具有独特类型的变体

Variant with unique types

本文关键字：类型      更新时间：2023-10-16

在编写 cpp 库时，我最终得出std::variants 应包含重复类型的可能性，例如，std::variant<int, double, int, std::string, int>。它在某些情况下可能很有用，但在我的图书馆的情况下，我觉得这是不必要的。所以我想像这样过滤类型：

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>

如何做到这一点？

#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;

演示

使用 Boost.Mp11，这是一个简短的单行代码(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>

我自己的版本使用 C++11(非提升(：

#include <type_traits>
template <typename T, typename... Args>
struct contains : std::integral_constant<bool, false> {};
template <typename T, typename T2, typename... Args>
struct contains<T, T2, Args...> : contains<T, Args...> {};
template <typename T, typename... Args>
struct contains<T, T, Args...> : std::integral_constant<bool, true> {};
template <typename W, typename... Ts>
struct __make_unique_impl {
using type = W;
};
template <template <typename...> class W, typename... Current, typename T,
typename... Ts>
struct __make_unique_impl<W<Current...>, T, Ts...>
: std::conditional<
contains<T, Ts...>::value,
typename __make_unique_impl<W<Current...>, Ts...>::type,
typename __make_unique_impl<W<Current..., T>, Ts...>::type> {};
template <typename W>
struct make_unique;
template <template <typename...> class W, typename... T>
struct make_unique<W<T...>> : __make_unique_impl<W<>, T...> {};

如何使用：

#include <variant>
using test_t = typename make_unique<std::variant<int, std::string, std::string, int>>::type; // std::variant<int, std::string>
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