std::wcsspn

#include <cwchar>
#include <iostream>
#include <locale>

int main()
{
    wchar_t dest[] = L"  ";
    const wchar_t src[] = L"   ";
    const std::size_t len = std::wcsspn(dest, src);
    dest[len] = L'\0'; //  ,   

    std::wcout.imbue(std::locale("en_US.utf8"));
    std::wcout << L"     " << len << L".\n";
    std::wcout << L" \"" << dest << L"\".\n";
}