Difference between revisions of "cpp/string/basic string/hash"
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< cpp | string | basic string
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===Defect reports=== | ===Defect reports=== | ||
{{dr list begin}} | {{dr list begin}} | ||
− | {{dr list item|wg=lwg|dr=2978|std=C++17|before=hash support for {{ | + | {{dr list item|wg=lwg|dr=2978|std=C++17|before=hash support for {{lc|std::pmr::string}} and its friends were not enabled|after=enabled}} |
{{dr list end}} | {{dr list end}} | ||
Revision as of 00:57, 30 May 2021
Defined in header <string>
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template<> struct hash<std::string>; template<> struct hash<std::wstring>; |
(since C++11) | |
template<> struct hash<std::pmr::string>; template<> struct hash<std::pmr::wstring>; |
(since C++17) | |
template<> struct hash<std::u8string>; template<> struct hash<std::pmr::u8string>; |
(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 |
(since C++17) |
Example
The following code shows one possible output of a hash function used on a string:
Run this code
#include <iostream> #include <string> #include <string_view> #include <functional> #include <memory_resource> 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 2978 | C++17 | hash support for std::pmr::string and its friends were not enabled | enabled |
See also
(C++11) |
hash function object (class template) |
hash support for string views (class template specialization) |