Does delete call the destructor?

Question

I have an class (A) which uses a heap memory allocation for one of it's fields. Class A is instantiated and stored as a pointer field in another class (B).

When I'm done with object B, I call delete, which I assume calls the destructor... But does this call the destructor in class A as well?

Edit:

From the answers, I take that (please edit if incorrect):

1. delete instance of B calls B::~B();
2. which calls A::~A();
3. and A::~A should explicitly delete all heap-allocated member variables of A;
4. and finally the memory block storing said instance of B is returned to the heap - when new was used, it first allocated a block of memory on heap, then invoked constructors to initialize it, now after all destructors have been invoked to finalize the object the block where the object resided is returned to the heap.

Answer 1

The destructor of A will run when its lifetime is over. If you want its memory to be freed and the destructor run, you have to delete it if it was allocated on the heap. If it was allocated on the stack this happens automatically (i.e. when it goes out of scope; see RAII). If it is a member of a class (not a pointer, but a full member), then this will happen when the containing object is destroyed.

class A
{
    char *someHeapMemory;
public:
    A() : someHeapMemory(new char[1000]) {}
    ~A() { delete[] someHeapMemory; }
};

class B
{
    A* APtr;
public:
    B() : APtr(new A()) {}
    ~B() { delete APtr; }
};

class C
{
    A Amember;
public:
    C() : Amember() {}
    ~C() {} // A is freed / destructed automatically.
};

int main()
{
    B* BPtr = new B();
    delete BPtr; // Calls ~B() which calls ~A() 
    C *CPtr = new C();
    delete CPtr;
    B b;
    C c;
} // b and c are freed/destructed automatically

In the above example, every delete and delete[] is needed. And no delete is needed (or indeed able to be used) where I did not use it.

auto_ptr, unique_ptr and shared_ptr etc... are great for making this lifetime management much easier:

class A
{
    shared_array<char> someHeapMemory;
public:
    A() : someHeapMemory(new char[1000]) {}
    ~A() { } // someHeapMemory is delete[]d automatically
};

class B
{
    shared_ptr<A> APtr;
public:
    B() : APtr(new A()) {}
    ~B() {  } // APtr is deleted automatically
};

int main()
{
    shared_ptr<B> BPtr = new B();
} // BPtr is deleted automatically

Answer 2

You should delete A yourself in the destructor of B.

Answer 3

It is named "destructor", not "deconstructor".

Inside the destructor of each class, you have to delete all other member variables that have been allocated with new.

edit: To clarify:

Say you have

struct A {}

class B {
    A *a;
public:
    B () : a (new A) {}
    ~B() { delete a; }
};

class C {
    A *a;
public:
    C () : a (new A) {}        
};

int main () {
    delete new B;
    delete new C;
}

Allocating an instance of B and then deleting is clean, because what B allocates internally will also be deleted in the destructor.

But instances of class C will leak memory, because it allocates an instance of A which it does not release (in this case C does not even have a destructor).

Answer 4

class B
{
public:
    B()
    {
       p = new int[1024];  
    }
    virtual ~B()
    {
        cout<<"B destructor"<<endl;
        //p will not be deleted EVER unless you do it manually.
    }
    int *p;
};


class D : public B
{
public:
    virtual ~D()
    {
        cout<<"D destructor"<<endl;
    }
};

When you do:

B *pD = new D();
delete pD;

The destructor will be called only if your base class has the virtual keyword.

Then if you did not have a virtual destructor only ~B() would be called. But since you have a virtual destructor, first ~D() will be called, then ~B().

No members of B or D allocated on the heap will be deallocated unless you explicitly delete them. And deleting them will call their destructor as well.

Answer 5

I was wondering why my class' destructor was not called. The reason was that I had forgot to include definition of that class (#include "class.h"). I only had a declaration like "class A;" and the compiler was happy with it and let me call "delete".

Answer 6

No. the pointer will be deleted. You should call the delete on A explicit in the destructor of B.