Binary output to file in random positions in C++

Question

I'm beginning to feel awkwardly stupid, but I have a problem with outputting binary data to a file. I have a file, let's say, 1000 bytes long. What I would like to do, in C++, is simply opening the file, replace ONE byte in a given position (let's say, the i-th byte), and close it.

File before operation: AAAAAA File after operation: AAABAA

What is the easiest way to do so? I tried to open it with a ofstream.open, with the following modes:

ios::binary|ios::out
ios::binary|ios::app
ios::binary|ios::ate

All of these affected the actual size of the file after the operation. What should I do? I'm beginning to feel desperate.

Thank you very much and merry christmas to everybody!

Matteo

Answer 1

Besides binary mode, you need to open it in out and in modes. This corresponds to the fopen mode "r+b" which opens a file for reading and writing, and doesn't truncate the file if it exist (in fact, it must exist or you will fail to open the file).

References:

• std::basic_filebuf::open
• std::fopen