namespace std {
template <class charT>
class collate : public locale::facet {
public:
typedef charT char_type;
typedef basic_string<charT> string_type;
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;
};
}
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 (Clause [algorithms]) and containers operating on strings. The specializations required in Table [tab:localization.category.facets] ([locale.category]), namely collate<char> and collate<wchar_t>, apply lexicographic ordering ([alg.lex.comparison]).
Each function compares a string of characters *p in the range [low,high).
int compare(const charT* low1, const charT* high1,
const charT* low2, const charT* high2) const;
Returns: do_compare(low1, high1, low2, high2)
string_type transform(const charT* low, const charT* high) const;
Returns: do_transform(low, high)
long hash(const charT* low, const charT* high) const;
Returns: do_hash(low, high)
int do_compare(const charT* low1, const charT* high1,
const charT* low2, const charT* high2) const;
Returns: 1 if the first string is greater than the second, -1 if less, zero otherwise. The specializations required in Table [tab:localization.category.facets] ([locale.category]), namely collate<char> and collate<wchar_t>, implement a lexicographical comparison ([alg.lex.comparison]).
string_type do_transform(const charT* low, const charT* high) const;
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.249
long do_hash(const charT* low, const charT* high) const;
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. [ Note: 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()). — end note ]
This function is useful when one string is being compared to many other strings.