Can you safely close a file that was never opened?

Question

If I have a std::ofstream that may or may not have been opened, is it safe to try to close regardless? In otherwords does close() do anything nasty (throw exception, etc) if !is_open(). For example

std::ofstream out;
if (some_condition)
{
    out.open(path, std::ios::out);
}

After I'm done with the file, can I just say

out.close();

Or should I first check

if (out.is_open())
    out.close();

The only description of std::basic_fstream::close on cppreference is

Closes the associated file.
Effectively calls rdbuf()->close(). If an error occurs during operation, setstate(failbit) is called.

Generally that is what I do, which is why I like `std::ofstream`, but in this particular case I would like the file to be closed earlier than it would otherwise be. — Cory Kramer, Oct 09 '15 at 16:21
If you look on cppreference for [`std::ofstream::close`](http://www.cplusplus.com/reference/fstream/ofstream/close/), you will see that the specific example of calling `close` on a not opened file is used. In summary, nothing nasty occurs, it just sets the failbit so anything you try to do with the `ofstream` afterwards might cause issues. — R_Kapp, Oct 09 '15 at 16:25
cppreference now links that rdbuf->close to the filebuf close page, which is a lot more detailed — Cubbi, Oct 09 '15 at 18:25

score 13 · Accepted Answer · answered Oct 09 '15 at 16:31

It does exactly what cppreference says it will: the failbit will be set, and you can inspect it with the fail() method. For instance, the following prints "fail\n":

#include <iostream>
#include <fstream>
int main(int argc, char ** argv)
{
        std::ofstream out;
        out.close();
        if (out.fail())
                std::cout << "fail" << std::endl;
        return 0;
}

In terms of interaction with the operating system, there's nothing there to close, but it's otherwise harmless.

score 8 · Answer 2 · answered Oct 09 '15 at 16:25

8

From the C++ standard, §27.9.1.4 [filebuf.members], paragraph 6:

basic_filebuf<charT,traits>* close();
Effects: If is_open() == false, returns a null pointer.…

answered Oct 09 '15 at 16:25

rici

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score 6 · Answer 3 · edited Oct 09 '15 at 16:51

6

Yes, you can close() the file always, even the file is not opened. No matter LINUX or Windows, I always close() without thinking they are opened or not.

From LINUX:

void
close()
{
     if (!_M_filebuf.close())
        this->setstate(ios_base::failbit);
}

edited Oct 09 '15 at 16:51

edmz

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answered Oct 09 '15 at 16:27

"From LINUX" you mean libstdc++? – edmz Oct 09 '15 at 16:51
yes. The indentation is wired because the libstdc++ is formatted like that – Oct 09 '15 at 16:55
3

Linux and Windows don't have anything to do with it. The question is about C++. – user207421 Oct 09 '15 at 23:27

Can you safely close a file that was never opened?

3 Answers3