4.1 Names
1
[
Names can denote declared
entities, whether declared explicitly or implicitly (see
3.1).
Names can also denote objects or subprograms
designated by access values; the results of
type_conversions
or
function_calls; subcomponents and slices
of objects and values; protected subprograms, single entries, entry families,
and entries in families of entries. Finally,
names
can denote attributes of any of the foregoing.]
Syntax
2
name ::=
direct_name |
explicit_dereference
|
indexed_component |
slice
|
selected_component |
attribute_reference
|
type_conversion |
function_call
|
character_literal
3
direct_name ::= identifier |
operator_symbol
3.a
Discussion: character_literal
is no longer a
direct_name.
character_literals
are usable even when the corresponding
enumeration_type_declaration
is not visible. See
4.2.
4
prefix ::= name |
implicit_dereference
5
explicit_dereference ::= name.
all
6
implicit_dereference ::= name
7
[Certain forms of name
(indexed_components, selected_components,
slices, and attributes)
include a prefix that is either itself a name
that denotes some related entity, or an implicit_dereference
of an access value that designates some related entity.]
Name Resolution Rules
8
{dereference}
{expected type (dereference
name) [partial]} The
name
in a
dereference (either an
implicit_dereference
or an
explicit_dereference) is expected to
be of any access type.
Static Semantics
9
{nominal subtype
(associated with a dereference) [partial]} If
the type of the
name in a dereference is some
access-to-object type
T, then the dereference denotes a view of
an object, the
nominal subtype of the view being the designated
subtype of
T.
9.a
Ramification: If the value of the
name
is the result of an access type conversion, the dereference denotes a
view created as part of the conversion. The nominal subtype of the view
is not necessarily the same as that used to create the designated object.
See
4.6.
9.b
To be honest: {
nominal subtype (of
a name) [partial]}
We sometimes refer to the
nominal subtype of a particular kind of
name
rather than the nominal subtype of the view denoted by the
name
(presuming the
name denotes a view of an object).
These two uses of nominal subtype are intended to mean the same thing.
9.c/2
{
AI95-00363-01}
If an allocator for
the access-to-object type T is one that creates objects that are
constrained by their initial value (see 4.8),
the subtype of the dereference is constrained even if the designated
subtype of T is not. We don't want the effect of the dereference
to depend on the designated object. This matters because general access-to-unconstrained
can designate both allocated objects (which are constrained at birth)
and aliased stack objects (which aren't necessarily constrained). This
is a wording bug that was discovered after the completion of Amendment
1 when it was too late to fix it; we expect that it will be corrected
by an early Ada 2005 AI. 9.d/2
Implementation Note:
{
AI95-00363-01}
Since we don't depend on whether the designated
object is constrained, it is not necessary to include a constrained bit
in every object that could be designated by a general access type.
10
{profile (associated
with a dereference) [partial]} If the
type of the
name in a dereference is some
access-to-subprogram type
S, then the dereference denotes a view
of a subprogram, the
profile of the view being the designated
profile of
S.
10.a
Ramification: This means that the formal
parameter names and default expressions to be used in a call whose name
or prefix is a dereference are those of the
designated profile, which need not be the same as those of the subprogram
designated by the access value, since 'Access requires only subtype conformance,
not full conformance.
Dynamic Semantics
11/2
{
AI95-00415-01}
{evaluation (name) [partial]}
The evaluation of a
name
determines the entity denoted by the
name name.
This evaluation has no other effect for a
name
that is a
direct_name or a
character_literal.
12
{evaluation (name
that has a prefix) [partial]} [The evaluation
of a
name that has a
prefix
includes the evaluation of the
prefix.]
{evaluation
(prefix) [partial]} The evaluation of
a
prefix consists of the evaluation of the
name or the
implicit_dereference.
The
prefix denotes the entity denoted by the
name or the
implicit_dereference.
13
{evaluation (dereference)
[partial]} The evaluation of a dereference
consists of the evaluation of the
name and
the determination of the object or subprogram that is designated by the
value of the
name.
{Access_Check
[partial]} {check,
language-defined (Access_Check)} A check
is made that the value of the
name is not
the null access value.
{Constraint_Error
(raised by failure of run-time check)} Constraint_Error
is raised if this check fails. The dereference denotes the object or
subprogram designated by the value of the
name.
Examples
14
Examples of direct
names:
15
Pi
-- the direct name of a number (see
3.3.2)
Limit
-- the direct name of a constant (see
3.3.1)
Count
-- the direct name of a scalar variable (see
3.3.1)
Board
-- the direct name of an array variable (see
3.6.1)
Matrix
-- the direct name of a type (see
3.6)
Random
-- the direct name of a function (see
6.1)
Error
-- the direct name of an exception (see
11.1)
16
Examples of dereferences:
17
Next_Car.
all --
explicit dereference denoting the object designated by
--
the access variable Next_Car (see 3.10.1)
Next_Car.Owner --
selected component with implicit dereference;
--
same as Next_Car.all.OwnerExtensions to Ada 83
17.a
{
extensions to Ada 83}
Type
conversions and function calls are now considered names that denote the
result of the operation. In the case of a type conversion used as an
actual parameter or that is of a tagged type, the type conversion is
considered a variable if the operand is a variable. This simplifies the
description of "parameters of the form of a type conversion"
as well as better supporting an important OOP paradigm that requires
the combination of a conversion from a class-wide type to some specific
type followed immediately by component selection. Function calls are
considered names so that a type conversion of a function call and the
function call itself are treated equivalently in the grammar. A function
call is considered the name of a constant, and can be used anywhere such
a name is permitted. See
6.5.
17.b/1
Type conversions of a tagged type are permitted
anywhere their operand is permitted. That is, if the operand is a variable,
then the type conversion can appear on the left-hand side of an
assignment_statement.
If the operand is an object, then the type conversion can appear in an
object renaming or as a
prefix prefix.
See
4.6.
Wording Changes from Ada 83
17.c/2
{
AI95-00114-01}
Everything of the general syntactic form
name(...)
is now syntactically a
name. In any realistic
parser, this would be a necessity since distinguishing among the various
name(...) constructs inevitably requires name
resolution. In cases where the construct yields a value rather than an
object, the name denotes
a the
value rather than an object. Names already denote values in Ada 83 with
named numbers, components of the result of a function call, etc. This
is partly just a wording change, and partly an extension of functionality
(see Extensions heading above).
17.d
The syntax rule for direct_name
is new. It is used in places where direct visibility is required. It's
kind of like Ada 83's simple_name, but simple_name
applied to both direct visibility and visibility by selection, and furthermore,
it didn't work right for operator_symbols.
The syntax rule for simple_name is removed,
since its use is covered by a combination of direct_name
and selector_name. The syntactic categories
direct_name and selector_name
are similar; it's mainly the visibility rules that distinguish the two.
The introduction of direct_name requires the
insertion of one new explicit textual rule: to forbid statement_identifiers
from being operator_symbols. This is the only
case where the explicit rule is needed, because this is the only case
where the declaration of the entity is implicit. For example, there is
no need to syntactically forbid (say) “X: "Rem";”,
because it is impossible to declare a type whose name is an operator_symbol
in the first place.
17.e
The syntax rules for explicit_dereference
and implicit_dereference are new; this makes
other rules simpler, since dereferencing an access value has substantially
different semantics from selected_components.
We also use name instead of prefix
in the explicit_dereference rule since that
seems clearer. Note that these rules rely on the fact that function calls
are now names, so we don't need to use prefix to allow functions calls
in front of .all.
17.f
Discussion: Actually, it would be reasonable
to allow any primary in front of .all,
since only the value is needed, but that would be a bit radical.
17.g
We no longer use the term appropriate for
a type since we now describe the semantics of a prefix in terms of
implicit dereference.
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