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 .

CodePudding user response：

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.

CodePudding user response：

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.

CodePudding user response：

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.