std::collate::transform, do_transform

 

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

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Technical Specifications

 

Localizations library

Locales and facets
locale
use_facet	has_facet
Character classification
isspace isblank (C++11) iscntrl isupper islower isalpha	isdigit ispunct isxdigit isalnum isprint isgraph
Conversions
toupper tolower	wstring_convert (C++11) wbuffer_convert (C++11)
Facet category base classes
ctype_base codecvt_base messages_base	time_base money_base
Facet categories
ctype ctype<char> codecvt collate messages time_get time_put	num_get num_put numpunct money_get money_put moneypunct
Locale-specific facets
ctype_byname codecvt_byname messages_byname collate_byname	time_get_byname time_put_byname numpunct_byname moneypunct_byname
Code conversion facets
codecvt_utf8 (C++11) codecvt_utf16 (C++11)	codecvt_utf8_utf16 (C++11) codecvt_mode (C++11)
C locale
setlocale localeconv lconv	LC_ALLLC_COLLATELC_CTYPELC_MONETARYLC_NUMERICLC_TIME

 

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 <iostream>
#include <iomanip>
#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)