c++ virtual public inheritance init strange result?

Question

The output of the program below is: 5 5

#include <iostream>
using namespace std;

struct A
{
public:
    int myInt;
    A(int n): myInt(n){}
    A(): myInt(5) {}
};


class B : virtual public A
{
public:
    B(int n):A(10) {}
    B():A(10) {}
};

class C : virtual public A
{
public:
    C(int n):A(3*n) {}
};

class D : public B, public C
{
public:
    D(int n=90) : C(2*n), B(n) {}
};

class E : public D
{
public:
    E(int n=20):D(n-1) {}
};

int main()
{
    D d(100);
    cout << d.myInt << endl;
    E e;
    cout << e.myInt << endl;
    return 0;
}

Consider the object d. From what I understand the inheritance is constructed based on the order of the inheritance list (rather than the initialization list) so B class is constructed first with the param 100 which goes to class A with the parameter 10. So now A sets myInt to the value 10. The same goes for Class c and because myInt is virtual then it is set to the number 600. I never expected 5. why is this happening?

why don't you set a breakpoint in your constructors to A, start the program up in the debugger and find out what is going on? — Thomas, Feb 16 '13 at 13:18
@Thomas: debugging won't explain _why_ that happens though, only what happens. — Mat, Feb 16 '13 at 13:22

score 0 · Accepted Answer · answered Feb 16 '13 at 13:25

See article in parashift:

Because a virtual base class subobject occurs only once in an instance, there are special rules to make sure the virtual base class's constructor and destructor get called exactly once per instance. The C++ rules say that virtual base classes are constructed before all non-virtual base classes. The thing you as a programmer need to know is this: constructors for virtual base classes anywhere in your class's inheritance hierarchy are called by the "most derived" class's constructor.

c++ virtual public inheritance init strange result?

1 Answers1