B.3.3 Pragma Unchecked_Union
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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:
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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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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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The completion of an incomplete or private type
declaration having a
known_discriminant_part
shall not be an unchecked union type.
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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.
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:
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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).
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.
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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:
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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.
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13 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:
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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