Multithreaded file writer in C++

mul·ti·thread·ing  

A technique by which a single set of code can be used by several processors at different stages of execution.
The class lock_guard is a mutex wrapper that provides a convenient RAII-style mechanism for owning a mutex or several mutexes (since C++17) for the duration of a scoped block.

When a lock_guard object is created, it attempts to take ownership of the mutex it is given. When control leaves the scope in which the lock_guard object was created, the lock_guard is destructed and the mutex is released.
If several mutexes are given, deadlock avoidance algorithm is used as if by std::lock. (since C++17)
The lock_guard class is non-copyable.

#include "stdafx.h"
#include <mutex>

CWinApp theApp;
using namespace std;


const int size_ = 100; //thread array size
std::mutex mymutex;
void printRailLock(int id) {
    printf("#ID :%d", id);
    lock_guard<std::mutex> lk(mymutex); // <- this is the lock
    CStdioFile lastLog;
    CString logfiledb{ "_FILE_2.txt" };
    CString str;
    str.Format(L"%d\n", id);
    bool opend = lastLog.Open(logfiledb, CFile::modeCreate | CFile::modeReadWrite | CFile::modeNoTruncate);
    if (opend) {
        lastLog.SeekToEnd();
        lastLog.WriteString(str);
        lastLog.Flush();
        lastLog.Close();
    }
}
int main()
{
    int nRetCode = 0;
    HMODULE hModule = ::GetModuleHandle(nullptr);

    if (hModule != nullptr)
    {       
        if (!AfxWinInit(hModule, nullptr, ::GetCommandLine(), 0))
        {            
            wprintf(L"Fatal Error: MFC initialization failed\n");
            nRetCode = 1;
        }
        else
        {
            std::thread threads[size_];
            for (int i = 0; i < size_; ++i) {
                threads[i] = std::thread(printRailLock, i + 1);
                Sleep(500);
            }
            for (auto& th : threads) { 
                //th.hardware_concurrency();
                th.join(); 
            }
        }
    }
    else
    {       
        wprintf(L"Fatal Error: GetModuleHandle failed\n");
        nRetCode = 1;
    }

    return nRetCode;
}