7.3.2 Type Invariants
For a private type,
private extension, or interface, the following language-defined aspects
may be specified with an
aspect_specification
(see
13.1.1):
Type_Invariant
This aspect shall be specified by an
expression,
called an
invariant expression.
Type_Invariant
may be specified on a
private_type_declaration,
on a
private_extension_declaration,
or on a
full_type_declaration
that declares the completion of a private type or private extension.
Type_Invariant'Class
This aspect shall be specified by an
expression,
called an
invariant expression. Type_Invariant'Class may be specified
on a
private_type_declaration,
a
private_extension_declaration,
or a
full_type_declaration
for an interface type.
Type_Invariant'Class
determines a
class-wide type invariant for a tagged type.
Name Resolution Rules
The expected type for an invariant expression is
any boolean type.
Within an invariant expression, the identifier of
the first subtype of the associated type denotes the current instance
of the type. Within an invariant expression for the Type_Invariant aspect
of a type T, the type of this current instance is T. Within
an invariant expression for the Type_Invariant'Class aspect of a type
T, the type of this current instance is interpreted as though
it had a (notional) type NT that is a visible formal derived type
whose ancestor type is T. The effect of this interpretation is
that the only operations that can be applied to this current instance
are those defined for such a formal derived type.
Legality Rules
The Type_Invariant'Class aspect shall not be specified
for an untagged type. The Type_Invariant aspect shall not be specified
for an abstract type.
If a type extension occurs at a point where a private
operation of some ancestor is visible and inherited, and a Type_Invariant'Class
expression applies to that ancestor, then the inherited operation shall
be abstract or shall be overridden.
Static Semantics
If the Type_Invariant aspect is specified for a type
T, then the invariant expression applies to T.
If the Type_Invariant'Class aspect is specified for
a tagged type T, then the invariant expression applies to all
descendants of T.
Dynamic Semantics
If one or more invariant
expressions apply to a nonabstract type
T, then an invariant check
is performed at the following places, on the specified object(s):
After successful initialization of an object of
type
T by default (see
3.3.1), the
check is performed on the new object unless the partial view of
T
has unknown discriminants;
After successful explicit initialization of the
completion of a deferred constant with a part of type T, if the
completion is inside the immediate scope of the full view of T,
and the deferred constant is visible outside the immediate scope of T,
the check is performed on the part(s) of type T;
After successful conversion to type T, the
check is performed on the result of the conversion;
For a view conversion, outside the immediate scope
of T, that converts from a descendant of T (including T
itself) to an ancestor of type T (other than T itself),
a check is performed on the part of the object that is of type T:
after assigning to the view conversion;
and
after successful return from a call
that passes the view conversion as an in out or out parameter.
After a successful call on the Read or Input stream-oriented
attribute of the type T, the check is performed on the object
initialized by the attribute;
An invariant is checked
upon successful return from a call on any subprogram or entry that:
is declared within the immediate scope
of type T (or by an instance of a generic unit, and the generic
is declared within the immediate scope of type T),
This paragraph
was deleted.
and either:
has a result with a part of type T,
or
has one or more out or in out
parameters with a part of type T, or
has an access-to-object parameter or result
whose designated type has a part of type T, or
is a procedure or entry that has an in
parameter with a part of type T,
and either:
T is a private type or a private
extension and the subprogram or entry is visible outside the immediate
scope of type T or overrides an inherited operation that is visible
outside the immediate scope of T, or
T is a record extension, and the
subprogram or entry is a primitive operation visible outside the immediate
scope of type T or overrides an inherited operation that is visible
outside the immediate scope of T.
The check is performed on each such part
of type T.
For a view conversion to a class-wide type occurring
within the immediate scope of T, from a specific type that is
a descendant of T (including T itself), a check is performed
on the part of the object that is of type T.
If performing checks is required by the Type_Invariant
or Type_Invariant'Class assertion policies (see
11.4.2)
in effect at the point of the corresponding aspect specification applicable
to a given type, then the respective invariant expression is considered
enabled.
The invariant check consists of the evaluation of
each enabled invariant expression that applies to
T, on each of
the objects specified above. If any of these evaluate to False, Assertions.Assertion_Error
is raised at the point of the object initialization, conversion, or call.
If a given call requires more than one evaluation of an invariant expression,
either for multiple objects of a single type or for multiple types with
invariants, the evaluations are performed in an arbitrary order, and
if one of them evaluates to False, it is not specified whether the others
are evaluated. Any invariant check is performed prior to copying back
any by-copy
in out or
out parameters. Invariant checks,
any postcondition check, and any constraint or predicate checks associated
with
in out or
out parameters are performed in an arbitrary
order.
For an invariant check on a value of type T1
based on a class-wide invariant expression inherited from an ancestor
type T, any operations within the invariant expression that were
resolved as primitive operations of the (notional) formal derived type
NT are bound to the corresponding operations of type T1
in the evaluation of the invariant expression for the check on T1.
The invariant checks performed on a call are determined
by the subprogram or entry actually invoked, whether directly, as part
of a dispatching call, or as part of a call through an access-to-subprogram
value.
13 For a call of a primitive subprogram
of type NT that is inherited from type T, the specified
checks of the specific invariants of both the types NT and T
are performed. For a call of a primitive subprogram of type NT
that is overridden for type NT, the specified checks of the specific
invariants of only type NT are performed.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
