7.3.2 Type Invariants
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.
Aspect Description for Type_Invariant:
A condition that will hold true for all objects of a type.
This aspect shall be specified by an
expression,
called an
invariant expression. Type_Invariant'Class may be specified
on a
private_type_declaration, or 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.]Reason: {
AI05-0254-1}
A class-wide type invariant cannot be hidden in the private part, as
the creator of an extension needs to know about it in order to conform
to it in any new or overriding operations. On the other hand, a specific
type invariant is not inherited, so that no operation outside of the
original package needs to conform to it; thus there is no need for it
to be visible.
Aspect Description for Type_Invariant'Class:
A condition that will hold true for all objects in a class of types.
Term entry: invariant
— assertion that is expected to be True for all objects of a given
private type when viewed from outside the defining package
Name Resolution Rules
{
AI05-0146-1}
The expected type for an invariant expression is any boolean type.
{
AI05-0146-1}
{
AI12-0150-1}
{
AI12-0159-1}
{
AI12-0199-1}
[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 associated with type
T, the type of
this the
current instance is
T. Within an invariant
expression for the Type_Invariant aspect
and T'Class 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.].Proof: The first sentence is given formally
in
13.1.1.
Reason: {
AI12-0159-1}
The rules for Type_Invariant'Class ensure that
the invariant expression is well-defined for any type descended from
T.
Legality Rules
{
AI05-0146-1}
[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.
Proof: The first sentence is given formally
in
13.1.1.
{
AI12-0042-1}
{
AI12-0382-1}
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
{
AI05-0250-1}
[If the Type_Invariant aspect is specified for a type
T, then
the invariant expression applies to
T.]
{
AI05-0146-1}
{
AI12-0199-1}
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: the
invariant expression applies to all descendants of T.{
AI12-0199-1}
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).Ramification: {
AI12-0199-1}
The only nondiscriminant components visible at
the point of such an aspect specification are necessarily inherited from
some nonprivate ancestor.
{
AI12-0199-1}
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). This paragraph
was deleted.Proof: "Applies"
is formally defined in 13.1.1.
Discussion: The
associated expression from which the corresponding expression is constructed
is the one that applies to the descendant type; "applies" is
formally defined in 13.1.1.
{
AI12-0075-1}
{
AI12-0191-1}
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.
Reason: {
AI12-0191-1}
A boundary entity for type T is one that
might require an invariant check for T. It includes subprograms
that don't (visibly) involve T; since we don't want to break privacy,
we can't statically know if some private type has some part of T.
We'll reduce the set when we describe the actual checks.
Dynamic Semantics
{
AI12-0133-1}
After successful
default 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;
Reason: {
AI12-0133-1}
The check applies everywhere, even in the package
body, because default initialization has to work the same for clients
as it does within the package. As such, checks within the package are
either harmless or will uncover a bug that could also happen to a client.
However, if the partial view of the type has unknown discriminants, no
client of the package can declare a default-initialized object. Therefore,
no invariant check is needed, as all default initialized objects are
necessarily inside the package.
{
AI12-0049-1}
{
AI12-0191-1}
After successful explicit initialization of the
completion of a deferred constant whose
nominal type has 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;
{
AI05-0146-1}
{
AI05-0269-1}
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.
Ramification: For a single view conversion
that converts between distantly related types, this rule could be triggered
for multiple types and thus multiple invariant checks may be needed.
Implementation Note: {
AI05-0299-1}
For calls to inherited subprograms (including dispatching calls), the
implied view conversions mean that a wrapper is probably needed. (See
the Note at the bottom of this subclause for more on the model of checks
for inherited subprograms.)
For view conversions involving class-wide types,
the exact checks needed may not be known at compile-time. One way to
deal with this is to have an implicit dispatching operation that is given
the object to check and the tag of the target of the conversion, and
which first checks if the passed tag is not for itself, and if not, checks
the its invariant on the object and then calls the operation of its parent
type. If the tag is for itself, the operation is complete.
{
AI12-0146-1}
{
AI12-0075-1}
{
AI12-0191-1}
{
AI12-0193-1}
Upon After
a successful
return from a call on
any callable entity which is a boundary entity
for T, an invariant the Read or Input
stream-oriented stream attribute of the type T, the 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
object initialized by the stream attribute.
The following objects of a callable entity are subject to an invariant
check for T:;Paragraph
16 was merged above.
{
AI12-0042-1}
{
AI12-0191-1}
an out or in out parameter whose
nominal type has a part of type T; is
visible outside the immediate scope of type T or overrides an
operation that is visible outside the immediate scope of T, and{
AI12-0075-1}
{
AI12-0191-1}
an access-to-object parameter or result whose designated
nominal type has a part of type T; or{
AI05-0289-1}
{
AI12-0042-1}
{
AI12-0044-1}
{
AI12-0075-1}
{
AI12-0191-1}
for a procedure or entry, an in parameter
whose nominal type has a part of type T. and
either: has
a result with a part of type T, or one or more parameters with
a part of type T, or an access to variable parameter whose designated
type has a part of type T.Ramification: {
AI12-0167-1}
{
AI12-0191-1}
This is a Dynamic Semantics rule, so we ignore
privacy when determining if a check is needed. We do, however, use the
nominal type of objects to determine if a part of type T is present;
therefore, parts that aren't known at compile-time (after ignoring privacy)
are never subject to an invariant check. This is preferable, as we don't
want overhead associated with the possibility that there might
exist an extension of a tagged type that has a part of type T.
(See the "leaks" note below for avoidance advice.)
Discussion: We
don't check in parameters for functions to avoid infinite recursion
for calls to public functions appearing in invariant expressions. Such
function calls are unavoidable for class-wide invariants and likely for
other invariants. This is the simplest rule that avoids trouble, and
functions are much more likely to be queries that don't modify their
parameters than other callable entities.
{
AI12-0075-1}
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{
AI12-0075-1}
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.
{
AI05-0146-1}
{
AI05-0269-1}
{
AI12-0075-1}
{
AI12-0338-1}
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 The
check is performed on each such part of type
T.
{
AI12-0042-1}
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.Reason: Class-wide
objects are treated as though they exist outside the scope of every type,
and may be passed across package "boundaries" freely without
further invariant checks.
{
AI12-0167-1}
Despite this model, if an object of type T
that is a component of a class-wide object is modified within the scope
of the full view of type T, then there is no invariant check for
T at that point. {
AI05-0290-1}
{
AI12-0080-1}
{
AI12-0159-1}
If performing checks is required by the
Type_Invariant Invariant
or
Type_Invariant'Class 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.
Ramification: If a class-wide invariant
expression is enabled for a type, it remains enabled when inherited by
descendants of that type, even if the policy in effect is Ignore for
the inheriting type.
{
AI05-0146-1}
{
AI05-0250-1}
{
AI05-0289-1}
{
AI05-0290-1}
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.
{
AI12-0150-1}
{
AI12-0159-1}
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.{
AI05-0146-1}
{
AI05-0247-1}
{
AI05-0250-1}
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.
Ramification: {
AI12-0149-1}
{
AI12-0167-1}
{
AI12-0210-1}
Type invariant checks are intended to prevent invariant-violating
values from inadvertently "leaking out"; that is, code which
cannot see the completion of the private type should not be able to reference
invariant-violating values of the type (assuming the type invariant condition
itself is well behaved - for example, no uses of global variables during
evaluation of the invariant expression). This goal is not completely
achieved; such leaking is possible but, importantly, it requires assistance
(deliberate or not) of some form from the package that declares the invariant-bearing
private type (or a child unit thereof); a client of a well-crafted package
cannot use these holes to obtain an invariant-violating value without
help. Invariant checks on subprogram return
are not performed on objects that are accessible only through access
values that are subcomponents
of some other object. It is also
possible to call through an access-to-subprogram value and reach a subprogram
body that has visibility on the full declaration of a type, from outside
the immediate scope of the type. No invariant checks will be performed
if the designated subprogram is not itself externally visible. These
cases represent "holes" in the protection provided by invariant
checks; but note that these holes cannot be caused by clients of the
type T with the invariant.
The designer of the package has to declare a visible type with an access-to-T
subcomponent and use it as a parameter or result to subprograms in the
package, or pass the client an access-to-subprogram value representing
a private operation of the package. In the absence of such things, all
values that the client can see will be checked for a private type or
extension without help for the designer of the package containing T.
{
AI12-0210-1}
The list of known techniques whereby this kind
of leak can occur (ignoring things like erroneous execution and the various
forms of unchecked type conversion) consists of:A boundary entity might
assign an invariant-violating value to a global variable that is visible
to client code.
{
AI12-0149-1}
Invariant checks on subprogram return are not performed
on objects that are accessible only through access-valued components
of other objects. This can only cause a leak if there is a type with
access-valued components that is used as a parameter or result type of
a boundary entity. For a type T that has a type invariant, avoiding
the declaration of types with access-valued components designating objects
with parts of T in the package that contains T is sufficient
to prevent this leak.A client could call
through an access-to-subprogram value and reach a subprogram body that
has visibility on the full declaration of a type; no invariant checks
will be performed if the designated subprogram is not itself a boundary
subprogram. This leak can only happen if an access-to-subprogram value
of a subprogram that is not visible to clients is passed out to clients.
{
AI12-0167-1}
{
AI12-0191-1}
Invariant checks are only performed for parts of
the nominal type for tagged parameters and function results. This means
that components of extensions are not checked (these would be very expensive
to check as any tagged type might have such an extension in the
future, even though that would be very unlikely). For this leak to occur
for a type T that has a type invariant, the body of a boundary
entity of T needs to have visibility on a type extension that
has components of T or access-to-T and also has an ancestor
type (or class) as a parameter or result of the subprogram.{
AI12-0338-1}
Invariant checks are not performed for parts of
incomplete types when the completion is not available. For this leak
to occur for a type T that has a type invariant and is declared
in a package P, one has to use a limited with on a package
that has P in its semantic closure, and then use a type from that
package as a parameter or result of a boundary subprogram for T
(or as the designated type of a parameter or result of such a subprogram).{
AI12-0210-1}
Consider a package P which declares an invariant-bearing
private type T and a generic package P.G1, which in turn
declares another generic package P.G1.G2. Outside of package P,
there are declarations of an instantiation I1 of P.G1 and
an instantiation I2 of I1.G2. I2 can declare visible
subprograms whose bodies see the full view of T and yet these
subprograms are not boundary subprograms (because the generic I1.G2
is not declared within the immediate scope of T - G1.G2
is, but that's irrelevant). So a call to one of these subprograms from
outside of P could yield an invariant-violating value. So long
as a nested generic of a nested generic unit of P is not declared,
no such leaks are possible.{
AI12-0210-1}
All of these leaks require cooperation of some
form (as detailed above) from within the immediate scope of the invariant-bearing
type.Implementation Note: The implementation
might want to produce a warning if a private extension has an ancestor
type that is a visible extension, and an invariant expression depends
on the value of one of the components from a visible extension part.
NOTE {
AI05-0250-1}
{
AI05-0269-1}
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.
Proof: This follows from the definition
of a call on an inherited subprogram as view conversions of the parameters
of the type and a call to the original subprogram (see
3.4),
along with the normal invariant checking rules. In particular, the call
to the original subprogram takes care of any checks needed on type
T,
and the checks required on view conversions take care of any checks needed
on type
NT, specifically on
in out and
out parameters.
We require this in order that the semantics of an explicitly defined
wrapper that does nothing but call the original subprogram is the same
as that of an inherited subprogram.
Examples
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;
Extensions to Ada 2005
Inconsistencies With Ada 2012
{
AI12-0042-1}
Corrigendum: Clarified
the definition of when invariant checks occur for inherited subprograms.
This might cause checks to be added or removed in some cases. These are
all rare cases involving class-wide type invariants and either record
extensions or multiple levels of derivation. Additionally, implementations
probably make the checks as the intent seems clear, even though the formal
language did not include them. So we do not expect this to be a problem
in practice.{
AI12-0042-1}
Corrigendum: Added invariant checks for
conversions to class-wide types. This might cause an invariant check
to fail in some cases where they would not be made in the original definition
of Ada 2012. Such cases represent a hole where a value that fails an
invariant could "leak out" of a package, and as such will detect
far more bugs than it causes.{
AI12-0044-1}
Corrigendum: Removed the invariant check
for in parameters of functions, so that typical invariants don't
cause infinite recursion. This is strictly inconsistent, as the Ada 2012
definition has this check; therefore, programs could depend on Assertion_Error
being raised upon the return from some call on a public function. However,
as the intent of assertion checking is to uncover bugs, a program that
depends on a bug occurring seems very unlikely.{
AI12-0049-1}
{
AI12-0149-1}
Corrigendum: Added an invariant check for
deferred constants and for access values returned from functions, so
they cannot be used to “leak” values that violate the invariant
from a package. This is strictly inconsistent, as the Ada 2012 definition
is missing these checks; therefore, programs could depend on using values
that violate an invariant outside of the package of definition. These
will not raise Assertion_Error in Ada 2012 as defined in the Ada 2012
Reference Manual, but ought to do so (as noted by this change). As these
are a violation of the intent of invariants, we think that this change
will mainly reveal bugs rather than cause them.{
AI12-0150-1}
{
AI12-0159-1}
Corrigendum: Eliminated unintentional redispatching
from class-wide type invariants. This means that a different body might
be evaluated for a type invariant check where the value has a different
tag than that of the type. The change means that the behavior of Type_Invariant
and Type_Invariant'Class will be the same for a particular subprogram,
and that the known behavior of the operations can be assumed. We expect
that this change will primarily fix bugs, as it will make class-wide
type invariants work more like expected. In the case where redispatching
is desired, an explicit conversion to a class-wide type can be used.{
AI12-0199-1}
Correction: Class-wide type invariants are
no longer checked for abstract types. Thus, a program that previously
raised Assertion_Error because of a call to a concrete subprogram of
an abstract type will no longer do so. However, programs that depend
on assertion failure are likely to be very rare, some explicit conversion
to the abstract type is needed to get static binding, and additionally
many such checks would call abstract functions (likely causing some compiler
failure). As such, this incompatibility most likely will never be seen
in practice. Incompatibilities With Ada 2012
{
AI12-0042-1}
{
AI12-0382-1}
Corrigendum: A private
operation that is inherited in the visible part of a package to which
a class-wide invariant applies now requires overriding. This is a very
unlikely situation, and will prevent problems with invariant checks being
added to routines that assume that they don't have them. Note: The original wording was missing the restriction to the visible
part of the package, this was added later for Ada 2022.
Extensions to Ada 2012
{
AI12-0041-1}
Corrigendum: Class-wide
type invariants can now be specified on interfaces as well as private
types. Wording Changes from Ada 2012
{
AI12-0133-1}
Corrigendum: Clarified that all objects
that are initialized by default should have an invariant check, and added
an exception for types with unknown discriminants, as in that case the
client cannot declare a default-initialized object. This exception to
the check is formally inconsistent, but since it is only removing an
assertion failure that occurs where no assertion should be checked anyway
(meaning it's more likely to fix a bug than cause one), and programs
depending on assertion failure should be very rare outside of test cases,
we don't document this as inconsistent.{
AI12-0075-1}
{
AI12-0191-1}
Defined the term “boundary entity”
to separate the static rules from the dynamic rules, and allow rules
in other subclauses to reference these rules. No semantic change is intended.{
AI12-0191-1}
Clarified that invariant checks only apply to parts
of the nominal type of objects.{
AI12-0193-1}
Correction: Clarified when type invariant
checks happen for protected actions and entry calls.{
AI12-0338-1}
Correction: Clarified that type invariant
checks do not occur for parts of incomplete types unless the completion
is available.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
