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.
]
Syntax
2
3
Name Resolution Rules
4
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
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
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:9.2/3
{
AI95-00252-01}
{
AI95-00407-01}
{
AI05-0090-1}
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 subprogram shall not be an implicitly
declared primitive operation of type T that overrides an inherited
subprogram implemented by an entry or protected subprogram 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. 9.b/3
Discussion: {
AI05-0090-1}
The part of the rule that excludes a primitive
overriding subprogram as a selector applies only to the wrapper subprogram
that is implicitly declared to override a subprogram inherited from a
synchronized interface that is implemented by an operation of a task
or protected type (see 9.1 and 9.4).
We don't want calls that use a prefixed view to be ambiguous between
the wrapper subprogram and the implementing entry or protected operation.
Note that it is illegal to declare an explicit primitive that has a prefixed
view that is homographic with one of the type's operations, so in normal
cases it isn't possible to have an ambiguity in a prefix call. However,
a class-wide operation of an ancestor type that is declared in the same
declaration list with the ancestor type is also considered, and that
can still make a call ambiguous.
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
15
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.
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
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
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}
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”.
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);
Wording Changes from Ada 2005
19.l/3
{
AI05-0090-1}
Correction: Corrected the definition of
a prefixed view to ignore the implicit subprograms declared for “implemented
by” entries and protected subprograms.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
