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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

hash (C++11)	hash function object (class template)
std::hash<std::string_view>std::hash<std::wstring_view>std::hash<std::u8string_view>std::hash<std::u16string_view>std::hash<std::u32string_view> (C++17)(C++17)(C++20)(C++17)(C++17)	hash support for string views (class template specialization)

std::hash(std::basic_string)

Example

Defect reports

See also