Digital Mars - c++ - Another template strangeness

↑ ↓ ← → Iliya Peregoudov <iliyap mail.ru> writes:

The idea of the following sample code is simple.  First, we keep track
of matrix features (square, diagonal, column, or row) as a type.
Second,  we want the type to divert minimally in the process of matrix
multiplication.  Specifically, this code tries to differentiate
multiplication of column matrix by row matrix into two cases:

1) when column matrix and row matrix are the same size the result is
    square matrix,
2) otherwise the result is general (rectangular) matrix.

This code compiles cleanly by g++ 2.96 and KAI C++ 4.0.  dmc gives
numerious errors about multiple non-identical declarations of
operator*.

/* matrix4.cpp */
#include <iostream.h>

template <class T, int n> class ColMatrix;
template <class T, int m> class RowMatrix;

template <class T, int n, int m>
class Matrix {
protected:
    T v[n][m];
public:
    template <class TT, int nn, int mm>
    friend Matrix<TT,nn,mm> operator*(const ColMatrix<TT,nn> &, const 
RowMatrix<TT,mm> &);
};

template <class T, int n>
class SquareMatrix: public Matrix<T,n,n> {
public:
    template <class TT, int nn>
    friend SquareMatrix<TT,nn> operator*(const ColMatrix<TT,nn> &, const 
RowMatrix<TT,nn> &);
};

template <class T, int n>
class ColMatrix: public Matrix<T,n,1> {
public:
    template <class TT, int nn, int mm>
    friend Matrix<TT,nn,mm> operator*(const ColMatrix<TT,nn> &, const 
RowMatrix<TT,mm> &);
    template <class TT, int nn>
    friend SquareMatrix<TT,nn> operator*(const ColMatrix<TT,nn> &, const 
RowMatrix<TT,nn> &);
};

template <class T, int m>
class RowMatrix: public Matrix<T,1,m> {
public:
    template <class TT, int nn, int mm>
    friend Matrix<TT,nn,mm> operator*(const ColMatrix<TT,nn> &, const 
RowMatrix<TT,mm> &);
    template <class TT, int nn>
    friend SquareMatrix<TT,nn> operator*(const ColMatrix<TT,nn> &, const 
RowMatrix<TT,nn> &);
};

template <class TT, int nn, int mm>
Matrix<TT,nn,mm> operator*(const ColMatrix<TT,nn> &a, const 
RowMatrix<TT,mm> &b)
{
    Matrix<TT,nn,mm> r;
    for (int i = 0; i < nn; ++i)
       for (int j = 0; j < mm; ++j)
          r.v[i][j] = a.v[i][0]*b.v[0][j];
    return r;
}

template <class TT, int nn>
SquareMatrix<TT,nn> operator*(const ColMatrix<TT,nn> &a, const 
RowMatrix<TT,nn> &b)
{
    SquareMatrix<TT,nn> r;
    for (int i = 0; i < nn; ++i)
       for (int j = 0; j < nn; ++j)
          r.v[i][j] = a.v[i][0]*b.v[0][j];
    return r;
}

int main()
{
    ColMatrix<int,2> m1;
    RowMatrix<int,2> m2;
    m1*m2;  // should give SquareMatrix

    return 0;
}

Sep 23 2002

↑ ↓ ← → Iliya Peregoudov <iliyap mail.ru> writes:

Walter wrote:
 The fix is posted now. -Walter


Ok, but if we change main() in the previous example to

int main()
{
    ColMatrix<int,2> m1;
    RowMatrix<int,2> m2;
    RowMatrix<int,3> m3;
    m1*m2;  // should give SquareMatrix
    m1*m3;  // should give Matrix

    return 0;
}

compiler say:

    m1*m3;  // should give Matrix
         ^
matrix4.cpp(66) : Error: need explicit cast for function parameter 1 to get
from: ColMatrix<int ,2>
to  : const ColMatrix<int ,3>

It seems that compiler doesn't see operator*() that return rectangular 
matrix.  [Why does it want to cast the first operator*() argument?  The 
second one should be the choice of the very same priority...]

Sep 24 2002

↑ ↓ ← → "Walter" <walter digitalmars.com> writes:

"Mark Evans" <Mark_member pathlink.com> wrote in message
news:amqq01$u2i$1 digitaldaemon.com...
 Just a thank you to Iliya for exploring these matters, and Walter for


 them!


Welcs!

 Hey Walter be sure to put even the most minor patches on c++-announce...I


 it all the time...I'd like to know what goes on between X.YY.ZZ.01 and
 X.YY.ZZ.02.


Done!

Sep 24 2002

↑ ↓ ← → "Walter" <walter digitalmars.com> writes:

"Iliya Peregoudov" <iliyap mail.ru> wrote in message
news:3D91F652.3060300 mail.ru...
 The fix for that is posted now too <g>.


 P.S.: I'm really amazed by the how fast all of this was fixed out.
 Looks more like a magic!


What was wrong was the template partial ordering code was well tested for
type arguments, but apparently none of the stlport tests tested it with
value arguments. So once I figured out what was going wrong, it wasn't too
hard to fix.

Sep 25 2002