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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../string/wide/wcsxfrm.html below:

std::wcsxfrm - cppreference.com

Transforms the null-terminated wide string pointed to by src into the implementation-defined form such that comparing two transformed strings with std::wcscmp gives the same result as comparing the original strings with std::wcscoll, in the current C locale.

The first count characters of the transformed string are written to destination, including the terminating null character, and the length of the full transformed string is returned, excluding the terminating null character.

If count is ​0​, then dest is allowed to be a null pointer.

[edit] Notes

The correct length of the buffer that can receive the entire transformed string is 1 + std::wcsxfrm(nullptr, src, 0).

This function is used when making multiple locale-dependent comparisons using the same wide string or set of wide strings, because it is more efficient to use std::wcsxfrm to transform all the strings just once, and subsequently compare the transformed wide strings with std::wcscmp.

[edit] Parameters dest - pointer to the first element of a wide null-terminated string to write the transformed string to src - pointer to the null-terminated wide character string to transform count - maximum number of characters to output [edit] Return value

The length of the transformed wide string, not including the terminating null-character.

[edit] Example 
#include <cwchar>
#include <iostream>
 
int main()
{
    std::setlocale(LC_ALL, "sv_SE.utf8");
 
    std::wstring in1 = L"\u00e5r";
    std::wstring out1(1 + std::wcsxfrm(nullptr, in1.c_str(), 0), L' ');
    std::wstring in2 = L"\u00e4ngel";
    std::wstring out2(1 + std::wcsxfrm(nullptr, in2.c_str(), 0), L' ');
 
    std::wcsxfrm(&out1[0], in1.c_str(), out1.size());
    std::wcsxfrm(&out2[0], in2.c_str(), out2.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 lexicographical 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 lexicographical comparison: ängel before år
[edit] See also transform a string so that strcmp would produce the same result as strcoll
(function) [edit] transforms a string so that collation can be replaced by comparison
(virtual protected member function of std::collate<CharT>) [edit] compares two wide strings in accordance to the current locale
(function) [edit]

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