How to overwrite only part of a file in c++

I want to make modifications to the middle of a text file using c++, without altering the rest of the file. How can I do that?

Use std::fstream.

The simpler std::ofstream would not work. It would truncate your file (unless you use option std::ios_base::app, which is not what you want anyway).

std::fstream s(my_file_path); // use option std::ios_base::binary if necessary
s.seekp(position_of_data_to_overwrite, std::ios_base::beg);
s.write(my_data, size_of_data_to_overwrite);

If the replacement string is the same length, you can make the change in place. If the replacement string is shorter, you may be able to pad it with zero-width spaces or similar to make it the same number of bytes, and make the change in place. If the replacement string is longer, there just isn't enough room unless you first move all remaining data.