Read from file, clear it, write to it

Question

I'm trying to read data from a text file, clear it, and then write to it, in that order using the fstream class.

My question is how to clear a file after reading from it. I know that I can open a file and clear it at the same time, but is there some function I can call on the stream to clear its contents?

Answer 1

You should open it, perform your input operations, and then close it and reopen it with the std::fstream::trunc flag set.

#include <fstream>

int main()
{
    std::fstream f;
    f.open("file", std::fstream::in);

    // read data

    f.close();
    f.open("file", std::fstream::out | std::fstream::trunc);

    // write data

    f.close();

    return 0;
}

Answer 2

If you want to be totally safe in the event of a crash or other disastrous event, you should do the write to a second, temporary file. Once finished, delete the first file and rename the temporary file to the first file. See the Boost Filesystem library for help in doing this.