把数字格式化成带有分号的字符串

void ScoreUI::setLabelString(const long score)
{
    char num_buff[128] = {0};
    //数字转化成字符
    sprintf(num_buff, "%ld", score);
    
    std::string string_format(num_buff);
    int lenght = string_format.length();
    
    for (int i = 0; i < (lenght-1)/3; ++i)
    {
        //在倒数三位数中插入一个‘，’
        string_format.insert(lenght - 3*(i+1), 1, ',');
    }
}