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copied!<p>You probably forgot to include <code>iostream</code> which makes <code>ostream</code> visible. You also need to change this into <code>std::ostream</code>, because C++ standard library names are within the namespace <code>std</code>. </p> <blockquote> <p>Do <strong>not</strong> write <code>using namespace std;</code> in a header-file, ever!</p> </blockquote> <p>It's ok to place it into the implementation file, if you want, or if you write up an example for a friend. Because any file that includes that header will have all of the standard library visible as global names, which is a huge mess and smells a lot. It suddenly increases the chance for name-clashes with other global names or other <code>using</code>'ed names - i would avoid using directives at all (see <a href="http://www.ddj.com/cpp/184401782" rel="nofollow noreferrer">Using me</a> by Herb Sutter). So change the code into this one</p> <pre><code>#include <iostream> // let ScaryDream be the interface class HereBeDragons : public ScaryDream { ... // mentioning virtual in the derived class again is not // strictly necessary, but is a good thing to do (documentary) virtual std::ostream & print( std::ostream & out ) const; ... }; </code></pre> <p>And in the implementation file (".cpp")</p> <pre><code>#include "HereBeDragons.h" // if you want, you could add "using namespace std;" here std::ostream & HereBeDragons::print( std::ostream & out ) const { return out << "flying animals" << std::endl; } </code></pre>

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