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#include <iostream>
using namespace std;
 
class Base
{
	public:
		virtual void Foo() = 0;
};
 
class Derived
	: public Base
{
	public:
		// void Foo() { } // Derived cannot be instantiated, it MUST define Foo()
		                  // OR its derived classes MUST define Foo(). The 
		                  // method is NOT optional when you want to instantiate
		                  // the class
};
 
int main() {
	// your code goes here
	Derived d;
	return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:22:10: error: cannot declare variable ‘d’ to be of abstract type ‘Derived’
  Derived d;
          ^
prog.cpp:10:7: note:   because the following virtual functions are pure within ‘Derived’:
 class Derived
       ^
prog.cpp:7:16: note: 	virtual void Base::Foo()
   virtual void Foo() = 0;
                ^

Standard output is empty