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std::collate<CharT>::hash, std::collate<CharT>::do_hash

Defined in header <locale> 
public:
long hash( const CharT* beg, const CharT* end ) const;
 (1) 
protected:
virtual long do_hash( const CharT* beg, const CharT* end ) const;
 (2)
1) Public member function, calls the protected virtual member function do_hash of the most derived class.
2) Converts the character sequence [beg, end) to an integer value that is equal to the hash obtained for all strings that collate equivalent in this locale (compare() returns ​0​). For two strings that do not collate equivalent, the probability that their hashes are equal should be very small, approaching 1.0/std::numeric_limits<unsigned long>::max().

Parameters

beg - pointer to the first character in the sequence to hash
end - one past the end pointer for the sequence to hash

Return value

The hash value that respects collation order.

Note

The system-supplied locales normally do not collate two strings as equivalent (compare() does not return ​0​) if basic_string::operator== returns false, but a user-installed std::collate facet may provide different collation rules, for example, it may treat strings as equivalent if they have the same Unicode normalized form.

Example

Demonstrates a locale-aware unordered container.

#include <iostream>
#include <string>
#include <locale>
#include <unordered_set>
 
struct CollateHash {
    template<typename CharT>
    std::size_t operator()(const std::basic_string<CharT>& s) const
    {
        return std::use_facet<std::collate<CharT>>(std::locale()).hash(
                   &s[0], &s[0] + s.size()
               );
    }
};
struct CollateEq {
    template<typename CharT>
    bool operator()(const std::basic_string<CharT>& s1,
                    const std::basic_string<CharT>& s2) const
    {
        return std::use_facet<std::collate<CharT>>(std::locale()).compare(
                     &s1[0], &s1[0] + s1.size(),
                     &s2[0], &s2[0] + s2.size()
               ) == 0;
    }
};
 
int main()
{
    std::locale::global(std::locale("en_US.utf8"));
    std::wcout.imbue(std::locale());
 
    std::unordered_set<std::wstring, CollateHash, CollateEq> s2 = {L"Foo", L"Bar"};
    for(auto& str: s2)
        std::wcout << str << ' ';
    std::cout << '\n';
}

Possible output:

Bar Foo

See also

(C++11)(C++20)(C++11)(C++11)(C++11)(C++20)(C++20)(C++20)(C++20)(C++20)
 hash support for strings
(class template specialization)

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