C++ set a pointer of pointer to null?

Question

I'm working on some kind of smart pointer technique but there is one piece I'm missing. I tried several combinations but the logic is as follow:

UInt *obj = new UInt;
UInt *ref;
ref = obj;

delete obj;
obj = NULL;

if (ref == NULL)
{
    // It works
}
else
{
    // It failed
}

Is there any way to hit "It Works" without setting ref to NULL explicitly?

EDIT:

A more appropriate scenario would be something like this:

class A
{
public:

    A(): ref(NULL) {}
    ~A()
    {
        if (ref != NULL)
            delete ref;
    }
    int *ref;
};

    int *obj = new int;
    A *host = new A();

    host->ref = obj; ???

    delete obj;
      obj = NULL;

    if (host->ref == NULL)
    {
        // It works.
    }
    else
    {
        // It failed.
    }

...

Can't use int*& ref as a class member though.... must be close.

Is UInt your smart pointer class? Smart pointers should not be allocated on the heap. — Seth Carnegie
– Seth Carnegie, Commented Feb 1, 2013 at 16:37
What does "pointer of pointer" stand for in the title of the question? — AnT stands with Russia
– AnT stands with Russia, Commented Feb 1, 2013 at 16:38
I'm sorry, forget about the smart pointer scenario. UInt could be int instead. — neosettler
– neosettler, Commented Feb 1, 2013 at 17:06
yeah i am going to have to concur with the other comments. those pointers seem pretty dumb to me. — thang
– thang, Commented Feb 1, 2013 at 17:07

ecatmur · Accepted Answer · 2013-02-01 16:47:08Z

4

As you say, you should be using a smart pointer:

#include <memory>

std::shared_ptr<UInt> obj = std::make_shared<UInt>();
std::weak_ptr<UInt> ref = obj;

obj.reset();

if (ref.expired())
{
    // It works
}
else
{
    // It failed
}

Don't try managing your own memory when the standard library has facilities to do it for you.

answered Feb 1, 2013 at 16:47

ecatmur

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2 Comments

Etherealone Over a year ago

Exactly, so that people who read your code will not need to read and understand your own implementation, when there is a standard that everybody knows. Also, most probably, standard will have it better than yours.

neosettler Over a year ago

True, I never use the std lib, I guess I could give it a try. I'm still interested how I could accomplish the example.

Andreas Brinck · Accepted Answer · 2013-02-01 16:39:52Z

3

Declare ref as a reference to pointer

Uint*& ref = obj;

ref will now refer to the obj pointer.

answered Feb 1, 2013 at 16:39

Andreas Brinck

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Comments

Veger · Accepted Answer · 2013-02-01 16:41:49Z

1

Intially both obj and ref point to the (same) UInt instance. Then you delete the instance and set obj to NULL. But ref is just like any other regular pointer, so it still points to the deleted instance.

Instead you can create a reference variable, or in this case a 'reference pointer' by adding & to the declaration:

Uint*& ref = obj;

Then ref really refers to obj and keeps the same value (pointer) as obj.

answered Feb 1, 2013 at 16:41

Veger

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2 Comments

neosettler Over a year ago

Oh excellent, I used this example for simplicity sake but what if ref is a class member?

Veger Over a year ago

stackoverflow.com/questions/892133/… provides some details, cons and pros of references as class members vs pointers

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