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std::hash<std::basic_string>

Defined in header <string> 
template< class A >
struct hash<std::basic_string<char, std::char_traits<char>, A>>;
 (1) (since C++11) 
template< class A >
struct hash<std::basic_string<char16_t, std::char_traits<char16_t>, A>>;
 (2) (since C++11) 
template< class A >
struct hash<std::basic_string<char32_t, std::char_traits<char32_t>, A>>;
 (3) (since C++11) 
template< class A >
struct hash<std::basic_string<wchar_t, std::char_traits<wchar_t>, A>>;
 (4) (since C++11) 
template< class A >
struct hash<std::basic_string<char8_t, std::char_traits<char8_t>, A>>;
 (5) (since C++20)

The template specializations of std::hash for the various string classes allow users to obtain hashes of strings.

These hashes equal the hashes of corresponding std::basic_string_view classes: If S is one of these string types, SV is the corresponding string view type, and s is an object of type S, then std::hash<S>()(s) == std::hash<SV>()(SV(s)).

 (since C++17)

Example

The following code shows one possible output of a hash function used on a string:

#include <functional>
#include <iostream>
#include <memory_resource>
#include <string>
#include <string_view>
using namespace std::literals;
 
int main()
{
    auto sv = "Stand back! I've got jimmies!"sv;
    std::string s(sv);
    std::pmr::string pmrs(sv); // use default allocator
 
    std::cout << std::hash<std::string_view>{}(sv) << '\n';
    std::cout << std::hash<std::string>{}(s) << '\n';
    std::cout << std::hash<std::pmr::string>{}(pmrs) << '\n';
}

Possible output:

3544599705012401047
3544599705012401047
3544599705012401047

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3705 C++11 hash support for std::basic_string with customized allocators was not enabled enabled

See also

(C++11)
hash function object
(class template)
(C++17)(C++17)(C++20)(C++17)(C++17)
hash support for string views
(class template specialization)

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