std::collate<CharT>::transform, do_transform

 

C++

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Symbols index

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Localization library

Locales and facets
Locales
has_facet use_facet	locale
Facet category base classes
ctype_base codecvt_base messages_base	time_base money_base
ctype facets
ctype ctype<char> ctype_byname	codecvt codecvt_byname
numeric facets
num_get num_put	numpunct numpunct_byname
collate facets
collate	collate_byname
time facets
time_get time_put	time_get_byname time_put_byname
monetary facets
money_get money_put	moneypunct moneypunct_byname
messages facets
messages	messages_byname

Character classification and conversion
Character classification
isspace isblank (C++11) iscntrl	isupper islower isalpha	ispunct isdigit isxdigit	isalnum isprint isgraph

Character conversions

toupper

tolower

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wstring_convert (C++11/17/26*)

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text_encoding (C++26)

Unicode conversion facets

codecvt_utf8 (C++11/17/26*)

codecvt_utf16 (C++11/17/26*)

codecvt_utf8_utf16 (C++11/17/26*)

codecvt_mode (C++11/17/26*)

C library locales
LC_ALLLC_COLLATELC_CTYPELC_MONETARYLC_NUMERICLC_TIME	setlocale localeconv lconv

 

std::collate

Member functions

collate::collate

collate::~collate

collate::comparecollate::do_compare

collate::transformcollate::do_transform

collate::hashcollate::do_hash

 

Defined in header <locale>
public: string_type transform( const CharT* low, const CharT* high ) const;	(1)
protected: virtual string_type do_transform( const CharT* low, const CharT* high ) const;	(2)

1) Public member function, calls the protected virtual member function do_transform of the most derived class.

2) Converts the character sequence [low, high) to a string that, compared lexicographically (e.g. with operator< for strings) with the result of calling transform() on another string, produces the same result as calling do_compare() on the same two strings.

Parameters

low	-	pointer to the first character in the sequence to transform
high	-	one past the end pointer for the sequence to transform

Return value

The string transformed so that lexicographic comparison of the transformed strings may be used instead of collating of the originals. In the "C" locale, the returned string is the exact copy of [low, high). In other locales, the contents of the returned string are implementation-defined, and the size may be considerably longer.

Notes

In addition to the use in collation, the implementation-specific format of the transformed string is known to std::regex_traits<>::transform_primary, which is able to extract the equivalence class information.

Example

Run this code

#include <iomanip>
#include <iostream>
#include <locale>
 
int main()
{
    std::locale::global(std::locale("sv_SE.utf8"));
    auto& f = std::use_facet<std::collate<wchar_t>>(std::locale());
 
    std::wstring in1 = L"\u00e4ngel";
    std::wstring in2 = L"\u00e5r";
 
    std::wstring out1 = f.transform(&in1[0], &in1[0] + in1.size());
    std::wstring out2 = f.transform(&in2[0], &in2[0] + in2.size());
 
    std::wcout << "In the Swedish locale: ";
    if (out1 < out2)
        std::wcout << in1 << " before " << in2 << '\n';
    else
        std::wcout << in2 << " before " << in1 << '\n';
 
    std::wcout << "In lexicographic comparison: ";
    if (in1 < in2)
        std::wcout << in1 << " before " << in2 << '\n';
    else
        std::wcout << in2 << " before " << in1 << '\n';
}

Output:

In the Swedish locale: år before ängel
In lexicographic comparison: ängel before år

See also

strxfrm	transform a string so that strcmp would produce the same result as strcoll (function)
wcsxfrm	transform a wide string so that wcscmp would produce the same result as wcscoll (function)