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:
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generic
type Source(<>)
is limited private;
type Target(<>)
is limited private;
function Ada.Unchecked_Conversion(S : Source)
return Target
with Pure, Nonblocking, Convention => Intrinsic;
Reason: {
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The aspect Convention implies that the attribute Access is not allowed
for instances of 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.
Ramification: Note that there is no requirement
that the Sizes be known at compile time.
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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.
{
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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 defined: The result of
unchecked conversion for instances with scalar result types whose result
is not defined by the language.
Implementation defined: The effect of
unchecked conversion for instances with nonscalar result types whose
effect is not defined by the language.
Reason: {
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Note the difference between these sentences; the first only says that
the bits returned are implementation defined, while the latter allows
any effect. The difference is because scalar objects should never be
abnormal unless their assignment was disrupted or if they are a subcomponent
of an abnormal composite object. Neither exception applies to instances
of Unchecked_Conversion.
Ramification: {
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Whenever unchecked conversions are used, it is the programmer's responsibility
to ensure that these conversions maintain the properties that are guaranteed
by the language for objects of the target type. For nonscalar types,
this requires the user to understand the underlying run-time model of
the implementation. The execution of a program that violates these properties
by means of unchecked conversions returning a nonscalar type is erroneous.
Properties of scalar types can be checked by using the Valid attribute
(see
13.9.2); programs can avoid violating
properties of the type (and erroneous execution) by careful use of this
attribute.
An instance of Unchecked_Conversion can be applied
to an object of a private type, assuming the implementation allows it.
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.]
Ramification: In other words, the result
object of a call on an instance of Unchecked_Conversion can occupy the
same storage as the formal parameter S.
An implementation may place restrictions on Unchecked_Conversion.
Ramification: For example, an instantiation
of Unchecked_Conversion for types for which unchecked conversion doesn't
make sense may be disallowed.
Implementation Advice
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Since the Size of an array object generally does not include its bounds,
the bounds should not be part of the converted data.
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 in an instance of Unchecked_Conversion.
Ramification: On the other hand, we have
no advice to offer about discriminants and tag fields.
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.
Implementation Advice: There should not
be unnecessary runtime checks on the result of an Unchecked_Conversion;
the result should be returned by reference when possible. Restrictions
on Unchecked_Conversions should be avoided.
Implementation Note: As an example of
an unnecessary runtime check, consider a record type with gaps between
components. The compiler might assume that such gaps are always zero
bits. If a value is produced that does not obey that assumption, then
the program might misbehave. The implementation should not generate extra
code to check for zero bits (except, perhaps, in a special error-checking
mode).
The
recommended level of support for unchecked conversions is:
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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.
Implementation Advice: The recommended
level of support for Unchecked_Conversion should be followed.
Wording Changes from Ada 95
{
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The implementation advice about the size of array objects was moved to
13.3 so that all of the advice about Size is in one place.
{
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Clarified that the result of Unchecked_Conversion for scalar types can
be invalid, but not abnormal.
Wording Changes from Ada 2005
{
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Correction: Relaxed the alignment requirement slightly, giving
a defined result in more cases.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
