title: identifier "ostream" is undefined error
body: <p>i need to implement a number class that support operator &lt;&lt; for output.
  i have an error: "identifier "ostream" is undefined" from some reason eventhough i included  and try also </p>
  
  <p>here the header file:</p>
  
  <p>Number.h</p>
  
  <pre><code>#ifndef NUMBER_H
  #define NUMBER_H
  #include &lt;iostream&gt;
  class Number{
  public:
  //an output method (for all type inheritance from number):
  virtual void show()=0;
  
  //an output operator:
  friend ostream&amp; operator &lt;&lt; (ostream &amp;os, const Number &amp;f);
  
  
  };
  
  #endif
  </code></pre>
  
  <p>why the compiler isnt recognize ostream in the friend function?</p>