Will "delete this" in a base class pointer delete the derived class object when you have a virtual destructor?

Question

I have a class hierarchy that's three levels deep, like this:

class A {
public: 
    virtual ~A() {}

}

class B : public A {
public:
    virtual ~B() {}
    void foo(E *e) {
        e->remove(this);
    }
}

class C : public B {
public:
    ~C() {}
}

class E {
public:
    void remove(A *a) {
        delete a;
    }
}

Ok so what I am wondering is what happens when I call foo() on an object of C. Is it going to remove the entire object or only the B and A part of the object, and leave the C part still in memory?

You have syntax errors. Please fix them. Also, avoid delete this like the plague. Also, void remove(A* a) in class E, but void remove(E* e) in class B? Are you sure? Yikes! — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Jan 6, 2013 at 14:28
You are destroying all the object, and releasing its memory. — Basile Starynkevitch
– Basile Starynkevitch, Commented Jan 6, 2013 at 14:28
Try access a method of an instance of class B after you removed A is an alternative way of checking this. — Floris Velleman
– Floris Velleman, Commented Jan 6, 2013 at 14:31

jpalecek · Accepted Answer · 2013-01-06 14:33:46Z

5

Is it going to remove the entire object or only the B and A part of the object, and leave the C part still in memory?

No. It will "do the right thing" (that is, delete the most derived subobject, run all its destructors etc.) provided A (that is, the static type of the pointee of the pointer you delete) has a virtual destructor (and if class A has a virtual destructor, all its descendants have it, too). This holds for multiple inheritance, too.

answered Jan 6, 2013 at 14:33

jpalecek

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Comments

NPE · Accepted Answer · 2013-01-06 14:34:45Z

2

Thanks to the virtual destructor in A the code would work correctly destroy the entire instance of C.

answered Jan 6, 2013 at 14:34

NPE

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1 Comment

Matthew Hoggan Over a year ago

Is this true for all compilers? I swear I have seen C destroyed without A having declared a virtual destructor. Mainly in gcc.

Sebastian Graf · Accepted Answer · 2013-01-06 14:41:00Z

delete will always release the memory for the whole object pointed to. This doesn't apply to called destructors however: It will try to call the destructor of the static type of the object to delete. In a polymorphistic scenario, this is often not what you want. Consider this:

struct Base { };

struct Derived : Base
{
    int* i;

    Derived() : i(new int) { }

    ~Derived() { delete i; }
}

void bad()
{
    Base* b = new Derived;
    delete b;
}

bad() causes a memory leak, because Deriveds destructor will never be called. This is because the static type of b is Base*, thus Base::~Base will be called. There is no destructor defined in Base, so the default implementation provided by the compiler executes, which does nothing in this particular example.

This however does not apply to your example: You made the root class' destructor virtual, so all derived classes' destructors will be executed as part of the destructor call.

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