3

How comes that std::hash does not exist for const std::string?

#include <string>
#include <functional>

int main() {
    std::hash<std::string> hasher1;       // works
    std::hash<const std::string> hasher2; // does not work
}

As expected, I can use the regular std::string specialization for example in a hash map:

#include <string>
#include <functional>
#include <unordered_map>

int main() {
    std::unordered_map<const std::string, int, std::hash<std::string>> a;
}

I feel like there is a better reason than "Well, they just didn't do it" to the const specialization missing. What is it?

Improve this question
6
  • 2
    Just a guess but an some point they had to stop the specialisations: en.cppreference.com/w/cpp/utility/hash Commented Jul 7, 2017 at 13:28
  • 1
    Well, they just didn't do it. Commented Jul 7, 2017 at 13:28
  • 1
    Doesn't operator() of a hash struct take a const argument anyway? Try passing a const string to your second example. It will probably work, meaning that it would have been redundant to specialize for const. Commented Jul 7, 2017 at 13:36
  • 1
    Why do you have unordered_map<const std::string, int> instead of just unordered_map<std::string, int>? Commented Jul 7, 2017 at 13:54
  • Yes, it does. That's why the unordered_map in the second snippet works as well. I am just surprised that those specializations did not exist. Commented Jul 7, 2017 at 13:54
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