Enable std::get support on class

Question

What are the templates that I have to specialize to support std::get?

struct MyClass {
  int a;
};

template <const size_t I>
struct MyContainer {
  MyClass array[I];
};

What do I have to specialize to be able to do:

MyContainer<16> mc;
std::get<0>(mc);

Answer 1

std::get is not a customization point for the standard library; the three function template overloads (for pair, tuple and array) do not explicitly allow for user-defined overloads, so 17.6.4.2.1p1 applies and adding a declaration of your own function template overload is undefined behaviour.

Note that get as an unqualified name is a customization point as of C++17; it is used by the structured binding declaration protocol to access tuple-like elements; but this is as an unqualified name, not the qualified name std::get.

That said, if you were to write:

namespace std {
   template<size_t I, size_t N> MyClass &get(MyContainer<N> &c) { return c.array[I]; }
}

and similarly for the rvalue reference and const reference overloads, your program would likely work as you expect.

However, there's little point seeing as the standard already supplies array:

template<size_t N> using MyContainer = std::array<MyClass, N>;

Answer 2

I'm guessing you want to implement some algorithms that need access to arbitrary array-like containers using compile-time indices and therefor aim to use some function (like std::get) to uniformly perform that task?! In that case, it is the same business as making begin and end available for your class. You simply declare a function get in the namespace you declared your container class in, and let ADL do its jobs.

    template <unsigned I, unsigned N>
    MyClass& get (MyContainer<N>& c) { return c.array[I]; }

    template <unsigned I, unsigned N>
    MyClass const& get (MyContainer<N> const& c) { return c.array[I]; }

In your algorithm you just use get (without the std namespace prefix) and ADL will call the correct function. So, for the standard structures like array, tuple and pair std::get is invoked and for your container the get function that you provided.

    int main(){
        std::array<int, 3> a {{0,1,2}};
        auto t = std::make_tuple(0.0, 1.0f, 2);
        auto p = std::make_pair('0', 4.4);
        MyContainer<3> c;

        std::cout << get<1>(a) << std::endl;
        std::cout << get<1>(t) << std::endl;
        std::cout << get<1>(p) << std::endl;
        std::cout << get<1>(c).a << std::endl;

        return 0;
    }

Example