B.3.3 Pragma Unchecked_Union
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{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
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The form of a pragma Unchecked_Union is as follows:
pragma Unchecked_Union
(
first_subtype_local_name);
Legality Rules
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Unchecked_Union is a representation pragma, specifying the unchecked
union aspect of representation.
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{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.
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All component subtypes of an unchecked union type shall be C-compatible.
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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.
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Any name that denotes a discriminant of an object of an unchecked union
type shall occur within the declarative region of the type.
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A component declared in a
variant_part
of an unchecked union type shall not have a controlled, protected, or
task part.
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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.
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
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An unchecked union type is eligible for convention C.
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All objects of an unchecked union type have the same size.
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Discriminants of objects of an unchecked union type are of size zero.
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Any check which would require reading a discriminant of an unchecked
union object is suppressed (see
11.5). These
checks include:
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).
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.
The subtype membership check associated with the
evaluation of a qualified expression (see
4.7)
or an uninitialized allocator (see
4.8).
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
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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} {
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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.
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Program_Error is raised in the following cases:
{Program_Error
(raised by failure of run-time check)} Evaluation of the predefined equality operator
for an unchecked union type if either of the operands lacks inferable
discriminants.
Evaluation of the predefined equality operator
for a type which has a subcomponent of an unchecked union type whose
nominal subtype is unconstrained.
Evaluation of a membership test if the
subtype_mark
denotes a constrained unchecked union subtype and the expression lacks
inferable discriminants.
Conversion from a derived unchecked union type
to an unconstrained non-unchecked-union type if the operand of the conversion
lacks inferable discriminants.
Execution of the default implementation of the
Write or Read attribute of an unchecked union type.
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
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An implementation may require that
pragma
Controlled be specified for the type of an access subcomponent of an
unchecked union type.
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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:
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);
X : T;
Y : Integer := X.F2; -- erroneous
Extensions to Ada 95
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