C++ : What is the Right way to delete a derived class instance?

Question

Assuming we have :

class A {
protected:
    int* iArr;
    float *fArr;
public:
    A(int* iArr,float *fArr);
    ~A();
}

class B : public A {
private:
    double *dArr;
public:
    B(int* iArr,float *fArr,double *dArr);
    ~B();
}

my intuation was that it would only invoke B's destructor but when i ran it on Visual C++ i saw that when destructing an instance of B it calls both A and then B destructor.

What is the correct way to write destructor in a derived class then? do i always need to assume parent class would take care of deleting everything but what only the derived class have?

Edit:

if it is so, then what if the child extending parent class only with overriding functions,does that mean that i leave the child's destructor empty?
what if i want to change that? i mean what if i want only child destructor to be called? is there a way to do that?

For future reference, the correct terms are parent and child over father and son, but when dealing with classes it's typically base and derived too. — chris, Jun 10 '12 at 18:42
after destructor of parent class has been called, child class's one will be calling! — Maziar Aboualizadehbehbahani, Jun 10 '12 at 18:44
@MaziarBouali: You sure that's not the other way around? Stuff typically gets destructed in the opposite order than it was constructed; that is, last constructed, first destructed... — cHao, Jun 10 '12 at 18:58
@cHao: i want to say: calling destructor of parent cause calling that of its child. im not talking about their calling order. you are right about their calling order. — Maziar Aboualizadehbehbahani, Jun 10 '12 at 19:05
@Maziar: That's not quite right either, though. See Goz's example below. If the destructor isn't `virtual`, the parent class destructor does not trigger the child class destructor -- see, the parent class doesn't even *know* about the child class, so the only way the child constructor could run is if it were `virtual` or someone tried to delete a `B*` (rather than an `A*` that happens to point at a `B`). In which case the *child* destructor runs and triggers the *parent* destructor to run. — cHao, Jun 10 '12 at 19:12

score 6 · Answer 1 · edited May 23 '17 at 10:28

In a word, yes.

The parent class's destructor always gets called automatically. Its job is to free up all resources used by the parent class.

The child class's destructor is responsible for freeing up things that are specific to the child class.

Additionally, if you ever delete instances of the child class through a pointer to the parent class, you need to make the parent class's destructor virtual. See When to use virtual destructors?

Goz · Accepted Answer · 2012-06-11T08:08:15.970

The destructor of A should be virtual. Your problem lies that if you do the following:

A* b = new B( iArr, fArr, dArr );
delete b;

Then B's destructor won't get called because b while being instantiated as a B looks like an A and B's destructor doesn't have an entry in the virtual table which means it doesn't know its there (ie it thinks its just an A)

Edit:

In answer to 1 you could leave the destructor empty or you could not define it.

In answer to 2, you can't really do that. You'd be best off having a virtual protected function called "Destroy" or something thats called from the base class destructor. That is the only way I can think of to control destruction like you describe.

C++ : What is the Right way to delete a derived class instance?

2 Answers2