Why doesn't delete destroy anything?

Question

I'm playing a little with memory dynamic allocation, but I don't get a point. When allocating some memory with the new statement, I'm supposed to be able to destroy the memory the pointer points to using delete.

But when I try, this delete command doesn't seem to work since the space the pointer is pointing at doesn't seem to have been emptied.

Let's take this truly basic piece of code as an example:

#include <iostream>  

using namespace std;

int main()  
{  
    //I create a pointer-to-integer pTest, make it point to some new space,  
    // and fulfill this free space with a number;  
    int* pTest;  
    pTest = new int;  
    *(pTest) = 3;  
    cout << *(pTest) << endl; 

    // things are working well so far. Let's destroy this
    // dynamically allocated space!
    delete pTest;

    //OK, now I guess the data pTest pointed to has been destroyed 
    cout << *(pTest) << endl; // Oh... Well, I was mistaking.  

    return 0;  
}  

Any clue ?

Answer 1

It's time to learn what undefined behavior is. :)

In C++, when you do something illegal/nonsensical/bad/etc. the standard often says that "it leads to undefined behavior." This means that from that point forward, the state of your program is completely non-guaranteed, and anything could happen.

At the point where you do your last *(pTest), you get undefined behavior. This is because pTest does not point to a valid object, and dereferencing such a pointer is undefined. So what you're seeing is totally allowed: undefined output.

All you've done is said "I'm finished with this allocation." Once you've said that, you shouldn't (and indeed, cannot) inspect or care about that memory any longer. It doesn't even make conceptual sense to deallocate something then try to use it; you've said you were done!

Your output is somewhat predictable though: likely, your OS simply says "okay, thanks for the memory" and that's it. It has no reason to actually "reset" the memory, or do anything special. That would indeed be a waste of time, when nobody (including your own program) is not using it.

But remember, this output is completely undefined. Don't try to use objects that don't exist. Perhaps a better test would have been:

#include <iostream>

struct foo
{
    ~foo()
    {
        std::cout << "foo is gone :(" << std::endl;
    }
};

int main(void)
{
    foo* f = new foo();
    delete f; // you'll see that the object is destroyed.
}

Although it seems you were looking to see what happens with the memory itself. Just remember that it makes no sense to get rid of memory then try to use it, so the answer is: who knows. It's up to your specific platform, which C++ doesn't care about.

Answer 2

Calling delete will mark the memory area as free. It won't necessary reset its old value.

You are advised to set your pointer to 0, after calling delete:

delete pTest;
pTest = 0;

Answer 3

delete operator calls the destructor of the object and deallocates the memory previously allocated to the object. It doesn't affect the pointer variable that points to the deleted object.

So when dereferencing a pointer pointing to a destroyed object, you'll get trouble.