Read the whole file, then overwrite all in same std::fstream

Question

Although I found a lot of stuff about std::fstream on SO, I still can't understand how it operates. Let's see this minimal example:

#include <fstream>
#include <string>

int main()
{
  std::fstream fs{ "somefile.txt", std::fstream::out | std::fstream::in };
  std::string line;

  while (std::getline(fs, line))
  {
    // reads the whole file
  }

  // the purpose is to overwrite the whole file
  fs.seekp(0, std::ios_base::beg); // moves at the beginning
  fs << "Hello world!" << std::endl; // writes in the file

  // possibly other read/write
}

This does not work, it seems that one cannot firstly read, and then write in the same stream according to everything I read about. I know the workaround that consists in closing the file, then opening it with the std::ios_base::trunc flag. However, that seems to be nonsensical: why is there such a limitation? Cannot we technically just overwrite the file after the reading?

Answer 1

After reading the whole file, the fs is now in eof state. You need to clear the state so it can do following IO operations.

Answer 2

The loop iterates until the fail bit gets set. Just clear() the flags prior to the seek:

fs.clear();

If you only want to overwrite the starting bit that is sufficient. If also want to truncate the stream I'd explicitly close() and open() it, where the open() would not set std::ios_base::in.