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Tip of the Day
Language: C++
Expertise: Intermediate
Jul 23, 1999

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Forward-Declaring I/O Classes and Templates

The standard header <iosfwd> contains forward-declarations of the I/O classes and templates. This header is sufficient to refer to any of the I/O classes and templates but not to apply operation to them. For example:

 
#include 
using namespace std;

class C{
public:
  friend ostream& operator<<(ostream& os, const C& d); 
};

ostream& operator<<(ostream& os, const C& d);

The declaration of the friend function does not need a complete definition of the ostream class; a forward declaration is sufficient in this case. Therefore, <iosfwd> is #included instead of the full-blown <iostream>, which significantly reduces compilation time.

Danny Kalev
 
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