
compatible with an unconstrained discriminated subtype if each
discriminant value belongs to the subtype of the corresponding discriminant.
Ramification: The "dependent compatibility
check" has been eliminated in Ada 95. Any checking on subcomponents
is performed when (and if) an object is created.
Discussion: There is no need to define
compatibility with a constrained discriminated subtype, because one is
not allowed to constrain it again.
{satisfies (a discriminant
constraint) [partial]} A composite value
satisfies a discriminant constraint if and only if each discriminant
of the composite value has the value imposed by the discriminant constraint.
{elaboration (discriminant_constraint)
[partial]} For the elaboration of a
discriminant_constraint,
the
expressions
in the
discriminant_associations
are evaluated in an arbitrary order and converted to the type of the
associated discriminant (which might raise Constraint_Error — see
4.6); the
expression
of a named association is evaluated (and converted) once for each associated
discriminant.
{implicit subtype conversion
(discriminant values) [partial]} The result
of each evaluation and conversion is the value imposed by the constraint
for the associated discriminant.
Reason: We convert to the type, not the
subtype, so that the definition of compatibility of discriminant constraints
is not vacuous.
56 The rules of the language ensure that
a discriminant of an object always has a value, either from explicit
or implicit initialization.
Discussion: Although it is illegal to
constrain a class-wide tagged subtype, it is possible to have a partially
constrained class-wide subtype: If the subtype S is defined by T(A =>
B), then S'Class is partially constrained in the sense that objects of
subtype S'Class have to have discriminants corresponding to A equal to
B, but there can be other discriminants defined in extensions that are
not constrained to any particular value.
Examples
Examples (using
types declared above in clause 3.7):
Large : Buffer(200); -- constrained, always 200 characters
-- (explicit discriminant value)
Message : Buffer; -- unconstrained, initially 100 characters
-- (default discriminant value)
Basis : Square(5); -- constrained, always 5 by 5
Illegal : Square; -- illegal, a Square has to be constrained
Inconsistencies With Ada 83
{
inconsistencies with Ada 83}
Dependent
compatibility checks are no longer performed on subtype declaration.
Instead they are deferred until object creation (see
3.3.1).
This is upward compatible for a program that does not raise Constraint_Error.
Wording Changes from Ada 83
Everything in RM83-3.7.2(7-12), which specifies
the initial values for discriminants, is now redundant with 3.3.1, 6.4.1,
8.5.1, and 12.4. Therefore, we don't repeat it here. Since the material
is largely intuitive, but nevertheless complicated to state formally,
it doesn't seem worth putting it in a "NOTE."
Incompatibilities With Ada 95
{
8652/0008}
{
AI95-00168-01}
{
AI95-00363-01}
{
incompatibilities with Ada 95}
The Corrigendum
added a restriction on
discriminant_constraints
for general access subtypes. Such constraints are prohibited if the designated
type can be treated as constrained somewhere in the program. Ada 2005
goes further and prohibits such
discriminant_constraints
if the designated type has (or might have, in the case of a formal type)
defaults for its discriminants. The use of general access subtypes is
rare, and this eliminates a boatload of problems that required many restrictions
on the use of aliased objects and components (now lifted). Similarly,
Ada 2005 prohibits
discriminant_constraints
on any access type whose designated type has a partial view that is constrained.
Such a type will not be constrained in the heap to avoid privacy problems.
Again, the use of such subtypes is rare (they can only happen within
the package and its child units).
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