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 must 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,
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.
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 must hold true for all objects in a class of types.
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}
[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.]
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}
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
{
AI05-0250-1}
[If the Type_Invariant aspect is specified for a type
T, then
the invariant expression applies to
T.]
{
AI05-0146-1}
[If the Type_Invariant'Class aspect is specified for a tagged type
T,
then the invariant expression applies to all descendants of
T.]
Proof: "Applies" is formally
defined in
13.1.1.
Dynamic Semantics
{
AI12-0133-1}
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;
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}
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;
{
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}
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;
{
AI05-0146-1}
{
AI05-0269-1}
An invariant is checked upon successful return from a call on any subprogram
or entry that:
{
AI05-0146-1}
{
AI05-0269-1}
{
AI12-0042-1}
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),
{
AI12-0044-1}
has one or more
out or
in out parameters with a part of
type
T, or
{
AI12-0044-1}
{
AI12-0149-1}
has an access-to-object parameter or result whose designated type has
a part of type
T, or
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.
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.
{
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.
{
AI05-0290-1}
{
AI12-0080-1}
{
AI12-0159-1}
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.
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}
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.
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.
13 {
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.
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 Standard, 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.
Incompatibilities With Ada 2012
{
AI12-0042-1}
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 need them.
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.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
