C++ INCLUDE Rule

C++ INCLUDE Rule : Use forward declaration when possible

Suppose you want to define a new class B that uses objects of class A.
B only uses references or pointers to A. Use forward declaration then : you don’t need to include . This will in turn speed a little bit the compilation.
class A ;

class B {
private:
A* fPtrA ;
public:
void mymethod(const& A) const ;
} ;

B derives from A or B explicitely (or implicitely) uses objects of class A. You then need to include
#include

class B : public A {

} ;

class C {
private:
A fA ;
public:
void mymethod(A par) ;
}

Ref: http://www-subatech.in2p3.fr/~photons/subatech/soft/carnac/CPP-INC-1.shtml

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