4.1.3 Selected Components
1
[
Selected_components are
used to denote components (including discriminants), entries, entry families,
and protected subprograms; they are also used as expanded names as described
below.
{dot selection: See selected_component}
]
Syntax
2
selected_component ::= prefix .
selector_name
3
selector_name ::= identifier |
character_literal |
operator_symbol
Name Resolution Rules
4
{expanded name}
A
selected_component is
called an
expanded name if, according to the visibility rules,
at least one possible interpretation of its
prefix
denotes a package or an enclosing named construct (directly, not through
a
subprogram_renaming_declaration or
generic_renaming_declaration).
4.a
Discussion: See AI83-00187.
5
A selected_component
that is not an expanded name shall resolve to denote one of the following:
5.a
Ramification: If the prefix
of a selected_component denotes an enclosing
named construct, then the selected_component
is interpreted only as an expanded name, even if the named construct
is a function that could be called without parameters.
6
- A component [(including a discriminant)]:
7
The prefix shall resolve
to denote an object or value of some non-array composite type (after
any implicit dereference). The selector_name
shall resolve to denote a discriminant_specification
of the type, or, unless the type is a protected type, a component_declaration
of the type. The selected_component denotes
the corresponding component of the object or value.
7.a/1
Reason: The components of a protected
object cannot be named except by an expanded name, even from within the
corresponding protected body. The protected body may not reference the
the private components of some arbitrary object of the protected
type; the protected body may reference components of the current instance
only (by an expanded name or a direct_name).
7.b
Ramification: Only the discriminants
and components visible at the place of the
selected_component
can be selected, since a
selector_name can
only denote declarations that are visible (see
8.3).
8
- A single entry, an entry family, or
a protected subprogram:
9
The prefix shall resolve
to denote an object or value of some task or protected type (after any
implicit dereference). The selector_name shall
resolve to denote an entry_declaration or
subprogram_declaration occurring (implicitly
or explicitly) within the visible part of that type. The selected_component
denotes the corresponding entry, entry family, or protected subprogram.
9.a
Reason: This explicitly says “visible
part” because even though the body has visibility on the private
part, it cannot call the private operations of some arbitrary object
of the task or protected type, only those of the current instance (and
expanded name notation has to be used for that).
9.1/2
- {AI95-00252-01}
{AI95-00407-01}
A view of a subprogram whose first formal parameter
is of a tagged type or is an access parameter whose designated type is
tagged:
9.2/2
The prefix
(after any implicit dereference) shall resolve to denote an object or
value of a specific tagged type T or class-wide type T'Class.
The selector_name shall resolve to denote
a view of a subprogram declared immediately within the declarative region
in which an ancestor of the type T is declared. The first formal
parameter of the subprogram shall be of type T, or a class-wide
type that covers T, or an access parameter designating one of
these types. The designator of the subprogram shall not be the same as
that of a component of the tagged type visible at the point of the selected_component.
The selected_component denotes a view of this
subprogram that omits the first formal parameter. This view is called
a prefixed view of the subprogram, and the prefix
of the selected_component (after any implicit
dereference) is called the prefix of the prefixed view. {prefixed
view} {prefix
(of a prefixed view)}
10
An expanded name shall
resolve to denote a declaration that occurs immediately within a named
declarative region, as follows:
11
- The prefix
shall resolve to denote either a package [(including the current instance
of a generic package, or a rename of a package)], or an enclosing named
construct.
12
- The selector_name
shall resolve to denote a declaration that occurs immediately within
the declarative region of the package or enclosing construct [(the declaration
shall be visible at the place of the expanded name — see 8.3)].
The expanded name denotes that declaration.
12.a
Ramification: Hence, a library unit or
subunit can use an expanded name to refer to the declarations within
the private part of its parent unit, as well as to other children that
have been mentioned in with_clauses.
13
- If the prefix
does not denote a package, then it shall be a direct_name
or an expanded name, and it shall resolve to denote a program unit (other
than a package), the current instance of a type, a block_statement,
a loop_statement, or an accept_statement
(in the case of an accept_statement or entry_body,
no family index is allowed); the expanded name shall occur within the
declarative region of this construct. Further, if this construct is a
callable construct and the prefix denotes
more than one such enclosing callable construct, then the expanded name
is ambiguous, independently of the selector_name.
Legality Rules
13.1/2
{
AI95-00252-01}
{
AI95-00407-01}
For a subprogram whose first parameter is an access
parameter, the prefix of any prefixed view shall denote an aliased view
of an object.13.2/2
{
AI95-00407-01}
For a subprogram whose first parameter is of mode
in out or out, or of an anonymous access-to-variable type,
the prefix of any prefixed view shall denote a variable.13.a/2
Reason: We want
calls through a prefixed view and through a normal view to have the same
legality. Thus, the implicit 'Access in this new notation needs the same
legality check that an explicit 'Access would have. Similarly, we need
to prohibit the object from being constant if the first parameter of
the subprogram is in out, because that is (obviously) prohibited
for passing a normal parameter.
Dynamic Semantics
14
{evaluation (selected_component)
[partial]} The evaluation of a
selected_component
includes the evaluation of the
prefix.
15
{Discriminant_Check
[partial]} {check,
language-defined (Discriminant_Check)} For
a
selected_component that denotes a component
of a
variant, a check is made that the values
of the discriminants are such that the value or object denoted by the
prefix has this component.
{Constraint_Error
(raised by failure of run-time check)} {Constraint_Error
(raised by failure of run-time check)} The
exception Constraint_Error is raised if this check fails.
Examples
16
Examples of selected
components:
17/2
{
AI95-00252-01}
{
AI95-00407-01}
Tomorrow.Month --
a record component (see 3.8)
Next_Car.Owner --
a record component (see 3.10.1)
Next_Car.Owner.Age --
a record component (see 3.10.1)
--
the previous two lines involve implicit dereferences
Writer.Unit --
a record component (a discriminant) (see 3.8.1)
Min_Cell(H).Value --
a record component of the result (see 6.1)
--
of the function call Min_Cell(H)
Cashier.Append -- a prefixed view of a procedure (see 3.9.4)
Control.Seize --
an entry of a protected object (see 9.4)
Pool(K).Write --
an entry of the task Pool(K) (see 9.4)18
Examples of expanded
names:
19
Key_Manager."<" --
an operator of the visible part of a package (see 7.3.1)
Dot_Product.Sum --
a variable declared in a function body (see 6.1)
Buffer.Pool --
a variable declared in a protected unit (see 9.11)
Buffer.Read --
an entry of a protected unit (see 9.11)
Swap.Temp --
a variable declared in a block statement (see 5.6)
Standard.Boolean --
the name of a predefined type (see A.1)Extensions to Ada 83
19.a
{
extensions to Ada 83}
We
now allow an expanded name to use a prefix that denotes a rename of a
package, even if the selector is for an entity local to the body or private
part of the package, so long as the entity is visible at the place of
the reference. This eliminates a preexisting anomaly where references
in a package body may refer to declarations of its visible part but not
those of its private part or body when the prefix is a rename of the
package.
Wording Changes from Ada 83
19.b
The syntax rule for
selector_name
is new. It is used in places where visibility, but not necessarily direct
visibility, is required. See
4.1, “
Names”
for more information.
19.c
The description of dereferencing an access type
has been moved to
4.1, “
Names”;
name.
all is no longer considered a
selected_component.
19.d
The rules have been restated to be consistent
with our new terminology, to accommodate class-wide types, etc.
Extensions to Ada 95
19.e/2
{
AI95-00252-01}
{extensions to Ada 95} The
prefixed view notation for tagged objects is new. This provides a similar
notation to that used in other popular languages, and also reduces the
need for use_clauses. This is sometimes known
as “distinguished receiver notation”. {distinguished receiver
notation} 19.f/2
Given
the following definitions for a tagged type T:
19.g/2
procedure Do_Something (Obj : in out T; Count : in Natural);
procedure Do_Something_Else (Obj : access T; Flag : in Boolean);
My_Object : aliased T;
19.h/2
the
following calls are equivalent:
19.i/2
Do_Something (My_Object, Count => 10);
My_Object.Do_Something (Count => 10);
19.j/2
as
are the following calls:
19.k/2
Do_Something_Else (My_Object'Access, Flag => True);
My_Object.Do_Something_Else (Flag => True);
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