Some confusion about the order of constructor call and inheritance

Question

I have the following code:

class A { public: A() { cout << "A-C" << endl; } ~A(){ cout << "A-D" << endl; } }; class B { public: B() { cout << "B-C" << endl; } ~B() { cout << "B-D" << endl; } }; class C { protected: A* a; B b; public: C() { a = new A; cout << "C-C" << endl; } ~C() { delete a; cout << "C-D" << endl; } }; class D : public C { protected: A a; B b; public: D(){ cout << "D-C" << endl; } ~D(){ cout << "D-D" << endl; } }; int main() { D* d = new D; B* b = new B; delete b; delete d; system("pause"); return 0; }

My initial thoughts on the output were:

A-C C-C A-C B-C D-C B-C B-D D-D B-D A-D A-D C-D

But it's wrong. The output is actually:

B-C A-C C-C A-C B-C D-C B-C B-D D-D B-D A-D A-D C-D B-D

I don't know why the program calls the B's constructor firstly and its destructor lastly. I think the order of constructor call should like this: C's constructor-> A's constructor -> B's constructor -> D's constructor.

And the order of destructor call is the reverse order of constructor call

Anyone can tell me the reason why the B's constructor is called at the beginning and B's destructor is called at last?

Wait, why should the output be B-C first? new D calls the C constructor which calls the A constructor. — Hatted Rooster
– Hatted Rooster, Commented Apr 22, 2017 at 16:45
@GillBates nuh-huh. C has a B data member that's initialized before the constructor's body. — Quentin
– Quentin, Commented Apr 22, 2017 at 16:47
@Quentin I thought that B data member in C was a pointer, my bad. — Hatted Rooster
– Hatted Rooster, Commented Apr 22, 2017 at 16:47
@aschepler Yeah, op meant that he's getting the 2nd part as actual output, just wasn't worded correctly. — Hatted Rooster
– Hatted Rooster, Commented Apr 22, 2017 at 16:56
it seems that my lecturer is wrong lol, he said that when the class inherit from another class, only the base class constructor will be called before the derived class constructor...He hasn't mentioned about the initialization of base class data member. — Kelvinyu1117
– Kelvinyu1117, Commented Apr 22, 2017 at 17:22

Hatted Rooster · Accepted Answer · 2017-04-22 16:52:29Z

B-C A-C C-C A-C B-C D-C

All of these are caused by D* d = new D;.

The D constructor calls the C constructor because C is a base of D.

Then, data members a and b of class C are initialized, a is a pointer so no constructor calls for that data member yet. b is an object of type B so the B parameterless constructor gets called, giving you the first B-C.

Then in the C constructor you say new A which calls the constructor of A which gives you A-C followed by C-C.

Then, data members a and b of class D are initialized, both are objects so both constructors are called, giving you A-C and B-C.

And finally, the D constructor is called, ending it with D-C.

Thank you very much, do you mean that when the class inherit from another class, let say D inherit from C, it will initialize the data member of the base class first and the constructor will be called after the initialization of the data member?

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