C++ map 根据value找key的实现

#include <algorithm>  #include <vector>   #include <map>#include <string>

std::map<int, std::string> t;  t.insert(std::make_pair(1, "a"));  t.insert(std::make_pair(2, "b"));  t.insert(std::make_pair(3, "c"));  t.insert(std::make_pair(4, "d"));

std::string s = "";  auto it = t.find(2);  if (it != t.end())  {    s = (*it).second;  }

std::string s = "c";  auto find_item = std::find_if(t.begin(), t.end(),    [s](const std::map<int, std::string>::value_type item)  {    return item.second == s;  });  int n = 0;  if (find_item!= t.end())  {    n = (*find_item).first;  }

class finder{public:  finder(const std::string &cmp_string) :s_(cmp_string){}  bool operator ()(const std::map<int, std::string>::value_type &item)  {    return item.second == s_;  }private:  const std::string &s_;};//调用int n = 0;auto it = std::find_if(t.begin(), t.end(), finder("d"));  if (it != t.end())  {    n = (*it).first;  }