c++ assignment operator implementation in templated class

Question

I'm building my own matrix class to solidify my c++ understanding. It's templated, so I can have an int matrix or a float, or boolean matrix. I wasn't going to implement a copy constructor or assignment operator or a destructor, because I'm not going to have any dynamic member elements but if I have:

Matrix<float,3,4> mat1;
Matrix<int,45,45> mat2;
mat1 = mat2;

it returns the following error:

/Users/Jake/Dropbox/C++/test.cpp: In function ‘bool test1()’:
/Users/Jake/Dropbox/C++/test.cpp:23: error: no match for ‘operator=’ in ‘m2 = m1’
/Users/Jake/Dropbox/C++/Matrix.h:22: note: candidates are: Matrix<float, 3u, 4u>& Matrix<float, 3u, 4u>::operator=(const Matrix<float, 3u, 4u>&)

which, if the two matrices are both float or both int, it's ok. Dimensions don't have to match. So the default assignment operator works great unless they are of different types. So I implement my own assignment operator:

template <class T, unsigned int rows, unsigned int cols>
template <class T2, unsigned int rows2, unsigned int cols2>
Matrix<T, rows2, cols2> & Matrix<T,rows,cols>::operator= (const Matrix<T2, rows2, cols2> & second_matrix){
        unsigned int i,j;
for (i=0; i < rows2; i++){
    for (j=0; j < cols2; j++){
        data[i][j] = second_matrix(i,j);    
    }
}
this->_rows = rows2;
this->_cols = cols2;
return *this;
}

This works if they are different types, but same dimensions- but the values in the second are converted from the second type to the first. My question is, how can I set it up so that they can be different types and different dimensions, and just set this to point at the second, or a copy of the second?

Answer 1

The problem you're having is that you have three types, but you only mention two. Fundamentally,

Matrix<float,3,4> mat1;
Matrix<int,45,45> mat2;
mat1 = mat2;

Can't work, because the result of the assignment should be Matrix<float, 45, 45>, but mat1 is of type Matrix<float, 3, 4>. This cannot possibly be changed.

Is there a reason the dimensions of the matrices must be part of the type? it seems like you really want to have this stuff change around dynamically. Just do:'

template <class T>
class Matrix
{
    unsigned int rows;
    unsigned int cols;
  public:
    Matrix(numrows, numcols): rows(numrows), cols(numcols) {}
};

etc... Then you can have the dimensions of the matrices change at runtime.

Answer 2

how can I set it up so that they can be different types and different dimensions, and just set this to point at the second, or a copy of the second?

You don't.

mat does not point at a Matrix<float,3,4>; it is a Matrix<float,3,4>. It can't ever be anything else. There is no way to turn mat into a Matrix<int,45,45>.

If you want to be able to point to different objects, you need to use pointers (or references). In order to be able to have a pointer that can point to any specialization of Matrix, you need to create a base class and derive Matrix from that base class:

class MatrixBase { };

template <typename T, unsigned Rows, unsigned Columns>
class Matrix : public MatrixBase { };

You can then use a MatrixBase* to point to an object of any Matrix type and you can use a MatrixBase& to refer to an object of any Matrix type. You will need to put as much shared functionality as you can into the MatrixBase class or make use of virtual functions.