Error(s):

source_file.cpp:16:15: error: cannot define member function ‘A::f’ within ‘Derived’
     void A::f() {
               ^
source_file.cpp:19:15: error: cannot define member function ‘B::f’ within ‘Derived’
     void B::f() { 
               ^
source_file.cpp: In function ‘int main()’:
source_file.cpp:26:13: error: cannot declare variable ‘derived’ to be of abstract type ‘Derived’
     Derived derived;
             ^
source_file.cpp:14:7: note:   because the following virtual functions are pure within ‘Derived’:
 class Derived :public A, public B {
       ^
source_file.cpp:12:18: note: 	virtual void B::f()
     virtual void f() = 0;
                  ^
source_file.cpp:8:18: note: 	virtual void A::f()
     virtual void f() = 0;
                  ^