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

One fun way to do this is to take your variant and turn it into a custom class hierarchy that all implement a particular static member function with a different result that you can query.

In other words, given variant, create a hierarchy that looks like:

struct base_A {
    static integral_constant get(tag);
};
struct base_B {
    static integral_constant get(tag);
};
struct base_C {
    static integral_constant get(tag);
};
struct getter : base_A, base_B, base_C {
    using base_A::get, base_B::get, base_C::get;
};

And then, decltype(getter::get(tag())) is the index (or doesn't compile). Hopefully that makes sense.

---

In real code, the above becomes:

template  struct tag { };

template 
struct base {
    static std::integral_constant get(tag);
};

template 
struct getter_impl;

template 
struct getter_impl, Ts...>
    : base...
{
    using base::get...;
};

template 
struct getter : getter_impl, Ts...>
{ };

And once you establish a getter, actually using it is much more straightforward:

template 
struct get_index;

template 
struct get_index>
    : decltype(getter::get(tag()))
{ };

---

That only works in the case where the types are distinct. If you need it to work with independent types, then the best you can do is probably a linear search?

template 
struct get_index;

template  
struct get_index_impl
{ };

template  
struct get_index_impl
    : std::integral_constant
{ };

template  
struct get_index_impl
    : get_index_impl
{ };

template  
struct get_index>
    : get_index_impl<0, T, Ts...>
{ };