7.3.1 Private Operations
[For a type declared in the visible part of a package
or generic package, certain operations on the type do not become visible
until later in the package — either in the private part or the
body.
{private operations}
Such
private operations are available only
inside the declarative region of the package or generic package.]
Static Semantics
The predefined operators that exist for a given type
are determined by the classes to which the type belongs. For example,
an integer type has a predefined "+" operator. In most cases,
the predefined operators of a type are declared immediately after the
definition of the type; the exceptions are explained below. Inherited
subprograms are also implicitly declared immediately after the definition
of the type, except as stated below.
{
8652/0019}
{
AI95-00033-01}
For a composite type, the characteristics (see
7.3)
of the type are determined in part by the characteristics of its component
types. At the place where the composite type is declared, the only characteristics
of component types used are those characteristics visible at that place.
If later immediately within the declarative region in which the composite
type is declared additional characteristics become visible for a component
type, then any corresponding characteristics become visible for the composite
type. Any additional predefined operators are implicitly declared at
that place.
{
8652/0019}
{
AI95-00033-01}
The corresponding rule applies to a type defined by a
derived_type_definition,
if there is a place immediately within the declarative region in which
the type is declared where additional characteristics of its parent type
become visible.
{
8652/0019}
{
AI95-00033-01}
{become nonlimited} {nonlimited
type (becoming nonlimited)} {limited
type (becoming nonlimited)} [For example,
an array type whose component type is limited private becomes nonlimited
if the full view of the component type is nonlimited and visible at some
later place immediately within the declarative region in which the array
type is declared. In such a case, the predefined "=" operator
is implicitly declared at that place, and assignment is allowed after
that place.]
{
8652/0019}
{
AI95-00033-01}
Inherited primitive subprograms follow a different rule. For a
derived_type_definition,
each inherited primitive subprogram is implicitly declared at the earliest
place, if any, immediately within the declarative region in which the
type_declaration
occurs, but after the
type_declaration,
where the corresponding declaration from the parent is visible. If there
is no such place, then the inherited subprogram is not declared at all.
[An inherited subprogram that is not declared at all cannot be named
in a call and cannot be overridden, but for a tagged type, it is possible
to dispatch to it.]
For a
private_extension_declaration,
each inherited subprogram is declared immediately after the
private_extension_declaration
if the corresponding declaration from the ancestor is visible at that
place. Otherwise, the inherited subprogram is not declared for the private
extension, [though it might be for the full type].
Discussion: The above rules matter only
when the component type (or parent type) is declared in the visible part
of a package, and the composite type (or derived type) is declared within
the declarative region of that package (possibly in a nested package
or a child package).
Consider:
package Parent is
type Root is tagged null record;
procedure Op1(X : Root);
type My_Int is range 1..10;
private
procedure Op2(X : Root);
type Another_Int is new My_Int;
procedure Int_Op(X : My_Int);
end Parent;
with Parent; use Parent;
package Unrelated is
type T2 is new Root with null record;
procedure Op2(X : T2);
end Unrelated;
package Parent.Child is
type T3 is new Root with null record;
-- Op1(T3) implicitly declared here.
package Nested is
type T4 is new Root with null record;
private
...
end Nested;
private
-- Op2(T3) implicitly declared here.
...
end Parent.Child;
with Unrelated; use Unrelated;
package body Parent.Child is
package body Nested is
-- Op2(T4) implicitly declared here.
end Nested;
type T5 is new T2 with null record;
end Parent.Child;
Another_Int does not inherit Int_Op, because
Int_Op does not “exist” at the place where Another_Int is
declared.
Type T2 inherits Op1 and Op2 from Root. However,
the inherited Op2 is never declared, because Parent.Op2 is never visible
immediately within the declarative region of T2. T2 explicitly declares
its own Op2, but this is unrelated to the inherited one — it does
not override the inherited one, and occupies a different slot in the
type descriptor.
T3 inherits both Op1 and Op2. Op1 is implicitly
declared immediately after the type declaration, whereas Op2 is declared
at the beginning of the private part. Note that if Child were a private
child of Parent, then Op1 and Op2 would both be implicitly declared immediately
after the type declaration.
T4 is similar to T3, except that the earliest
place immediately within the declarative region containing T4 where Root's
Op2 is visible is in the body of Nested.
If T3 or T4 were to declare a type-conformant
Op2, this would override the one inherited from Root. This is different
from the situation with T2.
T5 inherits Op1 and two Op2's from T2. Op1 is
implicitly declared immediately after the declaration of T5, as is the
Op2 that came from Unrelated.Op2. However, the Op2 that originally came
from Parent.Op2 is never implicitly declared for T5, since T2's version
of that Op2 is never visible (anywhere — it never got declared
either).
For all of these rules, implicit private parts
and bodies are assumed as needed.
It is possible
for characteristics of a type to be revealed in more than one place:
package P is
type Comp1 is private;
private
type Comp1 is new Boolean;
end P;
package P.Q is
package R is
type Comp2 is limited private;
type A is array(Integer range <>) of Comp2;
private
type Comp2 is new Comp1;
-- A becomes nonlimited here.
-- "="(A, A) return Boolean is implicitly declared here.
...
end R;
private
-- Now we find out what Comp1 really is, which reveals
-- more information about Comp2, but we're not within
-- the immediate scope of Comp2, so we don't do anything
-- about it yet.
end P.Q;
package body P.Q is
package body R is
-- Things like "xor"(A,A) return A are implicitly
-- declared here.
end R;
end P.Q;
{
8652/0019}
{
AI95-00033-01}
We say
immediately within the declarative region in order that
types do not gain operations within a nested scope. Consider:
package Outer is
package Inner is
type Inner_Type is private;
private
type Inner_Type is new Boolean;
end Inner;
type Outer_Type is array(Natural range <>) of Inner.Inner_Type;
end Outer;
package body Outer is
package body Inner is
-- At this point, we can see that Inner_Type is a Boolean type.
-- But we don't want Outer_Type to gain an "and" operator here.
end Inner;
end Outer;
[The Class attribute
is defined for tagged subtypes in
3.9. In addition,]
for every subtype S of an untagged private type whose full view is tagged,
the following attribute is defined:
S'Class
Denotes the class-wide subtype
corresponding to the full view of S. This attribute is allowed only from
the beginning of the private part in which the full view is declared,
until the declaration of the full view. [After the full view, the Class
attribute of the full view can be used.]
9 Because a partial view and a full view
are two different views of one and the same type, outside of the defining
package the characteristics of the type are those defined by the visible
part. Within these outside program units the type is just a private type
or private extension, and any language rule that applies only to another
class of types does not apply. The fact that the full declaration might
implement a private type with a type of a particular class (for example,
as an array type) is relevant only within the declarative region of the
package itself including any child units.
The consequences of this actual implementation are,
however, valid everywhere. For example: any default initialization of
components takes place; the attribute Size provides the size of the full
view; finalization is still done for controlled components of the full
view; task dependence rules still apply to components that are task objects.
10 {
AI95-00287-01}
Partial views provide initialization, membership tests, selected components
for the selection of discriminants and inherited components, qualification,
and explicit conversion. Nonlimited partial views also allow use of
assignment_statements.
11 For a subtype S of a partial view, S'Size
is defined (see
13.3). For an object A of
a partial view, the attributes A'Size and A'Address are defined (see
13.3). The Position, First_Bit, and Last_Bit
attributes are also defined for discriminants and inherited components.
Examples
Example of a type
with private operations:
package Key_Manager
is
type Key
is private;
Null_Key :
constant Key; --
a deferred constant declaration (see 7.4)
procedure Get_Key(K :
out Key);
function "<" (X, Y : Key)
return Boolean;
private
type Key
is new Natural;
Null_Key :
constant Key := Key'First;
end Key_Manager;
package body Key_Manager is
Last_Key : Key := Null_Key;
procedure Get_Key(K : out Key) is
begin
Last_Key := Last_Key + 1;
K := Last_Key;
end Get_Key;
function "<" (X, Y : Key) return Boolean is
begin
return Natural(X) < Natural(Y);
end "<";
end Key_Manager;
12 Notes on the example: Outside
of the package Key_Manager, the operations available for objects of type
Key include assignment, the comparison for equality or inequality, the
procedure Get_Key and the operator "<"; they do not include
other relational operators such as ">=", or arithmetic operators.
The explicitly declared operator "<"
hides the predefined operator "<" implicitly declared by
the
full_type_declaration.
Within the body of the function, an explicit conversion of X and Y to
the subtype Natural is necessary to invoke the "<" operator
of the parent type. Alternatively, the result of the function could be
written as not (X >= Y), since the operator ">=" is not
redefined.
The value of the variable Last_Key, declared in the
package body, remains unchanged between calls of the procedure Get_Key.
(See also the NOTES of
7.2.)
Wording Changes from Ada 83
The phrase in RM83-7.4.2(7), “...after
the full type declaration”, doesn't work in the presence of child
units, so we define that rule in terms of visibility.
The definition of the Constrained attribute
for private types has been moved to “Obsolescent Features.”
(The Constrained attribute of an object has not been moved there.)
Wording Changes from Ada 95
Sponsored by Ada-Europe