Question

I tried to implement the operator[] for a container of mine. But I am really new to c++, and it seems I have an error in my implementation.

I declared them like this:

float& operator[](const int &idx);
const float& operator[](const int &idx) const;

which should be fine, it’s pretty much copy/paste from tutorials. Now, Quaternion.cpp looks like this:

float& Quaternion::operator[](const int &idx)
{
    if(idx == 0)
    {
        return x;
    }
    if(idx == 1)
    {
        return y;
    }
    if(idx == 2)
    {
        return z;
    }
    if(idx == 3)
    {
        return w;
    }
    std::cerr << "Your Quaternion is only accessible at positions {0, 1, 2, 3}!" 
              << std::endl;
    return x;
}

const float& Quaternion::operator[](const int &idx)
{
    if(idx == 0)
    {
        return const x;
    }
    if(idx == 1)
    {
        return const y;
    }
    if(idx == 2)
    {
        return const z;
    }
    if(idx == 3)
    {
        return const w;
    }
    std::cerr << "Your Quaternion is only accessible at positions {0, 1, 2, 3}!" 
         << std::endl;
    return x;
}

I get the error for the signature “const float& Quaternion::operator[](const int &idx)”.

Another thing which happened before, was that I couldn’t return 0 in case the boundaries got exceeded. Maybe I will, once this issue is resolved, but it gave me an error message before. I just returned x then, which makes me really unhappy.

Answer 1

You left out the trailing const modifier from the second (const) operator implementation:

const float& Quaternion::operator[](const int &idx) const
{
    // ...
}