std::hash<std::basic_string>
From cppreference.com
< cpp | string | basic string
C++
Feature test macros (C++20)
Concepts library (C++20)
Metaprogramming library (C++11)
Ranges library (C++20)
Filesystem library (C++17)
Concurrency support library (C++11)
Execution control library (C++26)
std::basic_string
Literals Helper classes Deduction guides (C++17)
(C++23)
(DR*)
(DR*)
(C++23)
(DR*)
(C++23)
(C++23)
(C++20)
(C++20)
(C++23)
(C++20)(C++20)
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)
(C++11)(C++11)(C++11)
(C++11)(C++11)
(C++14)
hash<std::basic_string>
(C++11)
Defined in header
<string>
template< class A >
struct hash<std::basic_string <char, std::char_traits <char>, A>>;
(1)
(since C++11)
struct hash<std::basic_string <char, std::char_traits <char>, A>>;
template< class A >
struct hash<std::basic_string <char16_t, std::char_traits <char16_t>, A>>;
(2)
(since C++11)
struct hash<std::basic_string <char16_t, std::char_traits <char16_t>, A>>;
template< class A >
struct hash<std::basic_string <char32_t, std::char_traits <char32_t>, A>>;
(3)
(since C++11)
struct hash<std::basic_string <char32_t, std::char_traits <char32_t>, A>>;
template< class A >
struct hash<std::basic_string <wchar_t, std::char_traits <wchar_t>, A>>;
(4)
(since C++11)
struct hash<std::basic_string <wchar_t, std::char_traits <wchar_t>, A>>;
template< class A >
struct hash<std::basic_string <char8_t, std::char_traits <char8_t>, A>>;
(5)
(since C++20)
struct hash<std::basic_string <char8_t, std::char_traits <char8_t>, A>>;
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)).
[edit] Example
The following code shows one possible output of a hash function used on a string:
Run this code
#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
[edit] 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 |
[edit] See also
(C++17)(C++17)(C++20)(C++17)(C++17)
(class template specialization) [edit]