Does delete on a pointer to a subclass call the base class destructor?


Question

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

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

Edit:

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

  1. delete of an instance of B calls B::~B();
  2. which calls A::~A();
  3. A::~A should explicitly delete all heap-allocated member variables of the A object;
  4. Finally the memory block storing said instance of class 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.
1
161
4/16/2019 9:03:59 PM

Accepted Answer

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
180
12/14/2011 9:21:24 PM

When you call delete on a pointer allocated by new, the destructor of the object pointed to will be called.

A * p = new A;

delete p;    // A:~A() called for you on obkect pointed to by p

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