Why does std::unordered_map not work with const std::string key?

Question

The following code example will not compile, however it can be made to compile by removing the const specifier before std::string as the unordered map key.

#include <unordered_map>
#include <utility>
#include <string>
#include <iostream>

int main()
{

    int myint = 5;
    std::unordered_map<const std::string, int*> map;
    map.insert({"string", &myint});
    std::cout << *map.at("string") << std::endl;

    return 0;
}

Why does this code not compile when const std::string is used as a key, when std::string works?

What's the point of this? The key in the map is a copy of the original string. — Barmar
– Barmar, Commented May 29, 2018 at 21:16
I would like to know why map and unordered_map differ in this regard. It's a shame the duplicate doesn't seem to address this question. — Drew Dormann
– Drew Dormann, Commented May 29, 2018 at 21:18
@NO_NAME because it doesn't answer the question asked: WHY does it not compile — SergeyA
– SergeyA, Commented May 29, 2018 at 21:21

NathanOliver · Accepted Answer · 2020-11-21 23:29:57Z

4

std::unordered_map uses std::hash by default for the hash function. It uses the type of the key for the template type of std::hash. <string> specializes std::hash for std::string but since the key type is const std::string, there is no matching specialization and compilation fails.

Really though, using std::unordered_map<std::string, int*> will do exactly what you need. The key in all associative containers is const already for you so there is no reason to mark in const in the template parameters.

edited Nov 21, 2020 at 23:29

answered May 29, 2018 at 21:15

NathanOliver

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9 Comments

user2138149 Over a year ago

Is this a "bug"?

SergeyA Over a year ago

@user3728501 in where?

NathanOliver Over a year ago

@user3728501 It's not a code bug. You might call it a language bug (the standard should say there needs to be a const version) but since keys in associative containers are made const it's not really needed. std::unordered_map<std::string, int*> will do the exact same thing and actually compile.

user2138149 Over a year ago

@SergeyA Why would every developer in the world who wished to put the const specifier there also implement their own hash function?

SergeyA Over a year ago

Every developer in the world could think about 'why' they want to do it, and find out there is no good reason for it?

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