std::hash<std::basic_string>
Defined in header <string>
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template
<
class A >
struct hash< std::basic_string < char, std::char_traits < char >, A>> ; |
(1) | (since C++11) |
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template
<
class A >
struct hash< std::basic_string < char16_t, std::char_traits < char16_t >, A>> ; |
(2) | (since C++11) |
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template
<
class A >
struct hash< std::basic_string < char32_t, std::char_traits < char32_t >, A>> ; |
(3) | (since C++11) |
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template
<
class A >
struct hash< std::basic_string < wchar_t, std::char_traits < wchar_t >, A>> ; |
(4) | (since C++11) |
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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.
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These hashes equal the hashes of corresponding std::basic_string_view classes: If |
(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
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(C++11)
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hash function object (class template) |
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(C++17)
(C++17)
(C++20)
(C++17)
(C++17)
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hash support for string views (class template specialization) |