CODEWITHSUNDEEP
c++keystdstdstringstdmap
Chromex
Chromex

Reputation: 51

std::string as a key in std::map using a compare operator

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.

Thanks.

#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");
}

Upvotes: 5

Views: 31034

Answers (5)

Reed Hedges
Reed Hedges

Reputation: 1650

The custom compare operation is not usually required (you can normally just use std::map<std::string, int>), but for a bit of optimization when using (not very short) strings as keys and passing a string constant (char* constant) (or std::string_view or any other type that can be easily compared to std::string without converting to std::string) to contains() or find(), see https://www.cppstories.com/2021/heterogeneous-access-cpp20/.

(Also your comparer::operator() could take const string references, to avoid copying there as well: bool operator()(const std::string& x, const std::string& y) const...)

Upvotes: 0

李则博
李则博

Reputation: 7

This should work:

#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");
}

Upvotes: -1

Xeo
Xeo

Reputation: 131789

std::map (and set and their multi variants) enforce strict weak ordering.

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) < 0 or just leave your comparision functor out.

Upvotes: 3

Griwes
Griwes

Reputation: 9031

comparer::operator() should return value of operator <, not of operator ==.

Upvotes: 3

Michael Krelin - hacker
Michael Krelin - hacker

Reputation: 143071

Remove your comparer and it will work just fine. The thing is, you didn't implement it properly. It should return true if x is to be placed before y. Or change ==0 to <0 or >0 (it doesn't really matter).

Upvotes: 6

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