Why do I need to include both the iostream and fstream headers to open a file

#include <iostream>
#include <fstream>
using namespace std;

int main () {
  ofstream myfile;
  myfile.open ("test.txt");
  return 0;
}

fstream is derived from iostream, why should we include both in the code above?

I removed fstream, however, there is an error with ofstream. My question is ofstream is derived from ostream, why fstream is needed to make it compile?

You need to include fstream because that's where the definition of the ofstream class is.

You've kind of got this backwards: since ofstream derives from ostream, the fstream header includes the iostream header, so you could leave out iostream and it would still compile. But you can't leave out fstream because then you don't have a definition for ofstream.

Think about it this way. If I put this in a.h:

class A {
  public:
    A();
    foo();
};

And then I make a class that derives from A in b.h:

#include <a.h>

class B : public A {
  public:
    B();
    bar();
};

And then I want to write this program:

int main()
{
  B b;
  b.bar();

  return 0;
}

Which file would I have to include? b.h obviously. How could I include only a.h and expect to have a definition for B?

Remember that in C and C++, include is literal. It literally pastes the contents of the included file where the include statement was. It's not like a higher-level statement of "give me everything in this family of classes".