Why doesn't std::fstream write to a file?

Question

I'm getting different behavior between fstream vs. oftream which I cannot explain.

When I use fstream, nothing happens, i.e. no file is created:

int main()
{
    std::fstream file("myfile.txt");
    file << "some text"  << std::endl;
    return 0;
}

but when I change fstream to oftream, it works.

Why?

The second argument of fstream CTOR is ios_base::openmode mode = ios_base::in | ios_base::out which makes me think that the file is opened in read-write mode, right?

That should work. Buffering? I think we need a complete [mcve]. — Martin Bonner supports Monica, Sep 05 '16 at 07:43
I have just a function with this code and it does not work. I have nothing more to write. MVS2015. — Narek, Sep 05 '16 at 07:45
Maybe you are checking too early if it wrote to your file, e.g. "close()" is only called on destroying the fstream — CppChris, Sep 05 '16 at 07:46
Did you check if it's open? Like in `if (!file) cout <<"Error";`. Have you tried with `std::ofstream file(...`? — Bob__, Sep 05 '16 at 07:49
OF COURSE you have more to write: those two lines won't compile. You need at least one header file to define std::fstream and you need a `main` function. — Martin Bonner supports Monica, Sep 05 '16 at 07:51
Possible duplicate of [std::fstream doesn't create file](http://stackoverflow.com/questions/8835888/stdfstream-doesnt-create-file) — LogicStuff, Sep 05 '16 at 08:29
A broad lesson to learn from here: always check for error after I/O operations. — pts, Sep 05 '16 at 11:06

rustyx · Accepted Answer · 2016-09-05T08:22:22.957

ios_base::in requires the file to exist.

If you provide only ios_base::out, only then will the file be created if it doesn't exist.

+--------------------+-------------------------------+-------------------------------+
| openmode           | Action if file already exists | Action if file does not exist |
+--------------------+-------------------------------+-------------------------------+
| in                 | Read from start               | Failure to open               |
+--------------------+-------------------------------+-------------------------------+
| out, out|trunc     | Destroy contents              | Create new                    |
+--------------------+-------------------------------+-------------------------------+
| app, out|app       | Append to file                | Create new                    |
+--------------------+-------------------------------+-------------------------------+
| out|in             | Read from start               | Error                         |
+--------------------+-------------------------------+-------------------------------+
| out|in|trunc       | Destroy contents              | Create new                    |
+--------------------+-------------------------------+-------------------------------+
| out|in|app, in|app | Write to end                  | Create new                    |
+--------------------+-------------------------------+-------------------------------+

PS:

Some basic error handling could also prove useful in understanding what's going on:

#include <iostream>
#include <fstream>

int main()
{
  std::fstream file("triangle.txt");
  if (!file) {
    std::cerr << "file open failed: " << std::strerror(errno) << "\n";
    return 1;
  }
  file << "Some text " << std::endl;
}

Output:

 C:\temp> mytest.exe
 file open failed: No such file or directory

 C:\temp>

Why doesn't std::fstream write to a file?

1 Answers1