Question

I am opening a file with ifstream to check if it exists. Then I close it and open it with ofstream to write to it, and I think setting ios::trunc flag allows me to overwrite it.

However I’d like the ability to keep the file open if it exists, but I used an ifstream to open it so does that mean I can’t write to the file till I close and re-open using fstream or ofstream? I didn’t use fstream to begin with because that wouldn’t tell me if the file was already there or not.

Answer 1

Just open a read-write fstream on the file. You can test if the file previously existed (and was non-empty) by seeking to the end and seeing if you’re at a non-zero offset. If so, the file existed, and you can do whatever with it. If not, the file didn’t exist or was empty. Assuming you don’t need to distinguish between those two cases, you can then proceed as if it did not exist.

For example:

// Error checking omitted for expository purposes
std::fstream f("file.txt", std::ios::in | std::ios::out);
f.seekg(0, std::ios::end)
bool didFileExist = (f.tellg() > 0);
f.seekg(0, std::ios::beg);

// Now use the file in read-write mode.  If didFileExist is true, then the
// file previously existed (and has not yet been modified)