Hate to disappoint you, but..
There's no standard way of clearing the contents of a file from an open std::fstream, the straight forward way is therefore to handle the two operations as what they really are.. two operations.
First handle all the reading, and later the writing (through a different stream object).
The solution
In other words; first open the file in read-only mode (std::ifstream) and read the data you are interested in, then discard that file-handle and open the file again.. this time in write-only and truncation mode (std::ofstream), so that you will clear the contents of the file.
std::ifstream ifs ("some_file.txt");
... // read old data
ifs.close ();
std::ofstream ofs ("some_file.txt", std::ios::out | std::ios::trunc); // clear contents
... // write new data
ofs.close ();
