struct Colour{
    double r, g, b, a;
    double& operator[](int i){
        switch(i){
        case 0:
            return r;
            break;
        case 1:
            return g;
            break;
        case 2:
            return b;
            break;
        case 3:
            return a;
            break;
        }
    }
    /* ... */
};
/* ... */
int main(){
    Colour c = {1.0, 2.0, 3.0, 4.0};
    std::cout << c.g << std::endl;
    c[1] = 10.0;
    std::cout << c.g << std::endl;
    return 0;
}