28 Text processing library [text]

28.3 Localization library [localization]

28.3.4 Standard locale categories [locale.categories]

28.3.4.5 The collate category [category.collate]

28.3.4.5.1 Class template collate [locale.collate]

28.3.4.5.1.1 General [locale.collate.general]

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namespace std { template<class charT> class collate : public locale::facet { public: using char_type = charT; using string_type = basic_string<charT>; explicit collate(size_t refs = 0); int compare(const charT* low1, const charT* high1, const charT* low2, const charT* high2) const; string_type transform(const charT* low, const charT* high) const; long hash(const charT* low, const charT* high) const; static locale::id id; protected: ~collate(); virtual int do_compare(const charT* low1, const charT* high1, const charT* low2, const charT* high2) const; virtual string_type do_transform(const charT* low, const charT* high) const; virtual long do_hash (const charT* low, const charT* high) const; }; }
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The class collate<charT> provides features for use in the collation (comparison) and hashing of strings.
A locale member function template, operator(), uses the collate facet to allow a locale to act directly as the predicate argument for standard algorithms ([algorithms]) and containers operating on strings.
The specializations required in Table 86 ([locale.category]), namely collate<char> and collate<wchar_t>, apply lexicographical ordering ([alg.lex.comparison]).
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Each function compares a string of characters *p in the range [low, high).

28.3.4.5.1.2 Members [locale.collate.members]

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int compare(const charT* low1, const charT* high1, const charT* low2, const charT* high2) const;
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Returns: do_compare(low1, high1, low2, high2).
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string_type transform(const charT* low, const charT* high) const;
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Returns: do_transform(low, high).
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long hash(const charT* low, const charT* high) const;
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Returns: do_hash(low, high).

28.3.4.5.1.3 Virtual functions [locale.collate.virtuals]

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int do_compare(const charT* low1, const charT* high1, const charT* low2, const charT* high2) const;
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Returns: 1 if the first string is greater than the second, -1 if less, zero otherwise.
The specializations required in Table 86 ([locale.category]), namely collate<char> and collate<wchar_t>, implement a lexicographical comparison ([alg.lex.comparison]).
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string_type do_transform(const charT* low, const charT* high) const;
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Returns: A basic_string<charT> value that, compared lexicographically with the result of calling transform() on another string, yields the same result as calling do_compare() on the same two strings.231
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long do_hash(const charT* low, const charT* high) const;
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Returns: An integer value equal to the result of calling hash() on any other string for which do_compare() returns 0 (equal) when passed the two strings.
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Recommended practice: The probability that the result equals that for another string which does not compare equal should be very small, approaching (1.0/numeric_limits<unsigned long>​::​max()).
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This function is useful when one string is being compared to many other strings.