How can I open a file for reading & writing, creating it if it does not exist, without truncating it?

Question

What is the proper set of I/O flags for a std::fstream, where I want to be able to read from and write to the file, without truncating the file if it exists, but creating it if it does not?

I've tried

std::ios::binary | std::ios::in | std::ios::out
std::ios::binary | std::ios::in | std::ios::out | std::ios::ate

but neither of these create the file if it does not already exist.

I don't want std::ios::app, because I also need to be able to seek around the file at will, with both the get and put cursors.

One workaround, I suppose, would be to instantiate an std::ofstream first, then immediately close it and open the stream I really want, but that seems messy if it can be avoided with a single stream object.

Answer 1

At this time, I'm concluding that std::ios::in outright prevents this, and that I must use the workaround.

So:

if (!std::ostream(path.c_str()))
   throw std::runtime_error("Could not create/open file");

std::fstream fs(path.c_str(), std::ios::binary | std::ios::in | std::ios::out);
if (!fs)
   throw std::runtime_error("Could not open file");

// ... use `fs`

Answer 2

An investigation, from a Linux perspective (though much of this likely applies to other Unices):

At the syscall layer, you want open(O_RDWR | O_CREAT, 0666) (but not O_TRUNC or O_APPEND or a bunch of other flags, though arguably all files should be opened with O_CLOEXEC | O_LARGEFILE, but that's beside the point)

At the libc layer, there is no standard mode string that implies O_CREAT without O_TRUNC. However, you could use open followed by fdopen.

At the C++ library level, there is no standard way to pass the desired flags. However, using implementation-specific classes/functions or third-party libraries, it is possible; see How to construct a c++ fstream from a POSIX file descriptor?

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Personally, I tend to do all I/O at the C or even syscall level, since the API is a lot nicer and it's more predictable. For input/output of class instances, I have my own templates.