有没有办法在编译时替换元组元素？
例如,
using a_t = std::tuple<std::string,unsigned>; // start with some n-tuple using b_t = element_replace<a_t,1,double>; // std::tuple<std::string,double> using c_t = element_replace<b_t,char>; // std::tuple<char,double>
解决方法
你可以使用这个：
// the usual helpers (BTW: I wish these would be standardized!!)
template< std::size_t... Ns >
struct indices
{
typedef indices< Ns...,sizeof...( Ns ) > next;
};
template< std::size_t N >
struct make_indices
{
typedef typename make_indices< N - 1 >::type::next type;
};
template<>
struct make_indices< 0 >
{
typedef indices<> type;
};
// and now we use them
template< typename Tuple,std::size_t N,typename T,typename Indices = typename make_indices< std::tuple_size< Tuple >::value >::type >
struct element_replace;
template< typename... Ts,std::size_t... Ns >
struct element_replace< std::tuple< Ts... >,N,T,indices< Ns... > >
{
typedef std::tuple< typename std::conditional< Ns == N,Ts >::type... > type;
};
然后使用它：
using a_t = std::tuple<std::string,unsigned>; // start with some n-tuple using b_t = element_replace<a_t,double>::type; // std::tuple<std::string,char>::type; // std::tuple<char,double>
