我想要我的班级
template <class T,unsigned int n> class X;
创建一个包含n倍类型T的std ::元组.有没有一个特别整齐的方式呢？有没有一个很好的方法来做这个任意可变的模板类？
这是我先做的
#include <tuple>
template <class,unsigned int,class>
struct simple_repeat_helper;
template <class T,unsigned int n,class... Args>
struct simple_repeat_helper<T,n,std::tuple<Args...>>
{
typedef typename simple_repeat_helper<T,n-1,std::tuple<Args...,T>>::type type;
};
template <class T,std::tuple<Args...>>
{
typedef std::tuple<Args...> type;
};
template <class T,unsigned int n>
struct simple_repeat
{
using type = typename simple_repeat_helper<T,std::tuple<>>::type;
};
但实际上,我不需要这个std :: tuple,而是另一个类似的类.所以我以为我会创建一个更通用的版本：
template <class,template <class...> class,class>
struct repeat_helper;
template <class T,template <class...> class M,class... Args>
struct repeat_helper<T,M,M<Args...>>
{
typedef M<Args...> type;
};
template <class T,M<Args...>>
{
typedef typename repeat_helper<T,M<Args...,template <class...> class M = std::tuple>
struct repeat
{
using type = typename repeat_helper<T,M<>>::type;
};
我以为我可以这样使用：
repeat<double,5,std::tuple>::type x = std::make_tuple( 1.,2.,3.,4.,5. );
但不幸的是,由于：
ambiguous class template instantiation for ‘struct repeat_helper<double,0u,std::tuple,std::tuple<double,double,double> >’
任何关于这个错误的帮助将不胜感激！
解决方法
我会这样做：
template<typename,typename>
struct append_to_type_seq { };
template<typename T,typename... Ts,template<typename...> class TT>
struct append_to_type_seq<T,TT<Ts...>>
{
using type = TT<Ts...,T>;
};
template<typename T,unsigned int N,template<typename...> class TT>
struct repeat
{
using type = typename
append_to_type_seq<
T,typename repeat<T,N-1,TT>::type
>::type;
};
template<typename T,template<typename...> class TT>
struct repeat<T,TT>
{
using type = TT<>;
};
作为一个小考验：
#include <type_traits>
#include <tuple>
template<typename... Ts>
struct X { };
int main()
{
repeat<double,std::tuple>::type t = std::make_tuple(1.,5.);
static_assert(
std::is_same<
decltype(t),double>
>::value,"!");
repeat<double,3,X>::type y;
static_assert(
std::is_same<decltype(y),X<double,double>>::value,"!");
}
最后一个live example.
