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:
generic
type Source(<>)
is limited private;
type Target(<>)
is limited private;
function Ada.Unchecked_Conversion(S : Source)
return Target;
pragma Convention(Intrinsic, Ada.Unchecked_Conversion);
pragma Pure(Ada.Unchecked_Conversion);
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 = Target'Alignment.
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
run-time 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 clause 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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