文件写入函数

ios::app   The function performs a seek to the end of file. When new bytes are written to the file, they are always appended to the end, even if the position is moved with the ostream::seekp function.

当以 ios::app  打开文件时，seekp 函数不起作用

ios::out|ios::int 打开文件 可以读写       可以用来打开已有的文件，然后再写文件

ios::ate 打开会清除原有信息