C++ How to implement a compile time mapping from types to types?

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Is there a canonical/reference implementation of a compile time map, which maps types to types?

For example, I would need a type mapping from IBar -> IFoo or from int -> IFoo.

At compile time I can then select IFoo when given a IBar.

How would one go about this with C++17?

Edit: here is an example using structs https://godbolt.org/z/EEvrYd9PE

type-mapping template-meta-programming compile-time c++
1 Answers

You could define one with overloading and return types. This will act like a map data structure that you can initialize and reuse with many types for many purposes.

template<typename T>
struct type_tag {
    using type = T;
};

template<typename K, typename V>
struct pair {
    using first_type = K;
    using second_type = V;
};

template<typename Pair>
struct element {
    static auto value(type_tag<typename Pair::first_type>) -> type_tag<typename Pair::second_type>;
};

template<typename... elems>
struct type_map : element<elems>... {
    using element<elems>::value...;

    template<typename K>
    using find = typename decltype(type_map::value(type_tag<K>{}))::type;
};

You can use it like this:

using my_map = type_map<
    pair<int, float>,
    pair<char, double>,
    pair<long, short>
>;

static_assert(std::is_same_v<my_map::find<int>, float>);
static_assert(std::is_same_v<my_map::find<char>, double>);
static_assert(std::is_same_v<my_map::find<long>, short>);

Live example

It should be quite fast as the lookup is limited to the scope of the class only, and uses the compilers own overload resolution.

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