4.1.4 Attributes
1
Syntax
2
3/2
4
5
range_attribute_designator ::= Range[(
static_expression)]
Name Resolution Rules
6
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.
6.d/2
{
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: 6.e/2
function Get_It return Integer is ... -- (1)
function Get_It return Some_Record_Type is ... -- (2)
6.f/2
6.g/2
if Get_It'Valid then
6.h/3
{
AI05-0005-1}
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) had was been a procedure; even though the procedure does not denote an object,
the attribute_reference
is still illegal. 7
Legality Rules
8
Static Semantics
9/3
{
AI05-0006-1}
An
attribute_reference
denotes a value, an object, a subprogram, or some other kind of program
entity.
For an attribute_reference
that denotes a value or an object, if its type is scalar, then its nominal
subtype is the base subtype of the type; if its type is tagged, its nominal
subtype is the first subtype of the type; otherwise, its nominal subtype
is a subtype of the type without any constraint or null_exclusion.
Similarly, unless explicitly specified otherwise, for an attribute_reference
that denotes a function, when its result type is scalar, its result subtype
is the base subtype of the type, when its result type is tagged, the
result subtype is the first subtype of the type, and when the result
type is some other type, the result subtype is a subtype of the type
without any constraint or null_exclusion.
9.a
Ramification: The attributes defined
by the language are summarized in
K.2. Implementations
can define additional attributes.
9.b/3
Discussion: {
AI05-0006-1}
The nominal subtype is primarily a concern when
an attribute_reference,
or a call on an attribute_reference,
is used as the expression
of a case statement, due to the full coverage requirement based on the
nominal subtype. For non-discrete cases, we define the nominal subtype
mainly for completeness. Implementations may specify otherwise for implementation-defined
attribute functions.
9.c/3
The rule is written to
match the meaning of the italicized T in the definition of attributes
such as Input; see 4.5.1.
9.d/3
To be honest: {
AI05-0006-1}
We don't worry about the fact that “base
subtype” is not explicitly defined for the universal types. Since
it is not possible to constrain a universal numeric type, all subtypes
are unconstrained, and hence can be considered base subtypes. The wording
above could be altered to bypass this issue, but it doesn't seem necessary,
since universal integer is handled specially in the rules for case expression
full coverage, and we don't allow user-defined functions for attribute
functions whose result type is universal.
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
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
K.2.
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.
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
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
16.e
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). Wording Changes from Ada 2005
16.j/3
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
