Why normal virtual function required blank body in base class

[am 2009]

Hi, I have following block of code,

class Base
{
public:
Base()
{
cout<<"Constructor: Base"<<endl;
}
~Base()
{
cout<<"Destructor : Base"<<endl;
}
virtual void fun();

};
class Derived: public Base
{

   public:
       Derived()
	   { 
		   cout<<"Constructor: Derived"<<endl;
	   }
       ~Derived()
	   { 
		   cout<<"Destructor : Derived"<<endl;
	   }
	    void fun()
	  {
	    cout<<" in der";
	  }

};
int _tmain(int argc, _TCHAR* argv[])
{
Derived obj;
obj.fun();
getche();

In above code virtual fun() is showing error that it not resolved. If i add blank body then Derived class's Fun()is called. Is it necessary to have blank body for virtual function in base class and what is reason of this. :omg: Thanks abm

[Alain Rist]

Hi, Your Base::fun() member is declared but not defined, so the linker complains. You have to define it somewhere either as pure virtual virtual void fun() = 0; or otherwise:

//do nothing
virtual void Base::fun(){};

cheers, AR

When the wise (person) points at the moon the fool looks at the finger (Chinese proverb)