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?
From the answers, I take that (please edit if incorrect):
delete
of an instance of B calls B::~B();A::~A();
A::~A
should explicitly delete
all heap-allocated member variables of the A object;This question is related to
c++
memory-management
destructor
delete-operator
base-class
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".
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).
The destructor for the object of class A will only be called if delete is called for that object. Make sure to delete that pointer in the destructor of class B.
For a little more information on what happens when delete is called on an object, see: http://www.parashift.com/c++-faq-lite/freestore-mgmt.html#faq-16.9
You should delete A yourself in the destructor of B.
If you have a usual pointer (A*
) then the destructor will not be called (and memory for A
instance will not be freed either) unless you do delete
explicitly in B
's destructor. If you want automatic destruction look at smart pointers like auto_ptr
.
No. the pointer will be deleted. You should call the delete on A explicit in the destructor of B.
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.
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
no it will not call destructor for class A, you should call it explicitly (like PoweRoy told), delete line 'delete ptr;' in example to compare ...
#include <iostream>
class A
{
public:
A(){};
~A();
};
A::~A()
{
std::cout << "Destructor of A" << std::endl;
}
class B
{
public:
B(){ptr = new A();};
~B();
private:
A* ptr;
};
B::~B()
{
delete ptr;
std::cout << "Destructor of B" << std::endl;
}
int main()
{
B* b = new B();
delete b;
return 0;
}
You have something like
class B
{
A * a;
}
B * b = new B;
b->a = new A;
If you then call delete b;
, nothing happens to a, and you have a memory leak. Trying to remember to delete b->a;
is not a good solution, but there are a couple of others.
B::~B() {delete a;}
This is a destructor for B that will delete a. (If a is 0, that delete does nothing. If a is not 0 but doesn't point to memory from new, you get heap corruption.)
auto_ptr<A> a;
...
b->a.reset(new A);
This way you don't have a as a pointer, but rather an auto_ptr<> (shared_ptr<> will do as well, or other smart pointers), and it is automatically deleted when b is.
Either of these ways works well, and I've used both.
Source: Stackoverflow.com