3.7.2 Operations of Discriminated Types
[If a discriminated type has
default_expressions
for its discriminants, then unconstrained variables of the type are permitted,
and the discriminants of such a variable can be changed by assignment
to the variable. For a formal parameter of such a type, an attribute
is provided to determine whether the corresponding actual parameter is
constrained or unconstrained.]
Static Semantics
For a
prefix
A that is of a discriminated type [(after any implicit dereference)],
the following attribute is defined:
A'Constrained
Yields the value True if A denotes
a constant, a value, or a constrained variable, and False otherwise.
Implementation Note: This attribute is
primarily used on parameters, to determine whether the discriminants
can be changed as part of an assignment. The Constrained attribute is
statically True for in parameters. For in out and out
parameters of a discriminated type, the value of this attribute needs
to be passed as an implicit parameter, in general. However, if the type
does not have defaults for its discriminants, the attribute is statically
True, so no implicit parameter is needed. Parameters of a limited type
with defaulted discriminants need this implicit parameter, unless there
are no nonlimited views, because they might be passed to a subprogram
whose body has visibility on a nonlimited view of the type, and hence
might be able to assign to the object and change its discriminants.
Erroneous Execution
{erroneous execution
(cause) [partial]} The execution of a
construct is erroneous if the construct has a constituent that is a
name
denoting a subcomponent that depends on discriminants, and the value
of any of these discriminants is changed by this execution between evaluating
the
name and
the last use (within this execution) of the subcomponent denoted by the
name.
Ramification: This rule applies to
assignment_statements,
calls (except when the discriminant-dependent subcomponent is an
in
parameter passed by copy),
indexed_components,
and
slices.
Ada 83 only covered the first two cases. AI83-00585 pointed out the situation
with the last two cases. The cases of
object_renaming_declarations
and generic formal
in out objects are handled differently, by
disallowing the situation at compile time.
Extensions to Ada 83
{
extensions to Ada 83}
For
consistency with other attributes, we are allowing the
prefix
of Constrained to be a value as well as an object of a discriminated
type, and also an implicit dereference. These extensions are not important
capabilities, but there seems no reason to make this attribute different
from other similar attributes. We are curious what most Ada 83 compilers
do with F(1).X'Constrained.
We now handle in a general way the cases of
erroneousness identified by AI83-00585, where the
prefix
of an
indexed_component
or
slice
is discriminant-dependent, and the evaluation of the index or discrete
range changes the value of a discriminant.
Wording Changes from Ada 83
We have moved all discussion of erroneous use
of
names that
denote discriminant-dependent subcomponents to this subclause. In Ada
83, it used to appear separately under
assignment_statements
and subprogram calls.
Sponsored by Ada-Europe