"increment" `std::variant` alternative

Question

I want to increment / decrement a std::variant 's type alternative, essentially like so:

using var_t = std::variant</*...*/>;
var_t var;
var.emplace< (var.index()+1) % std::variant_size<var_t> >(); // "increment" case, wrapping for good measure

The problem here is that while emplace expects what clang's error message calls an "explicitly-specified argument", index does not appear to be constexpr .

The obvious alternative would be something like this:

switch(var.index()){
  0:
    var.emplace<1>();
    break;
  1:
    var.emplace<2>();
    break;
// ...
  variant_size<var_t>-1:
    var.emplace<0>();
}

But that's what I personally would call "extremely ugly" and "a massive pain in the behind to maintain" (especially since I'd have to maintain two almost-copies of those blocks off-by-two for both incrementing and decrementing).

Is there a better / "correct" way of doing this?

In case that information is important in any way, I'm targeting C++20 on clang with libstdc++ .

Answer 1

Another possible solution that's (in my opinion) a bit uglier than @Jarod42's one and rely on finding the index at compile time using a templated lambda in std::visit :

#include <variant>

template <class T, std::size_t I, class... Args>
struct index_of_;

template <class T, std::size_t I, class... Args>
struct index_of_<T, I, T, Args... >: std::integral_constant<std::size_t, I> {};

template <class T, std::size_t I, class U, class... Args>
struct index_of_<T, I, U, Args... >: index_of_<T, I + 1, Args... > {};

template <class T, class... Args>
struct next_index: std::integral_constant<
    std::size_t, 
    (index_of_<T, 0, Args... >::value + 1) % sizeof... (Args)> {};

template <class... Args>
void increment(std::variant<Args...>& variant) {
    std::visit([&](auto const& arg) {
        variant.template emplace<
            next_index<std::decay_t<decltype(arg)>, Args...>::value>();
    }, variant);
}

Unlike @Jarod42 solution, this solution will not work if you have duplicated types in your variant.

Answer 2

As usual, std::index_sequence might help:

#include <variant>

template <typename... Ts, std::size_t... Is>
void next(std::variant<Ts...>& v, std::index_sequence<Is...>)
{
    using Func = void (*)(std::variant<Ts...>&);
    Func funcs[] = {
        +[](std::variant<Ts...>& v){ v.template emplace<(Is + 1) % sizeof...(Is)>(); }...
    };
    funcs[v.index()](v);
}

template <typename... Ts>
void next(std::variant<Ts...>& v)
{
    next(v, std::make_index_sequence<sizeof...(Ts)>());
}

Demo

Note : for prev , Is + 1 should be replaced by Is + sizeof...(Is) - 1 .

Answer 3

This is one of the many situations that calls for index-based visitation. We can write that generically using Boost.Mp11 :

template <typename F, typename Variant>
decltype(auto) visit_with_index(F&& f, Variant&& v) {
    constexpr size_t N = mp_size<std::remove_cvref_t<Variant>>;
    return mp_with_index<N>(v.index(), [&](auto I){
        return f(I, std::get<I>(v));
    });
}

This passes both the index (which is some integral constant) and the element into the function. And now we can just write:

template <typename... Args>
void next_alt(std::variant<Args...>& v) {
    visit_with_index([&](auto I, auto&&){
        v.emplace<(I+1) % sizeof...(Args)>();
    }, v);
}

As a bonus, mp_with_index is a switch so it'll have better performance than std::visit , so it's a good solution there anyway. Note that this doesn't handle valueless_by_exception , but that's straightforward to add on top if desired.

Answer 4

The problem here is that while emplace expects what clang's error message calls an "explicitly-specified argument", index does not appear to be constexpr .

You can use std::variant to convert the run-time index into a compile-time constant, that is, std::integral_constant .

#include <variant>
#include <array>

template<std::size_t N>
using IC = std::integral_constant<std::size_t, N>;

template<std::size_t N>
constexpr auto gen_indices = []<std::size_t... Is>(
  std::index_sequence<Is...>) {
  return std::array{std::variant<IC<Is>...>(IC<Is>{})...};
}(std::make_index_sequence<N>{});

template <typename Variant>
constexpr void increment(Variant& v) {
  constexpr auto size = std::variant_size_v<Variant>;
  constexpr auto& indices = gen_indices<size>;
  std::visit(
    [&v](auto index) { v.template emplace<(index+1) % size>(); }, 
    indices[v.index()]);
}

Demo.