C++ fstream overwrite file

Question

This is my code (without error checking):

std::wfstream wfs("C:\\testfile.txt");
wfs.setf(std::ios_base::binary);
std::wstringstream wss;
wss << wfs.rdbuf();
wfs.seekp(0);
wfs << L"new";

Now, the first three characters in the file are replaced with "new". However, I would like "new" to be the only contents of the file.

How do I achieve that?

`setf` takes format flags as an argument, not open mode flags. You want `std::wfstream wfs("C:\\testfile.txt", std::ios_base::binary)`. — David G, Mar 15 '16 at 18:45
Oh, then I also need to copy the default arguments such as `std::ios::in | std::ios::out | std::ios::binary` right? — Felix Dombek, Mar 15 '16 at 18:47
Please don't add answers to the question. If you want to answer your own question, that is fully possible by adding a new answer. The question box is just for the actual question. — default, Mar 15 '16 at 18:53
Is that really a duplicate? That question specifies the read/trunk needs to be "atomic". — Galik, Mar 15 '16 at 19:15
This is possible using the new [Filesystem Technical Specification](http://en.cppreference.com/w/cpp/experimental/fs): **See:** http://stackoverflow.com/a/36020320/3807729 — Galik, Mar 15 '16 at 19:31

Samer · Answer 1 · 2016-03-15T18:44:51.320

0

Try this

wfs.setf(std::ios_base::binary,std::ios_base::trunc);

edited Mar 15 '16 at 18:44

answered Mar 15 '16 at 18:43

Samer

I had the impression that this erases the file before I can read; is this not correct? – Felix Dombek Mar 15 '16 at 18:43
yes, i thought this is what you want to do! – Samer Mar 15 '16 at 18:45
No, I first read the contents! Then overwrite the file. I thought the `seekp` followed by a `<<` would truncate it, but it doesn't. – Felix Dombek Mar 15 '16 at 18:45
1

why not after reading the contents, you close it and open it again with `trunc` enabled – Samer Mar 15 '16 at 18:46
Sure, that would be a workaround if it's not possible to get the stream to clear the file... – Felix Dombek Mar 15 '16 at 18:49

1 Answers1