如何建立编译时键/值存储？ [英] How to build a compile-time key/value store?

问题描述

我有一个问题，我需要在编译时将一个整数映射到另一个整数。基本上，我需要等价于 std :: map< int，int> 的编译时。如果在地图中找不到键，我想返回默认值。

I have a problem where I need to map an integer at compile time to another integer. Basically, I need the compile-time equivalent of std::map<int,int>. If a key is not found in the map, I'd like to return a default value.

我要使用的接口：

template<unsigned int default_value,
         unsigned int key0, unsigned int value0,
         unsigned int key1, unsigned int value1,
         ...>
struct static_map
{
  ...
};

template<unsigned int key, typename StaticMap>
struct lookup
{
  static unsigned int value = ...
};

lookup 返回与<$相关的值 StaticMap 中的c $ c> key 。如果未找到 key ，则返回 default_value 。

lookup returns the value associated with key in the StaticMap. If key is not found, then default_value is returned.

通常，键/值对的数量将受到一定限制>2。构建 static_map 和 lookup的最佳方法是什么？

In general, the number of key/value pairs will be bounded by some limit > 2. What is the best way to build static_map and lookup?

我还应该提到，我仅限于使用C ++ 03语言构造，因此没有C ++ 11，也没有外部库依赖项。

I should also mention that I'm limited to using C++03 language constructs, so no C++11, and no external library dependencies.

这是我受到nm启发的解决方案和DyP的答案如下：

Here's the solution I arrived at, inspired by n.m. and DyP's answers below:

#include <iostream>

template<unsigned int k, unsigned int v>
struct key_value
{
  static const unsigned int key = k;
  static const unsigned int value = v;
};


template<typename Head, typename Tail = void>
struct cons
{
  template<unsigned int key, unsigned int default_value>
  struct get
  {
    static const unsigned int value = (key == Head::key) ? (Head::value) : Tail::template get<key,default_value>::value;
  };
};


template<typename Head>
struct cons<Head,void>
{
  template<unsigned int key, unsigned int default_value>
  struct get
  {
    static const unsigned int value = (key == Head::key) ? (Head::value) : default_value;
  };
};


template<unsigned int default_value,
         unsigned int key0, unsigned int value0,
         unsigned int key1, unsigned int value1,
         unsigned int key2, unsigned int value2,
         unsigned int key3, unsigned int value3,
         unsigned int key4, unsigned int value4,
         unsigned int key5, unsigned int value5,
         unsigned int key6, unsigned int value6,
         unsigned int key7, unsigned int value7>
struct static_map
{
  template<unsigned int key>
  struct get
  {
    typedef cons<
      key_value<key0,value0>,
      cons<
        key_value<key1,value1>,
        cons<
          key_value<key2,value2>,
          cons<
            key_value<key3,value3>,
            cons<
              key_value<key4,value4>,
              cons<
                key_value<key5,value5>,
                cons<
                  key_value<key6,value6>,
                  cons<
                    key_value<key7,value7>
                  >
                >
              >
            >
          >
        >
      >
    > impl;

    static const unsigned int value = impl::template get<key,default_value>::value;
  };
};


template<unsigned int key, typename StaticMap>
struct lookup
{
  static const unsigned int value = StaticMap::template get<key>::value;
};


int main()
{
  typedef static_map<13, 
                     0, 0,
                     1, 10,
                     2, 20,
                     3, 30,
                     4, 40,
                     5, 50,
                     6, 60,
                     7, 70
  > my_static_map;

  std::cout << "0 maps to " << lookup<0, my_static_map>::value << std::endl;
  std::cout << "1 maps to " << lookup<1, my_static_map>::value << std::endl;
  std::cout << "2 maps to " << lookup<2, my_static_map>::value << std::endl;
  std::cout << "3 maps to " << lookup<3, my_static_map>::value << std::endl;
  std::cout << "4 maps to " << lookup<4, my_static_map>::value << std::endl;
  std::cout << "5 maps to " << lookup<5, my_static_map>::value << std::endl;
  std::cout << "6 maps to " << lookup<6, my_static_map>::value << std::endl;
  std::cout << "7 maps to " << lookup<7, my_static_map>::value << std::endl;
  std::cout << "100 maps to " << lookup<100, my_static_map>::value << std::endl;

  return 0;
}

推荐答案

在C ++ 11中：

In C++11:

template <int kk, int vv>
struct kv
{
    static const int k = kk, v = vv;
};

template <int dflt, typename...>
struct ct_map;

template <int dflt>
struct ct_map<dflt>
{
    template<int>
    struct get
    {
        static const int val = dflt;
    };
};

template<int dflt, int k, int v, typename... rest>
struct ct_map<dflt, kv<k, v>, rest...>
{
    template<int kk>
    struct get
    {
        static const int val =
            (kk == k) ?
            v :
            ct_map<dflt, rest...>::template get<kk>::val;
    };
};

typedef ct_map<42, kv<10, 20>, kv<11, 21>, kv<23, 7>> mymap;

#include <iostream>
int main()
{
    std::cout << mymap::get<10>::val << std::endl;
    std::cout << mymap::get<11>::val << std::endl;
    std::cout << mymap::get<23>::val << std::endl;
    std::cout << mymap::get<33>::val << std::endl;
}

这篇关于如何建立编译时键/值存储？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！