Ludonope - 7 months ago 25

C++ Question

I'm discovering C++ and I would like to create a mini mathematic Matrix librairy using templates.

Here, I want to overload the operator*.

If i describe a matrix like this:

`M(y, x)`

`M`

`y`

`x`

`M(a, b) * N(b, c) = R(a, c)`

Currently I have this code:

`template<unsigned int y, unsigned int x>`

class Matrix

{

public:

Matrix() { }

~Matrix() { }

Matrix<y, x2>& operator*(const Matrix<y2, x2>& right)

{

// code...

}

private:

std::array<std::array<double, x>, y> m_values;

};

So I would like to be able to multiply two differents matrix like that:

`Matrix<3, 4> m;`

Matrix<4, 2> n;

// fill the matrix with values

Matrix<3, 2> o = m * n;

I've searched but I didn't find the answer to this question (maybe because I don't really know what I must search exactly).

Thanks for your help :)

Answer

You need to make your `operator*`

a *template member function*, something like so:

```
template <unsigned int y2, unsigned int x2>
Matrix<y, x2> operator*(const Matrix<y2, x2>& right)
{
// code...
}
```

Notice that the return type is no longer a reference, as `operator*`

should return a new value -- if you like, you can define a complementary `operator*=`

which modifies the LHS matrix in-place.

Another thing to notice is that matrix multiplication only makes sense if the dimensions of the matrices agree: that is, if the number of columns in the LHS matches the number of rows in the RHS. To enforce this, you could use a `static_assert`

within your member function to make sure that the template parameters agree:

```
template <unsigned int y2, unsigned int x2>
Matrix<y, x2> operator*(const Matrix<y2, x2>& right)
{
static_assert(y2 == x, "Matrix dimensions mismatched");
// code...
}
```