编译时映射和逆映射值

prestokeys 发表于 Dev

钥匙

有人可以推荐一种更优雅的方法来实现这些编译时常量吗？

template <int> struct Map;
template <> struct Map<0> {static const int value = 4;};
template <> struct Map<1> {static const int value = 8;};
template <> struct Map<2> {static const int value = 15;};

template <int> struct MapInverse;
template <> struct MapInverse<4> {static const int value = 0;};
template <> struct MapInverse<8> {static const int value = 1;};
template <> struct MapInverse<15> {static const int value = 2;};

这些值需要在我的程序中为constexpr，但是反向映射的值更新起来变得很繁琐（而且容易出错或什至忘记做）。

沃尔特斯

使用C ++ 11进行线性搜索的另一种TMP方法：

#include <type_traits>

// === Types:
// Usage:
//    Function<Map<x1,y1>,Map<x2,y2>,...>
template<int D, int R> struct Map { enum { domain=D, range=R }; };
template<typename ...A> struct Function {};

// === Metafunctions:
// Usage:
//    ApplyFunction<x,F>::value
template<int I, typename M> struct ApplyFunction;
// Usage:
//    ApplyFunctionInverse<x,F>::value
template<int I, typename M> struct ApplyFunctionInverse;

// ==== Example:
// Define function M to the mapping in your original post.
typedef Function<Map<0,4>,Map<1,8>,Map<2,15>> M;

// ==== Implementation details
template<typename T> struct Identity { typedef T type; };
template<int I, typename A, typename ...B> struct ApplyFunction<I, Function<A,B...> > {
   typedef typename
      std::conditional <I==A::domain
                       , Identity<A>
                       , ApplyFunction<I,Function<B...>> >::type meta;
   typedef typename meta::type type;
   enum { value = type::range };
};
template<int I, typename A> struct ApplyFunction<I, Function<A>> {
   typedef typename
       std::conditional <I==A::domain
                        , Identity<A>
                        , void>::type meta;
   typedef typename meta::type type;
   enum { value = type::range };
};
// Linear search by range
template<int I, typename A> struct ApplyFunctionInverse<I, Function<A>> {
   typedef typename
       std::conditional <I==A::range
                        , Identity<A>
                        , void>::type meta;
   typedef typename meta::type type;
   enum { value = type::domain };
};
template<int I, typename A, typename ...B> struct ApplyFunctionInverse<I, Function<A,B...> > {
   typedef typename
       std::conditional <I==A::range
                        , Identity<A>
                        , ApplyFunctionInverse<I,Function<B...>> >::type meta;
   typedef typename meta::type type;
   enum { value = type::domain };
};

// ==============================
// Demonstration
#include <iostream>
int main()
{
   // Applying function M
   std::cout << ApplyFunction<0,M>::value << std::endl;
   std::cout << ApplyFunction<1,M>::value << std::endl;
   std::cout << ApplyFunction<2,M>::value << std::endl;

   // Applying function inverse M
   std::cout << ApplyFunctionInverse<4,M>::value << std::endl;
   std::cout << ApplyFunctionInverse<8,M>::value << std::endl;
   std::cout << ApplyFunctionInverse<15,M>::value << std::endl;
}