Pragma Unchecked_Union
Author: JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor

Ada Reference Manual — Legal Information

# 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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The first_subtype_ local_name of a pragma Unchecked_Union shall denote an unconstrained discriminated record subtype having a variant_part.
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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.
NOTES
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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

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