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:3/2
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.5/2
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The first_subtype_local_name
of a pragma Unchecked_Union shall denote an
unconstrained discriminated record subtype having a variant_part.6/2
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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.7/2
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All component subtypes of an unchecked union type
shall be C-compatible.8/2
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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.9/2
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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.10/2
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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.11/2
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The completion of an incomplete or private type
declaration having a known_discriminant_part
shall not be an unchecked union type.12/2
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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.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
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An unchecked union type is eligible for convention
C.14/2
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All objects of an unchecked union type have the
same size.15/2
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Discriminants of objects of an unchecked union
type are of size zero.16/2
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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: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).
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- 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.
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- The subtype
membership check associated with the evaluation of a qualified expression
(see 4.7) or an uninitialized allocator (see
4.8).
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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} 21/2
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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.22/2
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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.
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- Evaluation of
the predefined equality operator for a type which has a subcomponent
of an unchecked union type whose nominal subtype is unconstrained.
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- Evaluation of
a membership test if the subtype_mark denotes
a constrained unchecked union subtype and the expression lacks inferable
discriminants.
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- Conversion from
a derived unchecked union type to an unconstrained non-unchecked-union
type if the operand of the conversion lacks inferable discriminants.
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- Execution of
the default implementation of the Write or Read attribute of an unchecked
union type.
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- 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.30/2
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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: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);
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X : T;
Y : Integer := X.F2; -- erroneous
Extensions to Ada 95
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{extensions to Ada 95} Pragma
Unchecked_Union is new.
Sponsored by Ada-Europe