C++ delete - It deletes my objects but I can still access the data?

user_136896 0
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I have written a simple, working tetris game with each block as an instance of a class singleblock.

class SingleBlock
{
    public:
    SingleBlock(int, int);
    ~SingleBlock();

    int x;
    int y;
    SingleBlock *next;
};

class MultiBlock
{
    public:
    MultiBlock(int, int);

    SingleBlock *c, *d, *e, *f;
};

SingleBlock::SingleBlock(int a, int b)
{
    x = a;
    y = b;
}

SingleBlock::~SingleBlock()
{
    x = 222;
}

MultiBlock::MultiBlock(int a, int b)
{
    c = new SingleBlock (a,b);
    d = c->next = new SingleBlock (a+10,b);
    e = d->next = new SingleBlock (a+20,b);
    f = e->next = new SingleBlock (a+30,b);
}

I have a function that scans for a complete line, and runs through the linked list of blocks deleting the relevant ones and reassigning the ->next pointers.

SingleBlock *deleteBlock;
SingleBlock *tempBlock;

tempBlock = deleteBlock->next;
delete deleteBlock;

The game works, blocks are deleted correctly and everything functions as it is supposed to. However on inspection I can still access random bits of deleted data.

If I printf each of the deleted singleblocks "x" values AFTER their deletion, some of them return random garbage (confirming the deletion) and some of them return 222, telling me even though the destructor was called the data wasn't actually deleted from the heap. Many identical trials show it is always the same specific blocks that are not deleted properly.

The results:

Existing Blocks:
Block: 00E927A8
Block: 00E94290
Block: 00E942B0
Block: 00E942D0
Block: 00E942F0
Block: 00E94500
Block: 00E94520
Block: 00E94540
Block: 00E94560
Block: 00E945B0
Block: 00E945D0
Block: 00E945F0
Block: 00E94610
Block: 00E94660
Block: 00E94680
Block: 00E946A0

Deleting Blocks:
Deleting ... 00E942B0, X = 15288000
Deleting ... 00E942D0, X = 15286960
Deleting ... 00E94520, X = 15286992
Deleting ... 00E94540, X = 15270296
Deleting ... 00E94560, X = 222
Deleting ... 00E945D0, X = 15270296
Deleting ... 00E945F0, X = 222
Deleting ... 00E94610, X = 222
Deleting ... 00E94660, X = 15270296
Deleting ... 00E94680, X = 222

Is being able to access data from beyond the grave expected?

Sorry if this is a bit long winded.

undefined-behavior delete-operator pointers c++-faq c++
13 Answers

Is being able to access data from beyond the grave expected?

This is technically known as Undefined Behavior. Don't be surprised if it offers you a can of beer either.

user_666920

Is being able to access data from beyond the grave expected?

In most cases, yes. Calling delete doesn't zero the memory.

Note that the behavior is not defined. Using certain compilers, the memory may be zeroed. When you call delete, what happens is that the memory is marked as available, so the next time someone does new, the memory may be used.

If you think about it, it's logical - when you tell the compiler that you are no longer interested in the memory (using delete), why should the computer spend time on zeroing it.

user_2347960

Delete doesn't delete anything -- it just marks the memory as "being free for reuse". Until some other allocation call reserves and fills that space it will have the old data. However, relying on that is a big no-no, basically if you delete something forget about it.

One of the practices in this regard that is often encountered in libraries is a Delete function:

template< class T > void Delete( T*& pointer )
{
    delete pointer;
    pointer = NULL;
}

This prevents us from accidentally accessing invalid memory.

Note that it is perfectly okay to call delete NULL;.

user_2335220

It is what C++ calls undefined behaviour - you might be able to access the data, you might not. In any case, it is the wrong thing to do.

Admin1

Heap memory is like a bunch of blackboards. Imagine you are a teacher. While you're teaching your class, the blackboard belongs to you, and you can do whatever you want to do with it. You can scribble on it and overwrite stuff as you wish.

When the class is over and you are about to leave the room, there is no policy that requires you to erase the blackboard -- you simply hand the blackboard off to the next teacher who will generally be able to see what you wrote down.

user_2309260

The system does not clear the memory when you release it via delete(). The contents are therefore still accessible until the memory is assigned for reuse and overwritten.

user_2043980

After deleting an object it's not defined what will happen to the contents of the memory that it occupied. It does mean that that memory is free to be re-used, but the implementation doesn't have to overwrite the data that was there originally and it doesn't have to reuse the memory immediately.

You shouldn't access the memory after the object is gone but it shouldn't be surpising that some data remains in tact there.

Charles_Bailey_195630

delete deallocates the memory, but does not modify it or zero it out. Still you should not access deallocated memory.

user_1975140

Yes, it can be expected at times. Whereas new reserves space for data, delete simply invalidates a pointer created with new, allowing data to be written at the previously reserved locations; it doesn't necessarily delete the data. However, you shouldn't rely on that behaviour because the data at those locations could change at any time, possibly causing your program to misbehave. This is why after you use delete on a pointer (or delete[] on an array allocated with new[]), you should assign NULL to it so that you can't tamper with an invalid pointer, assuming you won't allocate memory using new or new[] before using that pointer again.

user_2332390

It won't zero/change memory just yet... but at some point, the rug is going to be pulled from under your feet.

No it is certainly not predictable: it depends on how fast memory allocation/deallocation is churned.

user_1714610

It will lead to undefined behaviour and delete deallocates memory , it does not reinitialize it with zero .

If you want to make it zero out then do :

SingleBlock::~SingleBlock()

{    x = y = 0 ; }
user_3039860
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