标签:c c11 templates variadic-templates template-meta-programming
由于this问题似乎没有涵盖所有有用的案例,我决定填补我的这个小问题的空白.如果两个类型的多个集合相等,有没有办法回答?
#include <tuple>
#include <type_traits>
template <typename, typename>
struct type_multiset_eq : std::false_type
{
};
template <typename ... Types1, typename ... Types2>
struct type_multiset_eq<std::tuple<Types1...>, std::tuple<Types2...>>
: std::true_type
{
// Should only be true_type if the multisets of types are equal
};
int main() {
static_assert(type_multiset_eq<std::tuple<char, int, double, float, int, float>, std::tuple<float, char, int, double, int, float>>::value, "err");
static_assert(!type_multiset_eq<std::tuple<char, int, double, float, int, float>, std::tuple<char, int, double, int, float>>::value, "err");
static_assert(type_multiset_eq<std::tuple<char, char, char, float, float, float>, std::tuple<char, float, char, float, char, float>>::value, "err");
static_assert(!type_multiset_eq<std::tuple<int, int>, std::tuple<int, int, int>>::value, "err");
}
解决方法:
有趣的问题……
从原始问题(type_set_eq one)中的答案中获取灵感(好吧……复制它),添加一个类型计数器(countT)并删除辅助结构和标签结构,我想你可以简单地写一些如下内容
#include <tuple>
#include <type_traits>
template <typename ...>
struct countT;
template <typename T>
struct countT<T>
{ static constexpr std::size_t value { 0U }; };
template <typename T, typename T0, typename ... Ts>
struct countT<T, T0, Ts...>
{ static constexpr std::size_t value { countT<T, Ts...>::value }; };
template <typename T, typename ... Ts>
struct countT<T, T, Ts...>
{ static constexpr std::size_t value { 1U + countT<T, Ts...>::value }; };
template <bool ...>
struct bool_pack
{ };
template <bool ... Bs>
using my_and = std::is_same<bool_pack<Bs..., true>, bool_pack<true, Bs...>>;
template <typename, typename, typename = void>
struct type_multiset_eq : std::false_type
{ };
template <template <typename ...> class C1, typename ... Ts1,
template <typename ...> class C2, typename ... Ts2>
struct type_multiset_eq<C1<Ts1...>, C2<Ts2...>,
typename std::enable_if<
(sizeof...(Ts1) == sizeof...(Ts2))
&& (my_and<( countT<Ts1, Ts1...>::value
== countT<Ts1, Ts2...>::value)...>::value)
>::type>
: std::true_type
{ };
int main()
{
static_assert( type_multiset_eq<
std::tuple<char, int, double, float, int, float>,
std::tuple<float, char, int, double, int, float>>::value, "err");
static_assert( ! type_multiset_eq<
std::tuple<char, int, double, float, int, float>,
std::tuple<char, int, double, int, float>>::value, "err");
static_assert( type_multiset_eq<
std::tuple<char, char, char, float, float, float>,
std::tuple<char, float, char, float, char, float>>::value, "err");
static_assert( ! type_multiset_eq<
std::tuple<int, int>,
std::tuple<int, int, int>>::value, "err");
}
如果您可以使用C 14,您可以使用以下constexpr函数替换countT类型特征
template <typename T, typename ... Ts>
constexpr std::size_t cntT ()
{
using unused = std::size_t[];
std::size_t ret { 0U };
(void)unused { 0U, ret += (std::is_same<T, Ts>::value ? 1U : 0U)... };
return ret;
}
标签:c,c11,templates,variadic-templates,template-meta-programming 来源: https://codeday.me/bug/20190828/1752520.html
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