C++ 使用比较运算符将 std::string 作为 std::map 中的键
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std::string as a key in std::map using a compare operator
提问by Chromex
I'm trying to use a std::string as a key in a std::map however, i'm unable to find() correctly. My code is somewhat complicated and large so this is a small program that demonstrates the problem I'm having. If someone could tell me why this doesn't work, i'd be very grateful.
我正在尝试使用 std::string 作为 std::map 中的键，但是，我无法正确 find() 。我的代码有点复杂和大，所以这是一个演示我遇到的问题的小程序。如果有人能告诉我为什么这不起作用，我将不胜感激。
Thanks.
谢谢。
#include <stdio.h>
#include <string>
#include <map>
struct comparer
{
public:
bool operator()(const std::string x, const std::string y)
{
return x.compare(y)==0;
}
};
int main(int argc, char *argv[])
{
std::map<std::string, int, comparer> numbers;
numbers.insert(std::pair<std::string,int>("One",1));
numbers.insert(std::pair<std::string,int>("Two",2));
numbers.insert(std::pair<std::string,int>("Three",3));
numbers.insert(std::pair<std::string,int>("Four",4));
numbers.insert(std::pair<std::string,int>("Five",5));
std::map<std::string, int, comparer>::iterator it=numbers.find("Three");
if(it!=numbers.end())
printf("The number is %d\n",(*it).second);
else
printf("Error, the number is not found\n");
}
回答by Michael Krelin - hacker
Remove your comparerand it will work just fine. The thing is, you didn't implement it properly. It should return trueif xis to be placed beforey. Or change ==0to <0or >0(it doesn't really matter).
删除你的comparer，它会工作得很好。问题是，你没有正确实施它。true如果x要放在之前，它应该返回y。或更改==0为<0或>0（这并不重要）。
回答by Griwes
comparer::operator()should return value of operator <, not of operator ==.
comparer::operator()应该返回运算符 < 的值，而不是运算符 == 的值。
回答by Xeo
std::map(and setand their multivariants) enforce strict weak ordering.
std::map（set及其multi变体）强制执行严格的弱排序。
x.compare(y) == 0;
Will return true if the strings are equal. The comparer should return whether the first string should go before the second string. Either return x.compare(y) < 0or just leave your comparision functor out.
如果字符串相等，则返回 true 。比较器应该返回第一个字符串是否应该在第二个字符串之前。要么返回，x.compare(y) < 0要么只留下你的比较函子。
回答by u9874315
This should work:
这应该有效：
#include <stdio.h>
#include <string>
#include <map>
struct comparer
{
public:
bool operator()(const std::string x, const std::string y) const
{
return x.compare(y)==0;
}
};
int main(int argc, char *argv[])
{
std::map<std::string, int, comparer> numbers;
numbers.insert(std::pair<std::string,int>("One",1));
numbers.insert(std::pair<std::string,int>("Two",2));
numbers.insert(std::pair<std::string,int>("Three",3));
numbers.insert(std::pair<std::string,int>("Four",4));
numbers.insert(std::pair<std::string,int>("Five",5));
std::map<std::string, int, comparer>::iterator it=numbers.find("Three");
if(it!=numbers.end())
printf("The number is %d\n",(*it).second);
else
printf("Error, the number is not found\n");
}
