Get index by type in std::variant

Question

Is there a utility in the standard library to get the index of a given type in std::variant? Or should I make one for myself? That is, I want

Answer 1

You can also do this with a fold expression:

template 
constexpr size_t get_index(std::variant const&) {
    size_t r = 0;
    auto test = [&](bool b){
        if (!b) ++r;
        return b;
    };
    (test(std::is_same_v) || ...);
    return r;
}

The fold expression stops the first time we match a type, at which point we stop incrementing r. This works even with duplicate types. If a type is not found, the size is returned. This could be easily changed to not return in this case if that's preferable, since missing return in a constexpr function is ill-formed.

If you dont want to take an instance of variant, the argument here could instead be a tag>.