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13.9 Unchecked Type Conversions

An unchecked type conversion can be achieved by a call to an instance of the generic function Unchecked_Conversion. 

Static Semantics

The following language-defined generic library function exists: 
   type Source(<>) is limited private;
   type Target(<>) is limited private;
function Ada.Unchecked_Conversion(S : Source) return Target
   with Pure, Nonblocking, Convention => Intrinsic;

Dynamic Semantics

The size of the formal parameter S in an instance of Unchecked_Conversion is that of its subtype. This is the actual subtype passed to Source, except when the actual is an unconstrained composite subtype, in which case the subtype is constrained by the bounds or discriminants of the value of the actual expression passed to S.
If all of the following are true, the effect of an unchecked conversion is to return the value of an object of the target subtype whose representation is the same as that of the source object S: 
S'Size = Target'Size. 
S'Alignment is a multiple of Target'Alignment or Target'Alignment is zero.
The target subtype is not an unconstrained composite subtype.
S and the target subtype both have a contiguous representation.
The representation of S is a representation of an object of the target subtype. 
Otherwise, if the result type is scalar, the result of the function is implementation defined, and can have an invalid representation (see 13.9.1). If the result type is nonscalar, the effect is implementation defined; in particular, the result can be abnormal (see 13.9.1).

Implementation Permissions

An implementation may return the result of an unchecked conversion by reference, if the Source type is not a by-copy type. In this case, the result of the unchecked conversion represents simply a different (read-only) view of the operand of the conversion. 
An implementation may place restrictions on Unchecked_Conversion.

Implementation Advice

Since the Size of an array object generally does not include its bounds, the bounds should not be part of the converted data. 
The implementation should not generate unnecessary runtime checks to ensure that the representation of S is a representation of the target type. It should take advantage of the permission to return by reference when possible. Restrictions on unchecked conversions should be avoided unless required by the target environment. 
The recommended level of support for unchecked conversions is: 
Unchecked conversions should be supported and should be reversible in the cases where this subclause defines the result. To enable meaningful use of unchecked conversion, a contiguous representation should be used for elementary subtypes, for statically constrained array subtypes whose component subtype is one of the subtypes described in this paragraph, and for record subtypes without discriminants whose component subtypes are described in this paragraph. 

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