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

Specializations

The standard library is guaranteed to provide the following specializations (they are required to be implemented by any locale object):

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

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

Run this code

#include <algorithm>
#include <iostream>
#include <locale>
#include <string>
#include <vector>
 
int main()
{
    std::locale::global(std::locale("en_US.utf8"));
    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

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

Character conversions
toupper
tolower
String and stream conversions
wstring_convert (C++11/17/26*)
wbuffer_convert (C++11/17/26*)

Member functions
collate::collate
collate::~collate
collate::comparecollate::do_compare
collate::transformcollate::do_transform
collate::hashcollate::do_hash

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