C++11 templates, determining return type

Question

I am building a matrix library and I am trying to use the policy-based design. So my base classes are classes that provide a storage method and some access functions. I also have a function matrix which provides the mathematical functions. This works great, but there is a major problem with the operator* because of the return type. I will explain it with some code.

Base class that provides a stack storage :

template < typename T, unsigned int rows, unsigned int cols>
class denseStackMatrix {
public:
    typedef T value_type;

private:
    value_type grid[rows][cols];
    const unsigned int rowSize;
    const unsigned int colSize;

Then I have my matrix class which provides mathematical functionality :

template <typename MatrixContainer >
class matrix : public MatrixContainer {
public:
    typedef MatrixContainer Mcontainer;

    matrix<Mcontainer>& operator +(const matrix<Mcontainer>&);
    matrix<Mcontainer>& operator *(const matrix<Mcontainer>&);

operator+ always works, operator* only works for square matrix. So we still need one for all matrices. And that's were it goes wrong. I have already tried few things, but nothings works. I look for something like this, with the help of c++0x (usage of c++0x is not a requirement) you shall notice the "???" :)

friend auto operator * (const matrix<T1>& matrix1, const matrix<T2>& matrix2)
-> decltype(matrix<???>);

An example of the problem

matrix<denseStackMatrix<int,3,2> > matrix1;
matrix<denseStackMatrix<int,2,4> > matrix2;
matrix<denseStackMatrix<int,3,4> > matrix3 = matrix1 * matrix2;

Here it will complain about the type, because it does not match any of the two parameter types. But the compiler needs to know the type at compile-time and I do not know how to provide it.

I know there are other options for the design, but I am really looking for a solution for this scenario..

Thank you !

Answer 1

Picking up on the idea of @hammar, but with partial specialization to allow the normal syntax like the question shows:

template<class MatrixContainer>
class matrix;

template<
  template<class,int,int> class MatrixContainer,
  class T, int rows, int cols
>
class matrix< MatrixContainer<T,rows,cols> >{
  typedef MatrixContainer<T,rows,cols> Mcontainer;
  typedef matrix<Mcontainer> this_type;
  static int const MyRows = rows;
  static int const MyCols = cols;

public:
  template<int OtherCols>
  matrix<MatrixContainer<T,MyRows,OtherColls> > operator*(matrix<MatrixContainer<T,MyCols,OtherCols> > const& other){
    typedef matrix<MatrixContainer<T,MyCols,OtherCols> > other_type;
    typedef matrix<MatrixContainer<T,MyRows,OtherCols> > result_type;
    // ...
  }
};

Edit: As you said in your comment, you can also use this to create a matrix that doesn't use a MatrixContainer which has row and column size as template parameters:

template<
  template<class> class MatrixContainer,
  class T
>
class matrix< MatrixContainer<T> >{
  typedef MatrixContainer<T> Mcontainer;
  typedef matrix<Mcontainer> this_type;

public:
  // normal matrix multiplication, return type is not a problem
  this_type operator*(this_type const& other){
    // ensure correct row and column sizes, e.g. with assert
  }

  // multiply dynamic matrix with stack-based one:
  template<
    template<class,int,int> class OtherContainer,
    int Rows, int Cols
  >
  this_type operator*(matrix<OtherContainer<T,Rows,Cols> > const& other){
    // ensure correct row and column sizes, e.g. with assert
  }
};

Usage:

// stack-based example
matrix<DenseStackMatrix<int,3,2> > m1;
matrix<DenseStackMatrix<int,2,4> > m2;
matrix<DenseStackMatrix<int,3,4> > m3 = m1 * m2;

// heap-based example
matrix<DenseHeapMatrix<int> > m1(3,2);
matrix<DenseHeapMatrix<int> > m2(2,4);
matrix<DenseHeapMatrix<int> > m3 = m1 * m2;

Answer 2

How about changing MatrixContainer to be a template template parameter?

template <class T, int Rows, int Cols>
class DenseStackMatrix {
public:
    typedef T value_type;

private:
    value_type grid[Rows][Cols];
};

template <class T, int Rows, int Cols, template<class, int, int> class MatrixContainer>
class Matrix : public MatrixContainer<T, Rows, Cols> {
public:
    template <int ResultCols>
    Matrix<T, Rows, ResultCols, MatrixContainer> & operator*(const Matrix<T, Cols, ResultCols, MatrixContainer> &);
};

int main() {
    Matrix<int, 3, 2, DenseStackMatrix> matrix1;
    Matrix<int, 2, 4, DenseStackMatrix> matrix2;
    Matrix<int, 3, 4, DenseStackMatrix> matrix3 = matrix1 * matrix2;
}

This way you not only get compile time dimensions checking, but you can also extend this to allow multiplications between matrices of different container types.