问题描述
我在查找有关如何将c ++ 11可变参数TypeList用作可移植参数包的容器方面遇到困难.可以说我有这段代码.
I'm having trouble finding information on how to use c++11 variadic TypeLists as containers for portable parameter packs. Lets say I had this piece of code.
#include <type_traits>
template <typename T, typename... Ts>
struct Index;
template <typename T, typename... Ts>
struct Index<T, T, Ts...> : std::integral_constant<std::size_t, 0> {};
template <typename T, typename U, typename... Ts>
struct Index<T, U, Ts...> : std::integral_constant<std::size_t, 1 + Index<T, Ts...>::value> {};
我可以用它来找到像这样的参数包内部类型的第一个索引.
I can use it to find the first index of a type inside of a parameter pack like this.
int main()
{
using namespace std;
cout << Index<int, int>::value << endl; //Prints 0
cout << Index<char, float, char>::value << endl; //Prints 1
cout << Index<int, double, char, int>::value << endl; //Prints 2
}
如何为TypeList实现此行为?我正在尝试创建类似这样的东西.
How could I achieve this behavior for a TypeList? I'm trying to create something like this.
template <typename ...>
struct TypeList {};
int main()
{
using namespace std;
using List = TypeList<char, short, long>
cout << Index<short, ConvertToParameterPack<List>::types...>::value << endl; //Prints 1
}
其中ConvertToParameterPack是将TypeList还原回其ParameterPack的某种方法.如果我要问的是不可能的，还有其他好的方法可以解决这个问题吗?
Where ConvertToParameterPack is some method of reverting a TypeList back to its ParameterPack. If what I'm asking is impossible, are there any other good ways to solve this?
推荐答案
template<std::size_t I>
using index_t=std::integral_constant<std::size_t,I>;
template<class T, class List>
struct Index{};
template<class T, class...Ts>
struct Index<T, TypeList<T,Ts...>>:
index_t<0>
{};
template<class T,class U, class...Ts>
struct Index<T, TypeList<U,Ts...>>:
index_t< 1+Index<T,TypeList<Ts...>::value >
{}
这篇关于如何将TypeList还原回其原始参数包的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！