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# std::collate::hash, do_hash

From cppreference.com
< cpp‎ | locale‎ | collate

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().

### [edit] Parameters

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

### [edit] Return value

The hash value that respects collation order

### [edit] 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.

### [edit] Example

Demonstrates a locale-aware unordered container

Run this code

    #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

### [edit] See also

std::hash<std::string>std::hash<std::u16string>std::hash<std::u32string>std::hash<std::wstring> (C++11)(C++11)(C++11)(C++11) | hash support for strings (class template specialization) [edit]

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