C++ map 根据value找key

#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;
}