运行时类型切换类型列表作为开关，而不是嵌套的if的？ [英] Runtime typeswitch for typelists as a switch instead of a nested if's?

问题描述

这是来自TTL：

////////////////////////////////////////////////////////////
//  run-time type switch
template <typename L, int N = 0, bool Stop=(N==length<L>::value) > struct type_switch;

template <typename L, int N, bool Stop>
  struct type_switch
  {
    template< typename F >
      void operator()( size_t i, F& f )
      {
        if( i == N )
        {
          f.operator()<typename impl::get<L,N>::type>();
        }
        else
        {
          type_switch<L, N+1> next;
          next(i, f);
        }
      }
  };

它用于类型列表上的类型切换。问题是 - 他们通过一系列嵌套的ifs做这个。有没有办法做这个类型切换作为一个单一的select语句？

It's used for typeswitching on a TypeList. Question is -- they are doing this via a series of nested if's. Is there a way to do this type switch as a single select statement instead?

谢谢！

推荐答案

你需要预处理器生成一个大的开关。您将需要 get<> 到无操作的超限查找。检查编译器输出以确保未使用的情况不产生输出，如果你在乎;

You'll need the preprocessor to generate a big switch. You'll need get<> to no-op out-of-bound lookups. Check the compiler output to be sure unused cases produce no output, if you care; adjust as necessary ;v) .

查看Boost预处理程序库，如果你关心这种事情...

Check out the Boost Preprocessor Library if you care to get good at this sort of thing…

template <typename L>
  struct type_switch
  {
    template< typename F >
      void operator()( size_t i, F& f )
      {
        switch ( i ) {
         #define CASE_N( N ) \
         case (N): return f.operator()<typename impl::get<L,N>::type>();
         CASE_N(0)
         CASE_N(1)
         CASE_N(2)
         CASE_N(3) // ad nauseam.
      }
  };

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