prog.cpp: In function ‘int main()’:
prog.cpp:15:4: error: cannot declare variable ‘c’ to be of abstract type ‘C’
C c;
^
prog.cpp:10:7: note: because the following virtual functions are pure within ‘C’:
class C : public B, public A {
^
prog.cpp:2:15: note: virtual void A::f()
virtual void f() = 0;
^