Empty file check on std::ofstream

Question

I've been using std::ofstream for writing purposes quite a bit. I open the file, do some operations based on certain conditions and close it.

Let's say that later I want to check if anything is really written into the file or not. There is not is_emtpy() kind of simple check available with std::ofstream.

One way I thought is of using the stat way which is independent of std::ofstream.

Wonder how do everyone else do it?

One way is to open it for reading, seek to the end, and check the position (which will equal the size of the file). — Some programmer dude, Jul 23 '19 at 18:01
If *you* are writing to it, and you handle errors properly, then you know it already, since `write()` returns the bytes written. — rustyx, Jul 23 '19 at 18:03
Hemant, your question is already answered, check the duplicates! Next time, you can save yourself some time and search a bit before asking.. I searched for "check if file is empty c++" and got the results right away. :D Hope that helps.. :) — gsamaras, Jul 23 '19 at 18:10
@gsamaras, He he. My question was to know if it could be done with std::ofstream though. — Hemant Bhargava, Jul 23 '19 at 18:16
Hemant my sincere apologies. I misunderstood your question, re-opened - my intentions were good. Good luck finding an answer! — gsamaras, Jul 23 '19 at 18:23
[std::filesystem::file_size](https://en.cppreference.com/w/cpp/filesystem/file_size) could be used instead of `stat` if you are using C++17. — Ted Lyngmo, Jul 23 '19 at 21:09

score 0 · Answer 1 · answered Oct 10 '19 at 14:58

Standard output streams provide a tellp() method which can be used to determine the current write position. The type and meaning of the return value is implementation-defined, but for it to be useful, it must return distinct values.

#include <iostream>
#include <fstream>

int main()
{
    std::ofstream out{"/tmp/test"};
    auto const empty_pos = out.tellp();

    std::clog.setf(std::ios_base::boolalpha);

    std::clog << "At start: " << (out.tellp() != empty_pos) << '\n';
    out << 'c';
    std::clog << "After writing 1 char: " << (out.tellp() != empty_pos) << '\n';
}

In principle, empty_pos may be different for each stream, so a truly portable program will take that into account.

Note also that this doesn't necessarily mean that the output is visible to other programs - use std::flush if that's important.

Empty file check on std::ofstream

1 Answers1