Re: Inheritance problem!

[Adam Beneschan <adam@irvine.com>]

On Dec 18, 7:14 am, AndreiK <andrei.krivos...@gmail.com> wrote:
> Hello everybody!
>
> I am quite new in Ada programming and I am trying to write simple (at
> the moment) OO program, but I have a problem with inheritance of the
> private type. I am using the GNAT+GPS (2008) and learning Ada by John
> Barnes "Ada 2005".
>
> The idea of the program is next: we have tagged private types Vector
> and Matrix. The Matrix inheritances all components and functions from
> Vector. Difference only in constructor: Mat is for Matrix and Vec is
> for Vector.
>
> In the test program the errors occures when I try to call inherited
> function, in the example it is Cell(...) and if I try to use function
> Put(This: Vect.Vector'Class).
>
> I don't understand where the mistake is?

The program isn't compiling because Matrix is a private type.  In
Vectors. Matrices (and therefore also in every instance of it), Matrix
is declared as private.  In the PRIVATE part, Matrix is declared as a
"new Vector"; however, other packages that use Vectors.Matrices don't
get to know that Matrix is actually a "Vector".  So they won't know
that the Cell function can be used on it, since Cell is for vectors,
but they don't know that a Matrix is a Vector.  And they won't be able
to use Put on it, because Put is defined for objects of types in
Vector'Class, but they don't know that a Matrix is a Vector and so
they won't know that Put can be used on it.

Most likely, what you want is for packages that use Vectors.Matrices
to know that a Matrix is a kind of Vector, but you want to keep all
the other implementation details hidden.  If that's the case, then
replace

   type Matrix is tagged private;

with

   type Matrix is new Vector with private;

                             -- Adam



>
> Thanks everybody for a help!
>
> The code of the test program:
>
> ----------------------------------------
> --   test.adb
> ----------------------------------------
> with Ada.Text_IO;
> with Vectors.Matrices;
>
> procedure test is
>
>    package TIO renames Ada.Text_IO;
>    package FIO is new Ada.Text_IO.Float_IO(Float);
>
>    package Vect is new Vectors(Float); --use Vect;
>    package Matr is new Vect.Matrices;-- use Matr;
>    M1: Matr.Matrix := Matr.Mat(4, 3, 1.0);
>    V1: Vect.Vector := Vect.Vec(3, 2.0);
>
>    --  Procedure, which intended to print out the Vector or Matrix
> objects.
>    --
>    procedure Put(This: in Vect.Vector'Class) is
>    begin
>       TIO.New_Line;
>       TIO.New_Line;
>       for J in 0 .. This.Size(0)-1 loop
>          TIO.Put("|");
>          for K in 0 .. This.Size(1)-1 loop
>             FIO.Put( Item => This.Cell(J, K),
>                     Fore => 2,
>                     Aft  => 3,
>                     Exp  => 3);
>             TIO.Put(" ");
>          end loop;
>          TIO.Put("|");
>          TIO.New_Line;
>       end loop;
>       null;
>    end;
>
> begin
>
>    FIO.Put(M1.Cell(0,0));
>    -- test.adb:37:14: error:
>    -- no selector "Cell" for private type "Matrix"
>    -- defined at vectors-matrices.ads:6, instance at line 10
>
>    Put(M1);
>    -- test.adb:39:08: error: expected type "Vector'class"
>    -- defined at vectors.ads:12, instance at line 9
>
>    -- test.adb:39:08: error: found private type "Matrix"
>    -- defined at vectors-matrices.ads:6, instance at line 10
>
>    -- test.adb:39:08: error:   ==> in call to "Put" at line 14
>
>    Put(V1);
>    -- No Errors for Vector V1.
>
>    FIO.Put(V1.Cell(0,0));
>    -- No Errors for Vector V1.
>
>    TIO.New_Line;
>
> end test;
>
> ----------------------------------------
> --  vectors.ads
> ----------------------------------------
> generic
>    type Real is digits <>;
>
> package Vectors is
>    pragma Preelaborate;
>
>    subtype Dim_Type is Natural;
>    type Sizes_Arr is array(Integer range <>) of Dim_Type;
>
>    --
>    --     The type <Vector> is private
>    --
>    type Vector is tagged private;
>    pragma Preelaborable_Initialization (Vector);
>
>    --
>    --  Vec - the constructor of the entity of type Vector.
>    --
>    function Vec(Elem_Num: Dim_Type; Fill_by_Value: Real'Base := 0.0)
> return Vector;
>
>    --
>    --  Function Cell returns the value of the element at position
> Index
>    --  of the object of the type Vector.
>    --
>    function Cell(
>                  This : in Vector;
>                  Row_Index: in Natural;
>                  Col_Index: in Natural
>                 ) return Real'Base;
>
>    function Row_Len(This: in Vector) return Natural;
>    function Col_Len(This: in Vector) return Natural;
>
>    --
>    --  Function Size returns the array (1D) of the dimension's sizes
> of the object
>    --  of the type Vector.
>    --
>    function Size(This: in Vector) return Sizes_Arr;
>
> private
>
>    type Elements_Array is array (Natural range <>) of Real'Base;
>    type Elements_Access is not null access Elements_Array;
>
>    type Vector is tagged
>       record
>          Elements_ac        : Elements_Access;
>          Count              : Natural := 0;
>          Sizes              : Sizes_Arr(0..1);
>          Transposed         : Boolean := False;
>          Busy               : Natural := 0;
>       end record;
>
> end Vectors;
>
> ----------------------------------------
> -- vectors-matrices.ads
> ----------------------------------------
> generic
>
> package Vectors.Matrices is
>    pragma Preelaborate;
>
>    type Matrix is tagged private;
>    pragma Preelaborable_Initialization (Matrix);
>
>    function Mat(Row_Num,Col_Num: in Dim_Type; Fill_by_Value:
> Real'Base := 0.0) return Matrix;
>
> private
>
>    type Matrix is new Vector with null record;
>
> end Vectors.Matrices;