具有唯一类型的变体

问题描述 投票:3回答:3

[编写cpp库时,我以std::variant应该包含重复类型(例如std::variant<int, double, int, std::string, int>)为最终结果。在某些情况下它可能很有用,但是对于我的图书馆,我认为这是不必要的。所以我想过滤这样的类型副本:

std::variant<int, double, int, std::string, int>-> std::variant<int, double, std::string>

std::variant<std::string, std::string, int>-> std::variant<std::string, int>

如何完成?

c++ c++17 variant
3个回答
3
投票
#include <type_traits>
#include <variant>

template <typename T, typename... Ts>
struct filter_duplicates { using type = T; };

template <template <typename...> class C, typename... Ts, typename U, typename... Us>
struct filter_duplicates<C<Ts...>, U, Us...>
    : std::conditional_t<(std::is_same_v<U, Ts> || ...)
                       , filter_duplicates<C<Ts...>, Us...>
                       , filter_duplicates<C<Ts..., U>, Us...>> {};

template <typename T>
struct unique_variant;

template <typename... Ts>
struct unique_variant<std::variant<Ts...>> : filter_duplicates<std::variant<>, Ts...> {};

template <typename T>
using unique_variant_t = typename unique_variant<T>::type;

DEMO


4
投票

使用Boost.Mp11,这是一条短线(一如既往):

using V1 = mp_unique<std::variant<int, double, int, std::string, int>>;
// V1 is std::variant<int, double, std::string>

using V2 = mp_unique<std::variant<std::string, std::string, int>>;
// V2 is std::variant<std::string, int>
© www.soinside.com 2019 - 2024. All rights reserved.