How to resolve ambiguity in overloaded functions using SFINAE

Question

I have an incredibly exciting library that can translate points: it should work with any point types

template
auto translate_point(T &p, int x         


        

Answer 1

If you want to give precedence to the case having public x/y, you can do this:

template
auto translate_point_impl(int, T &p, int x, int y) -> decltype(p.x, p.y, void())
{
    p.x += x;
    p.y += y;
}

template
auto translate_point_impl(char, T &p, int x, int y) -> decltype(p[0], void())
{
    p[0] += x;
    p[1] += y;
}

template
void translate_point(T &p, int x, int y) {
    translate_point_impl(0, p, x, y);
}

It goes without saying that the opposite configuration is given by switching the types of the first parameter.

---

If you have three or more options (says N), you can use a trick based on templates.
Here is the example above once switched to such a structure:

template
struct choice: choice {};

template<>
struct choice<0> {};

template
auto translate_point_impl(choice<1>, T &p, int x, int y) -> decltype(p.x, p.y, void()) {
    p.x += x; p.y += y;
}

template
auto translate_point_impl(choice<0>, T &p, int x, int y) -> decltype(p[0], void()) {
    p[0] += x;
    p[1] += y;
}

template
void translate_point(T &p, int x, int y) {
    // use choice as first argument
    translate_point_impl(choice<1>{}, p, x, y);
}

As you can see, now N can assume any value.