B.3.3 Pragma Unchecked_Union
1/2
{
AI95-00216-01}
{union (C)} [A
pragma Unchecked_Union specifies an interface correspondence between
a given discriminated type and some C union. The pragma specifies that
the associated type shall be given a representation that leaves no space
for its discriminant(s).]
Syntax
2/2
{
AI95-00216-01}
The form of a pragma Unchecked_Union is as follows:
3/2
pragma Unchecked_Union
(
first_subtype_local_name);
Legality Rules
4/2
{
AI95-00216-01}
Unchecked_Union is a representation pragma, specifying the unchecked
union aspect of representation.
5/2
6/2
{
AI95-00216-01}
{unchecked union type} {unchecked
union subtype} {unchecked
union object} A type to which a pragma
Unchecked_Union applies is called an
unchecked union type. A subtype
of an unchecked union type is defined to be an
unchecked union subtype.
An object of an unchecked union type is defined to be an
unchecked
union object.
7/2
{
AI95-00216-01}
All component subtypes of an unchecked union type shall be C-compatible.
8/2
{
AI95-00216-01}
If a component subtype of an unchecked union type is subject to a per-object
constraint, then the component subtype shall be an unchecked union subtype.
9/2
{
AI95-00216-01}
Any name that denotes a discriminant of an object of an unchecked union
type shall occur within the declarative region of the type.
10/2
{
AI95-00216-01}
A component declared in a
variant_part
of an unchecked union type shall not have a controlled, protected, or
task part.
11/2
12/2
{
AI95-00216-01}
An unchecked union subtype shall only be passed as a generic actual parameter
if the corresponding formal type has no known discriminants or is an
unchecked union type.
12.a/2
Ramification: This includes formal private
types without a
known_discriminant_part,
formal derived types that do not inherit any discriminants (formal derived
types do not have
known_discriminant_parts),
and formal derived types that are unchecked union types.
Static Semantics
13/2
{
AI95-00216-01}
An unchecked union type is eligible for convention C.
14/2
{
AI95-00216-01}
All objects of an unchecked union type have the same size.
15/2
{
AI95-00216-01}
Discriminants of objects of an unchecked union type are of size zero.
16/2
{
AI95-00216-01}
Any check which would require reading a discriminant of an unchecked
union object is suppressed (see
11.5). These
checks include:
17/2
The check performed when addressing a variant component
(i.e., a component that was declared in a variant part) of an unchecked
union object that the object has this component (see
4.1.3).
18/2
Any checks associated with a type or subtype conversion
of a value of an unchecked union type (see
4.6).
This includes, for example, the check associated with the implicit subtype
conversion of an assignment statement.
19/2
The subtype membership check associated with the
evaluation of a qualified expression (see
4.7)
or an uninitialized allocator (see
4.8).
19.a/2
Discussion: If a suppressed check would
have failed, execution is erroneous (see
11.5).
An implementation is always allowed to make a suppressed check if it
can somehow determine the discriminant value.
Dynamic Semantics
20/2
{
AI95-00216-01}
A view of an unchecked union object (including a type conversion or function
call) has
inferable discriminants if it has a constrained nominal
subtype, unless the object is a component of an enclosing unchecked union
object that is subject to a per-object constraint and the enclosing object
lacks inferable discriminants.
{inferable
discriminants}
21/2
{
AI95-00216-01}
An expression of an unchecked union type has inferable discriminants
if it is either a name of an object with inferable discriminants or a
qualified expression whose
subtype_mark
denotes a constrained subtype.
22/2
{
AI95-00216-01}
Program_Error is raised in the following cases:
{Program_Error
(raised by failure of run-time check)}
23/2
Evaluation of the predefined equality operator
for an unchecked union type if either of the operands lacks inferable
discriminants.
24/2
Evaluation of the predefined equality operator
for a type which has a subcomponent of an unchecked union type whose
nominal subtype is unconstrained.
25/2
Evaluation of a membership test if the
subtype_mark
denotes a constrained unchecked union subtype and the expression lacks
inferable discriminants.
26/2
Conversion from a derived unchecked union type
to an unconstrained non-unchecked-union type if the operand of the conversion
lacks inferable discriminants.
27/2
Execution of the default implementation of the
Write or Read attribute of an unchecked union type.
28/2
Execution of the default implementation of the
Output or Input attribute of an unchecked union type if the type lacks
default discriminant values.
Implementation Permissions
29/2
{
AI95-00216-01}
An implementation may require that
pragma
Controlled be specified for the type of an access subcomponent of an
unchecked union type.
30/2
13 {
AI95-00216-01}
The use of an unchecked union to obtain the effect of an unchecked conversion
results in erroneous execution (see
11.5).
Execution of the following example is erroneous even if Float'Size =
Integer'Size:
31/2
type T (Flag : Boolean := False) is
record
case Flag is
when False =>
F1 : Float := 0.0;
when True =>
F2 : Integer := 0;
end case;
end record;
pragma Unchecked_Union (T);
32/2
X : T;
Y : Integer := X.F2; -- erroneous
Extensions to Ada 95
32.a/2