c++ Virtual/Non-virtual Diamond inheritance -

given following code in c++:

struct {     a() { f(0); }     a(int i) { f(i); }     virtual void f(int i) { cout << i; } }; struct b1 : virtual {     b1(int i) : a(i) { f(i); }     virtual void f(int i) { cout << i+10; } }; struct b2 : virtual {     b2(int i) : a(i) { f(i); }     virtual void f(int i) { cout << i+20; } }; struct c : b1, virtual b2 {     int i;     c() : b1(6),b2(3),a(1){}     virtual void f(int i) { cout << i+30; } }; 

can explain why c* c = new c(); print 1 23 , 16 in order? how decide order print in? know nonvirtual b1 called last why a() called first? , explanation ahead of time.

because virtually inheriting b2, compiler construct first identify variables virtually inherited in c before constructs non-virtual inheritance (b1). of course, a gets constructed first because b2 needs before can constructed.