8.2 Scope of Declarations
[For each declaration, the language rules define
a certain portion of the program text called the scope of the
declaration. The scope of a declaration is also called the scope of any
view or entity declared by the declaration. Within the scope of an entity,
and only there, there are places where it is legal to refer to the declared
entity. These places are defined by the rules of visibility and overloading.]
Static Semantics
{immediate
scope (of a declaration)} The
immediate
scope of a declaration is a portion of the declarative region immediately
enclosing the declaration. The immediate scope starts at the beginning
of the declaration, except in the case of an overloadable declaration,
in which case the immediate scope starts just after the place where the
profile of the callable entity is determined (which is at the end of
the
_specification for the callable entity,
or at the end of the
generic_instantiation
if an instance). The immediate scope extends to the end of the declarative
region, with the following exceptions:
Reason: The reason for making overloadable
declarations with profiles special is to simplify compilation: until
the compiler has determined the profile, it doesn't know which other
declarations are homographs of this one, so it doesn't know which ones
this one should hide. Without this rule, two passes over the
_specification
or
generic_instantiation
would be required to resolve names that denote things with the same name
as this one.
The immediate scope of a
library_item
includes only its semantic dependents.
Reason: Section 10 defines only a partial
ordering of
library_items.
Therefore, it is a good idea to restrict the immediate scope (and the
scope, defined below) to semantic dependents.
Consider also
examples like this:
package P is end P;
package P.Q is
I : Integer := 0;
end P.Q;
with P;
package R is
package X renames P;
J : Integer := X.Q.I; -- Illegal!
end R;
The scope of P.Q does not contain R. Hence,
neither P.Q nor X.Q are visible within R. However, the name R.X.Q would
be visible in some other library unit where both R and P.Q are visible
(assuming R were made legal by removing the offending declaration).
Ramification: {
AI95-00217-06}
This rule applies to limited views as well as “normal” library
items. In that case, the semantic dependents are the units that have
a
limited_with_clause
for the limited view.
The immediate scope of a declaration in the private
part of a library unit does not include the visible part of any public
descendant of that library unit.
{descendant
(relationship with scope) [partial]} Ramification: In other words, a declaration
in the private part can be visible within the visible part, private part
and body of a private child unit. On the other hand, such a declaration
can be visible within only the private part and body of a public child
unit.
Reason: The purpose of this rule is to
prevent children from giving private information to clients.
Ramification: {
AI95-00231-01}
For a public child subprogram, this means that the parent's private part
is not visible in the profile of the declaration and of the body. This
is true even for
subprogram_bodies that are
not completions. For a public child generic unit, it means that the parent's
private part is not visible in the
generic_formal_part,
as well as in the first list of
basic_declarative_items
(for a generic package), or the (syntactic) profile (for a generic subprogram).
{visible part}
[The
visible part of (a view of) an entity
is a portion of the text of its declaration containing declarations that
are visible from outside.]
{private part
[distributed]} The
private part of
(a view of) an entity that has a visible part contains all declarations
within the declaration of (the view of) the entity, except those in the
visible part; [these are not visible from outside. Visible and private
parts are defined only for these kinds of entities: callable entities,
other program units, and composite types.]
{visible part (of
a view of a callable entity) [partial]} The
visible part of a view of a callable entity is its profile.
{visible part (of
a view of a composite type) [partial]} The
visible part of a composite type other than a task or protected type
consists of the declarations of all components declared [(explicitly
or implicitly)] within the
type_declaration.
Reason: Although there is no way to reference
anything but the formals from outside a generic unit, they are still
in the visible part in the sense that the corresponding declarations
in an instance can be referenced (at least in some cases). In other words,
these declarations have an effect on the outside world. The visible part
of a generic unit needs to be defined this way in order to properly support
the rule that makes a parent's private part invisible within a public
child's visible part.
Ramification: The visible part of an
instance of a generic unit is as defined for packages and subprograms;
it is not defined in terms of the visible part of a generic unit.
[The visible part of a package, task unit, or protected
unit consists of declarations in the program unit's declaration other
than those following the reserved word
private, if any; see
7.1
and
12.7 for packages,
9.1
for task units, and
9.4 for protected units.]
{scope (of a declaration)}
The scope of a declaration always contains the immediate
scope of the declaration. In addition, for a given declaration that occurs
immediately within the visible part of an outer declaration, or is a
public child of an outer declaration, the scope of the given declaration
extends to the end of the scope of the outer declaration, except that
the scope of a
library_item
includes only its semantic dependents.
Ramification: Note the recursion. If
a declaration appears in the visible part of a library unit, its scope
extends to the end of the scope of the library unit, but since that only
includes dependents of the declaration of the library unit, the scope
of the inner declaration also only includes those dependents. If X renames
library package P, which has a child Q, a
with_clause
mentioning P.Q is necessary to be able to refer to X.Q, even if P.Q is
visible at the place where X is declared.
{immediate scope
(of (a view of) an entity)} The immediate
scope of a declaration is also the immediate scope of the entity or view
declared by the declaration.
{scope (of
(a view of) an entity)} Similarly, the
scope of a declaration is also the scope of the entity or view declared
by the declaration.
Ramification:
The rule for immediate scope implies the following:
If the declaration is that of a library unit,
then the immediate scope includes the declarative region of the declaration
itself, but not other places, unless they are within the scope of a
with_clause
that mentions the library unit.
It is necessary to attach the semantics
of
with_clauses
to [immediate] scopes (as opposed to visibility), in order for various
rules to work properly. A library unit should hide a homographic implicit
declaration that appears in its parent, but only within the scope of
a
with_clause
that mentions the library unit. Otherwise, we would violate the "legality
determinable via semantic dependences" rule of
10,
“
Program Structure and Compilation Issues”.
The declaration of a library unit should be allowed to be a homograph
of an explicit declaration in its parent's body, so long as that body
does not mention the library unit in a
with_clause.
This means that one cannot denote
the declaration of the library unit, but one might still be able to denote
the library unit via another view.
A
with_clause
does not make the declaration of a library unit visible; the lack of
a
with_clause
prevents it from being visible. Even if a library unit is mentioned in
a
with_clause,
its declaration can still be hidden.
The completion of the declaration of a library
unit (assuming that's also a declaration) is not visible, neither directly
nor by selection, outside that completion.
The immediate scope of a declaration immediately
within the body of a library unit does not include any child of that
library unit.
This is needed to prevent children
from looking inside their parent's body. The children are in the declarative
region of the parent, and they might be after the parent's body. Therefore,
the scope of a declaration that occurs immediately within the body might
include some children.
4 There are notations for denoting visible
declarations that are not directly visible. For example,
parameter_specifications
are in the visible part of a
subprogram_declaration
so that they can be used in named-notation calls appearing outside the
called subprogram. For another example, declarations of the visible part
of a package can be denoted by expanded names appearing outside the package,
and can be made directly visible by a
use_clause.
Ramification: {
AI95-00114-01}
There are some obscure cases involving generics in which there is no
such notation. See Section 12.
Extensions to Ada 83
{
extensions to Ada 83}
The
fact that the immediate scope of an overloadable declaration does not
include its profile is new to Ada 95. It replaces RM83-8.3(16), which
said that within a subprogram specification and within the formal part
of an entry declaration or accept statement, all declarations with the
same designator as the subprogram or entry were hidden from all visibility.
The RM83-8.3(16) rule seemed to be overkill, and created both implementation
difficulties and unnecessary semantic complexity.
Wording Changes from Ada 83
The notion of "visible part" has been
extended in Ada 95. The syntax of task and protected units now allows
private parts, thus requiring us to be able to talk about the visible
part as well. It was necessary to extend the concept to subprograms and
to generic units, in order for the visibility rules related to child
library units to work properly. It was necessary to define the concept
separately for generic formal packages, since their visible part is slightly
different from that of a normal package. Extending the concept to composite
types made the definition of scope slightly simpler. We define visible
part for some things elsewhere, since it makes a big difference to the
user for those things. For composite types and subprograms, however,
the concept is used only in arcane visibility rules, so we localize it
to this clause.
In Ada 83, the semantics of
with_clauses
was described in terms of visibility. It is now described in terms of
[immediate] scope.
We have clarified
that the following is illegal (where Q and R are library units):
package Q is
I : Integer := 0;
end Q;
package R is
package X renames Standard;
X.Q.I := 17; -- Illegal!
end R;
even though Q is declared in the declarative
region of Standard, because R does not mention Q in a
with_clause.
Wording Changes from Ada 95
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