C++ write file and read file via fstream

void Util::logFile21(string fileName)
{
    fstream wFile(fileName, ios::app);
    if (!wFile.is_open())
    {
        cout << "Create or open " << fileName << " failed!" << endl;
        return;
    }

    int loops = 200, repeatedTimes = 1000000;
    stringstream ss;
    long double num = 0;
    for (int i = 0; i < loops; i++)
    {
        for (int j = 0; j < repeatedTimes; j++)
        {
            ss << fixed << ++num << "," << getUuid27() << endl;
        }
        wFile << ss.str();
        ss = stringstream();
        cout << fixed << "Num=" << num << ",now is " << getTimeNow() << endl;
    }
    wFile.close();
    cout << "Finished in void Util::logFile21(string fileName) and now is " << getTimeNow() << endl;
}

char *Util::getUuid27()
{
    uuid_t newUUID;
    uuid_generate(newUUID);
    uuid_unparse(newUUID, Util::uuidValue);
    return Util::uuidValue;
}

g++ -g -std=c++2a -I. *.cpp ./Model/*.cpp -o h1 -luuid -lpthread

time ./h1 log.txt

void Util::readFile22(string fileName)
{
    fstream rFile(fileName, ios::in);
    if (!rFile.is_open())
    {
        cout << "Open " << fileName << " failed!" << endl;
        return;
    }

    long double num = 0;
    string line;
    while (getline(rFile, line))
    {
        if (!fmod(++num, 1000000))
        {
            cout << fixed << "Num=" << num << ",value=" << line << ",now is " << getTimeNow() << endl;
        }
    }
    rFile.clear();
    rFile.close();
    cout << "Finished in void Util::readFile22(string fileName) and now is " << getTimeNow() << endl;
}