fstream不会打印到文件

Question

The following code will print something to a file

    std::fstream fout ("D_addr.txt", std::fstream::app);
    fout <<  pkt->Addr() << std::endl;
    flush(fout);
    fout.close();

While debugging, I watched pkt->Addr() and it has some values. The fout line is passed without problem. Also the file D_addr.txt is created. However after closing the file, the file size is zero! nothing has been written to it.

Where is the problem?

Answer 1

This is not your actual code I guess and if it is I would start with that Addr() function of yours.

Note that fstream::close "closes the file currently associated with the object, disassociating it from the stream. Any pending output sequence is written to the physical file." flush(fout); can be omitted.

You should also specify std::fstream::out flag. "If the function is called with any value in that parameter the default mode is overridden, not combined." So instead of std::fstream::app you should pass std::fstream::app | std::fstream::out std::fstream::app | std::fstream::out .

Answer 2

I'm wondering if you're not using the wrong class. If you want to write to a file, use std::ofstream , and not std::fstream . In particular, the constructor of std::ofstream forces the ios_base::out bit when calling rdbuf()->open ; the constructor of std::fstream doesn't (so you're opening the file with neither read nor write access).

And you probably want to check the error status: did the open succeed, and after the close (or the flush), did all of the writes succeed. The usual way of doing this is just:

if ( fout ) {
    //  All OK...
}

if ( !fout ) {
    //  Something went wrong.
}

After the open (the constructor), you can use fout.is_open() , which has the advantage of being a little bit more explicit with regards to what you are checking for.

Answer 3

First of all, flush() and fout.close() do not make any harm, but are not needed here, because when fout gets destroyed the file will be closed (and flushed) as part of fstream destructor.

Second, you should use an ofstream or alternatively add the flag std::ios::out to the openmode parameter.

Try something along the lines of:

{
  uint64_t x = 42;
  std::fstream of("test.txt", std::ios::app);
  of << x << std::endl;
}