专用于地图容器的功能 [英] Specialize function for map like containers

问题描述

我想要一个功能模板的矢量和地图像容器。对于向量我可以做下面，但我不知道如何可以有一个专门的版本的函数，将只用于地图像容器。

I want specialize a function template for vector and map like containers. For vector I can do like below but I don't know how can I have a specialized version of the function that will be used only for map like containers.

#include <iostream>
#include <vector>
#include <map>

using namespace std;

template<typename Iterator>
void print(Iterator begin, Iterator end)
{
    while (begin != end)
    {
        cout << *begin << endl; // compiler error for map like containers
        ++begin;
    }
}

int main()
{
    vector<int> noVec = { 1, 2, 3 };

    print(noVec.begin(), noVec.end());

    map<int, int> nosMap;
    nosMap[0] = 1;
    nosMap[1] = 2;
    nosMap[3] = 3;

    print(nosMap.begin(), nosMap.end());

    return 0;
}

这个问题是类似的，但它建议使用在我不想做的向量中的对。我知道专业化可以用SFINAE做，但不知道什么条件检查。

This question is similar but it suggests to use pair in vector which I don't want to do. I know the specialization can be done with SFINAE but don't know what condition to check for. It would be great if I can achieve this with C++ 11 type_traits.

推荐答案

value_type 映射是一些对，所以你可以检查 value_type 的迭代器是 std :: pair 或不，例如

The value_type of a map is some pair so you could check if the value_type of the iterator is a std::pair or not, e.g.

#include <vector>
#include <map>
#include <iostream>

template <typename> 
struct is_pair : std::false_type 
{ };

template <typename T, typename U>
struct is_pair<std::pair<T, U>> : std::true_type 
{ };



template <typename Iter>
typename std::enable_if<is_pair<typename std::iterator_traits<Iter>::value_type>::value>::type
print(Iter begin, Iter end)
{
  std::cout << "called with map-like" << std::endl;
  for (; begin != end; ++begin)
  {
    std::cout << begin->second;
  }
  std::cout << std::endl;
}

template <typename Iter>
typename std::enable_if<!is_pair<typename std::iterator_traits<Iter>::value_type>::value>::type
print(Iter begin, Iter end)
{
  std::cout << "called with vector-like" << std::endl;
  for (; begin != end; ++begin)
  {
    std::cout << *begin;
  }
  std::cout << std::endl;
}



int main()
{
  std::vector<int> vec { 1, 2, 3 };
  std::map<int, int> map {{0, 0}, {1, 1}, {2, 4}, {3, 9}};

  print(vec.begin(), vec.end());
  print(map.begin(), map.end());
}

打印

called with vector-like
123
called with map-like
0149

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