CODEWITHSUNDEEP
c++templatesmatrix
moongoal
moongoal

Reputation: 2956

Multiply C++ template matrix

I have a matrix class defined this way:

template<int M, int N, typename T>
class Matrix
{
 typedef Matrix<M, N, T> MTYPE;
 /*...*/
};

I have to implement the matrix multiplication but I do not know how to do the operator overriding..

Something like

MTYPE operator *(MTYPE& m) { /*...*/ }

Would accept only a N*M matrix. So how can I overcome this problem?

Upvotes: 2

Views: 2087

Answers (2)

Mike Bailey
Mike Bailey

Reputation: 12817

You'll need to create a templated operator, either inside or outside of the class.

For example, to multiply a N x M by a M x M you might want to do:

  template <int N, int M, class T>
  friend Matrix<N, M, T> operator*(const Matrix<N, M, T> &lhs, const Matrix<M, M, T> &rhs);

Other versions look similar. It'd probably most useful to define this outside of the class.

To multiply a (N1 x M) by (M x N2) you'd do:

 template<int N1, int N2, int M, class T>
 friend Matrix<N1, N2, T> operator*(const Matrix<N1, M, T> &lhs, const Matrix<M, N2, T> &rhs);

Upvotes: 3

wolfgang
wolfgang

Reputation: 4953

As has been pointed out in a comment, *= doesn't make sense for non-square matrices.

For the general case,

template<int M, int N, typename T>
class Matrix
{
    typedef Matrix<M, N, T> MTYPE;
    /*...*/
public:
    template<int L>
    Matrix<M,L,T> operator*(const Matrix<N,L,T>& second) const
    {
       Matrix<M,L,T> result;
       for(...)
           for(...)
               for(...)
                   // ...
       return result;
    }
};

Or, if you prefer, use a free function operator* with two parameters (and template arguments M,N,L, and T), and make it a friend of your matrix class.

Upvotes: 3

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