4.1.4 Attributes
1
{attribute}
[An
attribute is a characteristic of an entity
that can be queried via an
attribute_reference
or a
range_attribute_reference.]
Syntax
2
attribute_reference ::= prefix'
attribute_designator
3
attribute_designator ::=
identifier[(
static_expression)]
| Access | Delta | Digits
4
range_attribute_reference ::= prefix'
range_attribute_designator
5
range_attribute_designator ::= Range[(
static_expression)]
Name Resolution Rules
6
In an attribute_reference,
if the attribute_designator is for an attribute
defined for (at least some) objects of an access type, then the prefix
is never interpreted as an implicit_dereference;
otherwise (and for all range_attribute_references),
if the type of the name within the prefix
is of an access type, the prefix is interpreted
as an implicit_dereference. Similarly, if
the attribute_designator is for an attribute
defined for (at least some) functions, then the prefix
is never interpreted as a parameterless function_call;
otherwise (and for all range_attribute_references),
if the prefix consists of a name
that denotes a function, it is interpreted as a parameterless function_call.
6.a
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.
6.b
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.
6.c/1
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 prefix.
7
{expected type (attribute_designator
expression) [partial]} {expected
type (range_attribute_designator expression) [partial]} The
expression, if any, in an
attribute_designator
or
range_attribute_designator is expected
to be of any integer type.
Legality Rules
8
The expression, if any,
in an attribute_designator or range_attribute_designator
shall be static.
Static Semantics
9
An attribute_reference
denotes a value, an object, a subprogram, or some other kind of program
entity.
9.a
Ramification: The attributes defined
by the language are summarized in
Annex K. Implementations
can define additional attributes.
10
[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
11
{evaluation (attribute_reference)
[partial]} {evaluation
(range_attribute_reference) [partial]} The
evaluation of an
attribute_reference (or
range_attribute_reference)
consists of the evaluation of the
prefix.
Implementation Permissions
12/1
{
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.
12.a
Implementation defined: Implementation-defined
attributes.
12.b
Ramification: They cannot be reserved
words because reserved words are not legal identifiers.
12.c
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.
12.c.1/1
{
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. 13
4 Attributes are defined throughout this
International Standard, and are summarized in
Annex
K.
14/2
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 prefix prefix
has to be a single access type, and
if it is an
access-to-subprogram type (see 3.10.2) then
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)} 14.a/2
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.
14.b
Reason: 'Access is a special case, because
without it, it would be very difficult to take 'Access of an overloaded
subprogram.
Examples
15
Examples of attributes:
16
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
16.a
{
extensions to Ada 83}
We
now uniformly treat X'Range as X'First..X'Last, allowing its use with
scalar subtypes.
16.b
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
16.c
We use the syntactic category attribute_reference
rather than simply "attribute" to avoid confusing the name
of something with the thing itself.
16.d
The syntax rule for attribute_reference
now uses identifier instead of simple_name,
because attribute identifiers are not required
to follow the normal visibility rules.
16.e
We now separate attribute_reference
from range_attribute_reference, and enumerate
the reserved words that are legal attribute or range attribute designators.
We do this because identifier no longer includes
reserved words.
16.f
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.
16.g
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
16.h/2
{
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.16.i/2
{
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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