如何在编译时替换元组元素？

Question

Is there a way to replace a tuple element at compile time ?

For example,

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,0,char>;       // std::tuple<char,double>

Answer 1

You can use this:

// 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 N, typename T, std::size_t... Ns >
struct element_replace< std::tuple< Ts... >, N, T, indices< Ns... > >
{
    typedef std::tuple< typename std::conditional< Ns == N, T, Ts >::type... > type;
};

and then use it like this:

using a_t = std::tuple<std::string,unsigned>;     // start with some n-tuple
using b_t = element_replace<a_t,1,double>::type;  // std::tuple<std::string,double>
using c_t = element_replace<b_t,0,char>::type;    // std::tuple<char,double>

Answer 2

看看boost MPL transform或replace algos http://www.boost.org/doc/libs/1_40_0/libs/mpl/doc/refmanual/transformation-algorithms.html

Answer 3

You can access the types of the elements of a tuple type using std::tuple_element . This does not actually allow you to replace tuple element types, but it allows you to define tuple types in terms of types used as element types in other tuple types.