Why 'ifstream' and 'ofstream' are added to "std", while 'fstream' can serve both the purposes?

Question

Using std::fstream one can declare objects of both the types ifstream and ofstream. The only difference is that, with fstream we need to provide in, out, app as a parameter which may not always require for other two.

Is there anything special about ifstream,ofstream which cannot be accomplished with fstream or just a coding convenience ?

Answer 1

If anything, fstream is the one that's just a convenience. In particular, what you have is basically:

namespace std { 
class ifstream { /* ... */ };

class ofstream { /* ... */ };

class fstream : public ifstream, public ofstream { /* ... */ };
}

[obviously skipping over a lot of irrelevant details].

In short, the fstream provides all of the input capabilities of an ifstream and all the output capabilities of a ofstream by deriving from both ifstream and ofstream. Without ifstream and ofstream, an fstream (at least in anything resembling its current form) couldn't exist at all.

Answer 2

It's a bit like asking why we'd want const when you can read and write from variables anyway. It allows compile-time checking, an invaluable feature for reducing bugs. It's also more self-documenting, as when looking at a declaration without the constructor call you can see whether it's an input, output or both: the parameters you mention can often only be seen in the implementation file which may not be to hand. Also, each type of stream may have a few differences in the data members they need - potentially using the minimally-functional class matching your actual needs could save memory, time initialising or checking those other variables etc..