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

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

translate_point will work with points that have public x and y members, and it will also work with tuples/indexable containers where x and y are represented by the first and second element, respectively.

The problem is, another library defines a point class with public x and y, but also allows indexing:

struct StupidPoint
{
    int x, y;

    int operator[](int i) const
    {
        if(i == 0) return x;
        else if(i == 1) return y;
        else throw "you're terrible";
    }

};

My application, using both libraries, is the following:

int main(int argc, char **argv)
{
    StupidPoint stupid { 8, 3 };
    translate_point(stupid, 5, 2);
    return EXIT_SUCCESS;
}

but this makes GCC (and clang) unhappy:

error: call of overloaded ‘translate_point(StupidPoint&, int, int)’ is ambiguous

Now I can see why this is happening, but I want to know how to fix this (assuming I can't change the internals of StupidPoint), and, if there is no easy workaround, how I might as a library implementer make this easier to deal with.

Answer 1

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

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

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

template<class T>
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<std::size_t N>
struct choice: choice<N-1> {};

template<>
struct choice<0> {};

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

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

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

Answer 2

You could provide an overload for StupidPoint:

auto translate_point(StupidPoint &p, int x, int y)
{
    p.x += x;
    p.y += y;
}

live example

---

Another solution:

Since operator[] is const for StupidPoint, you can check this in your SFINAE condition:

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

live example

---

You could also use a different type-traits based approach to select the appropriate translate_point function:

template<typename T, typename = void>
struct has_x_y : std::false_type { };

template<typename T>
struct has_x_y<T, decltype(std::declval<T>().x, std::declval<T>().y, void())> : std::true_type { };

template<typename T, typename = void>
struct has_index : std::false_type { };

template<typename T>
struct has_index<T, decltype(std::declval<T>().operator[](0), void())> : std::true_type { };

template<class T>
std::enable_if_t<has_x_y<T>::value> translate_point(T &p, int x, int y)
{
    p.x += x;
    p.y += y;
}

template<class T>
std::enable_if_t<!has_x_y<T>::value && has_index<T>::value> translate_point(T &p, int x, int y)
{
    p[0] += x;
    p[1] += y;
}

live example