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I'm taking a quiz on an intro level class in C++ and I'm trying to understand a question. After searching the internet and not getting an answer, so here I am.

Which of the following function declarations will accept either cout or a file stream object as its argument? A. void output( fstream &outFile); B. void output( ofstream &outFile); C. void output( ostream &outFile); D. void output( iostream &outFile);

The answer is C.

I know the difference between: fstream, ofstream, ostream, iostream.

What I don't understand is why none of the other options are able to accept the cout or file stream object as an argument.

Is the answer as simple as ostream objects contain data (char,etc) that are able to be passed as arguments?

Any information would be greatly appreciated.

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    Perhaps this C++ I/O reference and its inheritance hierarchy diagram can help? Commented Oct 11, 2017 at 14:04
  • Related: Is it possible to pass derived classes by reference to a function taking base class as a parameter Commented Oct 11, 2017 at 14:07
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    I would choose D because a file stream can be input and output. Answer C is output only. Commented Oct 11, 2017 at 14:09
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    @ThomasMatthews: D doesn't work because you can't pass std::cout to it; it's not an std::istream. Commented Oct 11, 2017 at 14:09
  • If I have an ifstream for reading, which is a file stream, I can't use C. The question should be reworded. :-) Commented Oct 11, 2017 at 14:12

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The answer is C. The question is about the inheritance hierarchy. std::cout is an instance of std::ostream. All other functions accept subclasses of std::ostream and can therefore not handle std::cout. std::fstream could be passed to all of them but the question was about both.

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