No viable overloaded '='

[fafner]

I'm writing a small linear algebra library to use with some OpenGL stuff. In my Matrix class I have the following two operators defined:

Matrix4 Matrix4::operator*(Matrix4& m) {
    Matrix4 r;
    int index = 0;
    
    for(int col = 0; col < 4; col++) {
        for(int row = 0; row < 4; row++) {
            float sum = 0.0f;
            for(int i = 0; i < 4; i++) {
                sum += m.raw()[i + col * 4]*mat[row + i * 4];
            }
            r.raw()[index] = sum;
            index++;
        }
    }
    
    return r;
}

Matrix4& Matrix4::operator=(Matrix4& m) {
    for(int i = 0; i < 16; i++) {
        mat[i] = m.raw()[i];
    }
    return *this;
}

And I'm trying to translate an existing matrix every frame, by doing

Matrix4 t = Matrix4::translation(elapsed * speed + lastPos, 0.0, 0.0);
matrix = matrix * t; //ERROR HERE 

But I'm getting a compiler error saying 'No viable overloaded '=''. Why would it say that when I clearly have overloaded the operator, and the types are correct?

[Peter87]

A temporary object can't bind to a non-const reference. You have to mark the parameter m of operator= as const.

Matrix4& Matrix4::operator=(const Matrix4& m) {

You should also mark operator* as const (both the parameter and the function).

Matrix4 Matrix4::operator*(const Matrix4& m) const {

This is not needed for your example code to compile but in some cases it will be needed (e.g. if you multiply three matrices at once).

[keskiverto]

Look at the canonical copy assignment operators in http://en.cppreference.com/w/cpp/language/operators

There are

Matrix4& Matrix4::operator=( Matrix4 m )
Matrix4& Matrix4::operator=( const Matrix4 & m )

but you have only
Matrix4& Matrix4::operator=( Matrix4 & m )

The thing is that the matrix * t creates an unnamed, temporary Matrix4 object and taking the address of that is not ok.

Forcing a copy/move is ok and taking address to const is ok.

[fafner]

OK, thanks :)