4.1.4 Attributes
Syntax
range_attribute_designator ::= Range[(
static_expression)]
Name Resolution Rules
Discussion: The first part of this rule
is essentially a "preference" against implicit dereference,
so that it is possible to ask for, say, 'Size of an access object, without
automatically getting the size of the object designated by the access
object. This rule applies to 'Access, 'Unchecked_Access, 'Size, and 'Address,
and any other attributes that are defined for at least some access objects.
The second part of this rule implies that, for
a parameterless function F, F'Address is the address of F, whereas F'Size
is the size of the anonymous constant returned by F.
We normally talk in terms of expected type or
profile for name resolution rules, but we don't do this for attributes
because certain attributes are legal independent of the type or the profile
of the
prefix.
{
AI95-00114-01}
Other than the rules given above, the Name Resolution Rules for the
prefix
of each attribute are defined as Name Resolution Rules for that attribute.
If no such rules are defined, then no context at all should be used when
resolving the
prefix.
In particular, any knowledge about the kind of entities required must
not be used for resolution unless that is required by Name Resolution
Rules. This matters in obscure cases; for instance, given the following
declarations:
function Get_It return Integer is ... -- (1)
function Get_It return Some_Record_Type is ... -- (2)
if Get_It'Valid then
even though the Valid attribute is only defined
for objects of scalar types, and thus cannot be applied to the result
of function (2). That information cannot be used to resolve the
prefix.
The same would be true if (2) was been a procedure; even though the procedure
does not denote an object, the
attribute_reference
is still illegal.
Legality Rules
Static Semantics
An
attribute_reference
denotes a value, an object, a subprogram, or some other kind of program
entity.
Ramification: The attributes defined
by the language are summarized in
Annex K. Implementations
can define additional attributes.
[A
range_attribute_reference
X'Range(N) is equivalent to the
range
X'First(N) .. X'Last(N), except that the
prefix
is only evaluated once. Similarly, X'Range is equivalent to X'First ..
X'Last, except that the
prefix
is only evaluated once.]
Dynamic Semantics
Implementation Permissions
{
8652/0015}
{
AI95-00093-01}
An implementation may provide implementation-defined attributes; the
identifier
for an implementation-defined attribute shall differ from those of the
language-defined attributes unless supplied for compatibility with a
previous edition of this International Standard.
Implementation defined: Implementation-defined
attributes.
Ramification: They cannot be reserved
words because reserved words are not legal identifiers.
The semantics of implementation-defined attributes,
and any associated rules, are, of course, implementation defined. For
example, the implementation defines whether a given implementation-defined
attribute can be used in a static expression.
{
8652/0015}
{
AI95-00093-01}
Implementations are allowed to support the Small attribute for floating
types, as this was defined in Ada 83, even though the name would conflict
with a language-defined attribute.
4 Attributes are defined throughout this
International Standard, and are summarized in
Annex
K.
5 {
AI95-00235}
In general, the
name
in a
prefix
of an
attribute_reference
(or a
range_attribute_reference)
has to be resolved without using any context. However, in the case of
the Access attribute, the expected type for the
attribute_reference
has to be a single access type, and the resolution of the
name
can use the fact that the type of the object or the profile of the callable
entity denoted by the
prefix
has to match the designated type or be type conformant with the designated
profile of the access type.
{type conformance
(required)} Proof: {
AI95-00235}
In the general case, there is no “expected type” for the
prefix of
an
attribute_reference.
In the special case of 'Access, there is an “expected type”
or “expected profile” for the
prefix.
Reason: 'Access is a special case, because
without it, it would be very difficult to take 'Access of an overloaded
subprogram.
Examples
Examples of attributes:
Color'First --
minimum value of the enumeration type Color (see 3.5.1)
Rainbow'Base'First --
same as Color'First (see 3.5.1)
Real'Digits --
precision of the type Real (see 3.5.7)
Board'Last(2) --
upper bound of the second dimension of Board (see 3.6.1)
Board'Range(1) --
index range of the first dimension of Board (see 3.6.1)
Pool(K)'Terminated --
True if task Pool(K) is terminated (see 9.1)
Date'Size --
number of bits for records of type Date (see 3.8)
Message'Address --
address of the record variable Message (see 3.7.1)Extensions to Ada 83
{
extensions to Ada 83}
We
now uniformly treat X'Range as X'First..X'Last, allowing its use with
scalar subtypes.
We allow any integer type in the
static_expression
of an attribute designator, not just a value of
universal_integer.
The preference rules ensure upward compatibility.
Wording Changes from Ada 83
We use the syntactic category
attribute_reference
rather than simply "attribute" to avoid confusing the name
of something with the thing itself.
The Ada 95 name resolution rules are a bit more
explicit than in Ada 83. The Ada 83 rule said that the "meaning
of the prefix of an attribute must be determinable independently of the
attribute designator and independently of the fact that it is the prefix
of an attribute." That isn't quite right since the meaning even
in Ada 83 embodies whether or not the prefix is interpreted as a parameterless
function call, and in Ada 95, it also embodies whether or not the prefix
is interpreted as an implicit_dereference. So the attribute designator
does make a difference — just not much.
Note however that if the attribute designator
is Access, it makes a big difference in the interpretation of the prefix
(see
3.10.2).
Wording Changes from Ada 95
{
8652/0015}
{
AI95-00093-01}
Corrigendum: The wording was changed to allow implementations
to continue to implement the Ada 83 Small attribute. This was always
intended to be allowed.
{
AI95-00235-01}
The note about resolving prefixes of attributes was updated to reflect
that the prefix of an Access attribute now has an expected type (see
3.10.2).
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