How to validate deleted pointer?

Question

May be this is very dumb question. But I don't know answer to this.

If object to which pointer is referring is deleted by someone( we don't know who is deleting it ), but I still have raw pointer to it. The how to validate if its a valid or not. I though null check would help, but it does not.

Here is an example:

int main()
{
   class A
   {
   public:
      A() = default;
      ~A() = default;
   private:
      int a;
      long b;
   };

   A* a = new A();
   delete a; // deleted here
   A* b = reinterpret_cast< A* >( a ); // still trying to use it 
   if( b == nullptr )
      std::cout << "B is Null"; // b should have come as null, but it does not and I do not see this line in output.

   // same here
   A* c = ( A* ) a;
   if( c == nullptr )
      std::cout << "C is null";
   getchar();
    return 0;
}

If you see in above picture, a is not null, and has some garbage values.

I know this is very naïve code, but this show problem that I have with actual code.

Thanks, Kailas

You cannot validate whether a pointer points to valid memory or not, you simply have to know yourself. — SirGuy, May 09 '17 at 19:34
On a per system basis, many people do code asserts that check the pointer to see if it points to valid memory. What is valid though varies a lot system to system so you have to code it native to each. It's honestly a great check to have though, — Michael Dorgan, May 09 '17 at 19:40
You should store the pointer created by `new A()` into a [`std::unique_ptr`](http://www.cplusplus.com/reference/memory/unique_ptr). That way it will get destructed automatically when it goes out of scope (you can still delete it manually). This does not fully answer your question, but in your code snippet it would have solved your problem. — chtz, May 09 '17 at 20:20

score 3 · Answer 1 · edited May 23 '17 at 12:17

3

No, you can't know whether a pointer points to valid memory.

From here:

Because it would be expensive to maintain meta data about what constitutes a valid pointer and what doesn't, and in C++ you don't pay for what you don't want.

Moreover, you don't really need to check if a pointer is valid because you, as programmer, know where the pointers come from.

edited May 23 '17 at 12:17

Community

1
1

answered May 09 '17 at 19:37

granmirupa

2,780
16
27

score -5 · Answer 2 · answered May 09 '17 at 19:38

-5

You can check using if(!a) or if(a) for validating.

answered May 09 '17 at 19:38

Raging bull

16
2

What about on systems where 0 is a valid address (vector table pointers and such)? This is a bit too broad of a statement as is commented elsewhere. – Michael Dorgan May 09 '17 at 19:50

How to validate deleted pointer?

2 Answers2