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#include <iostream>
 
class base{
public:
	virtual int first(int a) = 0;
	int final_result(int a) {
		return 3*first(a);
	}
//	virtual int multiply(int a, int b, int c) = 0;
 
};
 
class derived : public base {
public:
	int first(int a) {
		return 2*a;
	}
};
 
class another_derived : public base {
public:
	int first(int a, int b) {
		return a + b;
	}
 
};
 
int main() {
 
	derived d;
	std::cout << d.final_result(1) << "\n";
	another_derived d2; // doesn't work
	return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:32:18: error: cannot declare variable ‘d2’ to be of abstract type ‘another_derived’
  another_derived d2; // doesn't work
                  ^~
prog.cpp:20:7: note:   because the following virtual functions are pure within ‘another_derived’:
 class another_derived : public base {
       ^~~~~~~~~~~~~~~
prog.cpp:5:14: note: 	‘virtual int base::first(int)’
  virtual int first(int a) = 0;
              ^~~~~

Standard output is empty