Why does writing empty std::istringstream.rdbuf() set failbit?

Question

I have learned, that I can copy a C++ std::istream to an C++ std::ostream by outputting the istreams' rdbuf(). I used it several times and it worked fine.

Today I got in trouble, because this operation sets badbit, if the std::istream is empty (at least for std::istringstream). I have written the following code to demonstrate my problem:

#include <stdio.h>
#include <sstream>

int main(int argc, char *argv[])
{
    std::ostringstream ss;

    ss << std::istringstream(" ").rdbuf(); // this does not set failbit
    printf("fail=%d\n", ss.fail());

    ss << std::istringstream("").rdbuf(); // why does this set failbit ???
    printf("fail=%d\n", ss.fail());
}

I tried Windows/VS2017 and Linux/gcc-9.20 and both behave the same.

I am using std::istream& in a method signature with a default value of std::istringstream(""). The calling code shall be able to pass an optional istream, which is appended to some other data.

Can anybody explain, why badbit is set?
Is there better way to implement this optional std::istream& parameter?

I know, I could write two methods, one with an additional std::istream& parameter, but I want to avoid duplicate code.

Thanks in advance,

Mario

Update 22-Apr-20

I now use the following code:

#include <stdio.h>
#include <sstream>

int main(int argc, char *argv[])
{
    std::ostringstream out;
    std::istringstream in("");
    while (in)
    {
        char Buffer[4096];
        in.read(Buffer, sizeof(Buffer));
        out.write(Buffer, in.gcount());
    }
    printf("fail=%d\n", out.fail());
}

I also added a warning about setting failbit when copying empty files to https://stackoverflow.com/a/10195497/6832488

score 2 · Accepted Answer · edited Jun 20 '20 at 09:12

The documentation of ostream::operator<< describes the following behavior for reading streams:

Behaves as an UnformattedOutputFunction. After constructing and checking the sentry object, checks if sb is a null pointer. If it is, executes setstate(badbit) and exits. Otherwise, extracts characters from the input sequence controlled by sb and inserts them into *this until one of the following conditions are met:
   * end-of-file occurs on the input sequence;
   * inserting in the output sequence fails (in which case the character to be inserted is not extracted);
   * an exception occurs (in which case the exception is caught). 
If no characters were inserted, executes setstate(failbit). If an exception was thrown while extracting, sets failbit and, if failbit is set in exceptions(), rethrows the exception.

As you can tell, that explicitly says that trying to insert an empty buffer will set the failbit. If you want to essentially make it "optional", just check that the stream is good before inserting the buffer and do ss.clear() afterwards to clear the failbit.

Why does writing empty std::istringstream.rdbuf() set failbit?

Update 22-Apr-20

1 Answers1