如何在编译期间切换/选择类型？

Question

Is there a standard way for me to select a type at compile-time on an unsigned index in c++11?

For example, something like:

using type_0 = static_switch<0,T,U>;  // yields type T
using type_1 = static_switch<1,T,U>;  // yields type U

If there is a variadic-template version, it would be very useful.

Answer 1

This should work:

template<std::size_t N, typename... T>
using static_switch = typename std::tuple_element<N, std::tuple<T...> >::type;

Another method:

template<std::size_t N, typename T, typename... Ts>
struct static_switch {
  using type = typename static_switch<N - 1, Ts...>::type;
};
template<typename T, typename... Ts>
struct static_switch<0, T, Ts...> {
  using type = T;
};

Answer 2

You could probably use a boost::mpl::vector to store your types and use boost::mpl::at<v,n>::type to get a type with from the index.

template<std::size_t N, typename... T>
using static_switch = typename boost::mpl::at<boost::mpl::vector<T...>, N>::type;

Answer 3

How about

 template<size_t N, typename T, typename U>
 struct static_switch {};

 template<typename T, typename U>
 struct static_switch<0, T, U>{typedef T type;};

 template<typename T, typename U>
 struct static_switch<1, T, U>{typedef U type;};

You would use it as follows:

using type_0 = static_switch<0,T,U>::type;  // yields type T
using type_1 = static_switch<1,T,U>::type;  // yields type U

This is more or less implemented for you in std::conditional .

Answer 4

With C++17 you can also go about this another way. Instead of calculating the type explicitly you can use constexpr if and do different things (including returning different types) directly:

template<size_t N>
decltype(auto) foo(){
  if constexpr(N%2==0){
      return std::string("Hello I'm even");
  }else{
      return std::pair(
           std::vector<char>{'O','d','d',' ','v','a','l','u','e'},
           [](){ return N; });         
  }
}

foo<0>()           // "Hello I'm even"
foo<21>().second() // 21

You can also use this to get just the type:

using type_0 = decltype(foo<0>());
using type_1 = decltype(foo<1>());