Ada Reference Manual (Ada 2022)

7.3.2 Type Invariants

For a private type, private extension, or interface, the following language-defined assertion 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. The Type_Invariant'Class aspect is not inherited, but its effects are additive, as defined below.

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) nonabstract 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 immediately within the visible part of a package specification, 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 a corresponding expression also applies to each nonabstract descendant T1 of T (including T itself if it is nonabstract). The corresponding expression is constructed from the associated expression as follows:

References to nondiscriminant components of T (or to T itself) are replaced with references to the corresponding components of T1 (or to T1 as a whole).

References to discriminants of T are replaced with references to the corresponding discriminant of T1, or to the specified value for the discriminant, if the discriminant is specified by the derived_type_definition for some type that is an ancestor of T1 and a descendant of T (see 3.7).

For a nonabstract type T, a callable entity is said to be a boundary entity for T if it is declared within the immediate scope of T (or by an instance of a generic unit, and the generic is declared within the immediate scope of type T), or is the Read or Input stream-oriented attribute of type T, and either:

T is a private type or a private extension and the callable entity 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 callable entity 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.

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 whose nominal type has 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.

Upon successful return from a call on any callable entity which is a boundary entity for T, an invariant check is performed on each object which is subject to an invariant check for T. In the case of a call to a protected operation, the check is performed before the end of the protected action. In the case of a call to a task entry, the check is performed before the end of the rendezvous. The following objects of a callable entity are subject to an invariant check for T:

Paragraph 16 was merged above.

a result with a nominal type that has a part of type T;

an out or in out parameter whose nominal type has a part of type T;

an access-to-object parameter or result whose designated nominal type has a part of type T; or

for a procedure or entry, an in parameter whose nominal type has a part of type T.

If the nominal type of a formal parameter (or the designated nominal type of an access-to-object parameter or result) is incomplete at the point of the declaration of the callable entity, and if the completion of that incomplete type does not occur in the same declaration list as the incomplete declaration, then for purposes of the preceding rules the nominal type is considered to have no parts 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.

NOTE 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.

Examples

Example of a work scheduler where only urgent work can be scheduled for weekends:

package Work_Orders is

-- See 3.5.1 for type declarations of Level, Day, and Weekday

type Work_Order is private with
Type_Invariant => Day_Scheduled (Work_Order) in Weekday
or else Priority (Work_Order) = Urgent;

function Schedule_Work (Urgency : in Level;
To_Occur : in Day) return Work_Order
with Pre => Urgency = Urgent or else To_Occur in Weekday;

function Day_Scheduled (Order : in Work_Order) return Day;

function Priority (Order : in Work_Order) return Level;

procedure Change_Priority (Order : in out Work_Order;
New_Priority : in Level;
Changed : out Boolean)
with Post => Changed = (Day_Scheduled(Order) in Weekday
or else Priority(Order) = Urgent);

private

type Work_Order is record
Scheduled : Day;
Urgency : Level;
end record;

end Work_Orders;

package body Work_Orders is

function Schedule_Work (Urgency : in Level;
To_Occur : in Day) return Work_Order is
(Scheduled => To_Occur, Urgency => Urgency);

function Day_Scheduled (Order : in Work_Order) return Day is
(Order.Scheduled);

function Priority (Order : in Work_Order) return Level is
(Order.Urgency);

procedure Change_Priority (Order : in out Work_Order;
New_Priority : in Level;
Changed : out Boolean) is
begin
-- Ensure type invariant is not violated
if Order.Urgency = Urgent or else (Order.Scheduled in Weekday) then
Changed := True;
Order.Urgency := New_Priority;
else
Changed := False;
end if;
end Change_Priority;

end Work_Orders;