identifier “ostream” is undefined error [closed]

Answer 1

You need to fully qualify the name ostream with the name of the namespace that class lives in:

    std::ostream
//  ^^^^^

So your operator declaration should become:

friend std::ostream& operator << (std::ostream &os, const Number &f);
//     ^^^^^                      ^^^^^

Alternatively, you could have a using declaration before the unqualified name ostream appears:

using std::ostream;

This would allow you to write the ostream name without full qualification, as in your current version of the program.

Answer 2

Andy Prowl's answer is great but please resist putting "using std::ostream" in a header. If you do this then other compilation units using your header file will also have this namespace 'used' by default and you can get nasty compilation errors with namespace clashes.