std::collate

Defined in header `<locale>`
template< class CharT > class collate;

Class std::collate encapsulates locale-specific collation (comparison) and hashing of strings. This facet is used by std::basic_regex and can be applied, by means of std::locale::operator(), directly to all standard algorithms that expect a string comparison predicate.

Inheritance diagram.

Two standalone (locale-independent) specializations are provided by the standard library:

Defined in header `<locale>`
`std::collate<char>`	implements lexicographical ordering of byte strings
`std::collate<wchar_t>`	implements lexicographical ordering of wide strings

In addition, every locale object constructed in a C++ program implements its own (locale-specific) versions of these specializations.

Member types

Member type	Definition
`char_type`	`CharT`
`string_type`	`std::basic_string<CharT>`

Member functions

(constructor)	constructs a new collate facet (public member function)
(destructor)	destructs a collate facet (protected member function)
compare	invokes `do_compare` (public member function)
transform	invokes `do_transform` (public member function)
hash	invokes `do_hash` (public member function)

Member objects

static std::locale::id id

id of the locale
(public member object)

Protected member functions

do_compare [virtual]	compares two strings using this facet's collation rules (virtual protected member function)
do_transform [virtual]	transforms a string so that collation can be replaced by comparison (virtual protected member function)
do_hash [virtual]	generates an integer hash value using this facet's collation rules (virtual protected member function)

Example

#include <locale>
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
 
int main()
{
    std::wcout.imbue(std::locale(""));
    std::vector<std::wstring> v = {L"ar", L"zebra", L"\u00f6grupp", L"Zebra", L"\u00e4ngel",
                                   L"\u00e5r", L"f\u00f6rnamn"};
 
    std::wcout << "Default locale collation order: ";
    std::sort(v.begin(), v.end());
    for (auto s : v) std::wcout << s << ' '; std::wcout << '\n';
 
    std::wcout << "English locale collation order: ";
    std::sort(v.begin(), v.end(), std::locale("en_US.UTF-8"));
    for (auto s : v) std::wcout << s << ' '; std::wcout << '\n';
 
    std::wcout << "Swedish locale collation order: ";
    std::sort(v.begin(), v.end(), std::locale("sv_SE.UTF-8"));
    for (auto s : v) std::wcout << s << ' '; std::wcout << '\n';
}

Output:

Default locale collation order: Zebra ar förnamn zebra ängel år ögrupp
English locale collation order: ängel ar år förnamn ögrupp zebra Zebra
Swedish locale collation order: ar förnamn zebra Zebra år ängel ögrupp

operator()	lexicographically compares two strings using this locale's collate facet (public member function of `std::locale`)
collate_byname	creates a collate facet for the named locale (class template)