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[locale.collate]
# 28 Text processing library [[text]](./#text)
## 28.3 Localization library [[localization]](localization#locale.collate)
### 28.3.4 Standard locale categories [[locale.categories]](locale.categories#locale.collate)
#### 28.3.4.5 The collate category [[category.collate]](category.collate#locale.collate)
#### 28.3.4.5.1 Class template collate [locale.collate]
#### [28.3.4.5.1.1](#general) General [[locale.collate.general]](locale.collate.general)
[🔗](#lib:collate)
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; };}
[1](#general-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/text.tex#L3381)
The class collate<charT> provides features
for use in the collation (comparison) and hashing of strings[.](#general-1.sentence-1)
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]](algorithms "26Algorithms library")) and
containers operating on strings[.](#general-1.sentence-2)
The specializations
required in Table [91](locale.category#tab:locale.category.facets "Table 91: Locale category facets") ([[locale.category]](locale.category "28.3.3.1.2.1Type locale::category")),
namely collate<char> and collate<wchar_t>,
apply lexicographical ordering ([[alg.lex.comparison]](alg.lex.comparison "26.8.11Lexicographical comparison"))[.](#general-1.sentence-3)
[2](#general-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/text.tex#L3393)
Each function compares a string of characters *p in the range [low, high)[.](#general-2.sentence-1)
#### [28.3.4.5.1.2](#members) Members [[locale.collate.members]](locale.collate.members)
[🔗](#lib:collate,compare)
`int compare(const charT* low1, const charT* high1,
const charT* low2, const charT* high2) const;
`
[1](#members-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/text.tex#L3406)
*Returns*: do_compare(low1, high1, low2, high2)[.](#members-1.sentence-1)
[🔗](#lib:collate,transform)
`string_type transform(const charT* low, const charT* high) const;
`
[2](#members-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/text.tex#L3417)
*Returns*: do_transform(low, high)[.](#members-2.sentence-1)
[🔗](#lib:collate,hash)
`long hash(const charT* low, const charT* high) const;
`
[3](#members-3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/text.tex#L3428)
*Returns*: do_hash(low, high)[.](#members-3.sentence-1)
#### [28.3.4.5.1.3](#virtuals) Virtual functions [[locale.collate.virtuals]](locale.collate.virtuals)
[🔗](#lib:collate,do_compare)
`int do_compare(const charT* low1, const charT* high1,
const charT* low2, const charT* high2) const;
`
[1](#virtuals-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/text.tex#L3442)
*Returns*: 1 if the first string is greater than the second,-1 if less,
zero otherwise[.](#virtuals-1.sentence-1)
The specializations
required in Table [91](locale.category#tab:locale.category.facets "Table 91: Locale category facets") ([[locale.category]](locale.category "28.3.3.1.2.1Type locale::category")),
namely collate<char> and collate<wchar_t>,
implement a lexicographical comparison ([[alg.lex.comparison]](alg.lex.comparison "26.8.11Lexicographical comparison"))[.](#virtuals-1.sentence-2)
[🔗](#lib:collate,do_transform)
`string_type do_transform(const charT* low, const charT* high) const;
`
[2](#virtuals-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/text.tex#L3459)
*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[.](#virtuals-2.sentence-1)[226](#footnote-226 "This function is useful when one string is being compared to many other strings.")
[🔗](#lib:collate,do_hash)
`long do_hash(const charT* low, const charT* high) const;
`
[3](#virtuals-3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/text.tex#L3476)
*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[.](#virtuals-3.sentence-1)
[4](#virtuals-4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/text.tex#L3482)
*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())[.](#virtuals-4.sentence-1)
[226)](#footnote-226)[226)](#footnoteref-226)
This function is useful when one string is being compared to many other strings[.](#footnote-226.sentence-1)