Annotated Ada Reference ManualLegal Information
Contents   Index   References   Search   Previous   Next 

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. 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} {AI05-0029-1} 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 within the immediate scope of the composite type 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. If there is no such place, then additional predefined operators are not declared at all, but they still exist.
Reason: {AI05-0029-1} We say that the predefined operators exist because they can emerge in some unusual generic instantiations. See 12.5.
Discussion: {AI05-0029-1} The predefined operators for the underlying class of a type always exist, even if there is no visibility on that underlying class. This rule is simply about where (if ever) those operators are declared (and thus become usable). The “additional predefined operators” defined by this rule are any that are not declared at the point of the original type declaration. For instance, a type derived from a private type whose full type is type String always will have a ">" operator, but where that operator is declared (and thus whether it is visible) will depend on the visibility of the full type of the parent type. 
{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 within its immediate scope where additional characteristics of its parent type become visible.
{8652/0019} {AI95-00033-01} [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. within the immediate scope of the array type. In such a case, the predefined "=" operator is implicitly declared at that place, and assignment is allowed after that place.]
  {AI05-0115-1} {AI05-0269-1} A type is a descendant of the full view of some ancestor of its parent type only if the current view it has of its parent is a descendant of the full view of that ancestor. More generally, at any given place, a type is descended from the same view of an ancestor as that from which the current view of its parent is descended. This view determines what characteristics are inherited from the ancestor[, and, for example, whether the type is considered to be a descendant of a record type, or a descendant only through record extensions of a more distant ancestor].
  {AI05-0115-1} [It is possible for there to be places where a derived type is visibly a descendant of an ancestor type, but not a descendant of even a partial view of the ancestor type, because the parent of the derived type is not visibly a descendant of the ancestor. In this case, the derived type inherits no characteristics from that ancestor, but nevertheless is within the derivation class of the ancestor for the purposes of type conversion, the "covers" relationship, and matching against a formal derived type. In this case the derived type is considered to be a descendant of an incomplete view of the ancestor.]
Discussion: Here is an example of this situation: 
package P is
   type T is private;
   C : constant T;
   type T is new Integer;
   C : constant T := 42;
end P;
with P;
package Q is
    type T2 is new P.T;
end Q;
with Q;
package P.Child is
    type T3 is new Q.T2;
    Int : Integer := 52;
    V : T3 := T3(P.C);  -- Legal: conversion allowed
    W : T3 := T3(Int);  -- Legal: conversion allowed
    X : T3 := T3(42);   -- Error: T3 is not a numeric type
    Y : T3 := X + 1;    -- Error: no visible "+" operator
    Z : T3 := T3(Integer(W) + 1);   -- Legal: convert to Integer first
end P.Child;
{8652/0019} {AI95-00033-01} {AI05-0029-1} 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 within the immediate scope of 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, but it still exists. [For a tagged type, it is possible to dispatch to an An inherited subprogram that is not declared at allcannot 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]. 
Reason: There is no need for the “earliest place immediately within the declarative region within the immediate scope” business here, because a private_extension_declaration will be completed with a full_type_declaration, so we can hang the necessary private implicit declarations on the full_type_declaration.
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).
package Parent is
    type Root is tagged null record;
    procedure Op1(X : Root);
    type My_Int is range 1..10;
    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;
    end Nested;
    -- 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 within the immediate scope 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 within T4's immediate scope 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;
    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;
        type Comp2 is new Comp1;
        -- A becomes nonlimited here.
        -- "="(A, A) return Boolean is implicitly declared here.
    end R;
    -- 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;
        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: 
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 assignment (unless the view is limited), 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.


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;
   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
      Last_Key := Last_Key + 1;
      K := Last_Key;
   end Get_Key;
   function "<" (X, Y : Key) return Boolean is
      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

{8652/0018} {AI95-00033-01} Corrigendum: Clarified when additional operations are declared.
{AI95-00287-01} Revised the note on operations of partial views to reflect that limited types do have an assignment operation, but not assignment_statements.

Wording Changes from Ada 2005

{AI05-0029-1} Correction: Revised the wording to say that predefined operations still exist even if they are never declared, because it is possible to reference them in a generic unit.
{AI05-0115-1} Correction: Clarified that the characteristics of a descendant of a private type depend on the visibility of the full view of the direct ancestor. This has to be the case (so that privacy is not violated), but it wasn't spelled out in earlier versions of Ada.

Contents   Index   References   Search   Previous   Next 
Ada-Europe Ada 2005 and 2012 Editions sponsored in part by Ada-Europe