Can I delete[] a pointer that points into an allocated array, but not to the start of it?

Question

I'm wondering specifically about the following situation (which I discovered in some code I have to work with):

SomeClass *ar = new SomeClass[2];
ar++;
delete[] ar;

This code seems to be working fine - i.e. not crashing (win32, built with VS2005).

Is this "legal"? It certainly doesn't feel right.

Answer 1

No, it is undefined to pass any address to delete which was not returned by new.
Here is the quote from the Standard.

§ 3.7.4.2-3

If a deallocation function terminates by throwing an exception, the behavior is undefined. The value of the first argument supplied to a deallocation function may be a null pointer value; if so, and if the deallocation function is one supplied in the standard library, the call has no effect. Otherwise, the value supplied to operator delete(void*) in the standard library shall be one of the values returned by a previous invocation of either operator new(std::size_t) or operator new(std::size_t, const std::nothrow_-t&) in the standard library, and the value supplied to operator delete[](void*) in the standard library shall be one of the values returned by a previous invocation of either operator new[](std::size_t) or operator new[](std::size_t, const std::nothrow_t&) in the standard library.

Answer 2

No, not legal. You can only delete what you got back from new, and the exact same applies to new[] and delete[]

Answer 3

No it is not. You must call delete[] on the very same address (or pointer) that you received from new[]. You might just get lucky that it does not crash, but it definitely does not clear the memory appropriately.

References :

From http://www.cplusplus.com/doc/tutorial/dynamic/

The value passed as argument to delete must be either a pointer to a memory block
previously allocated with new, or a null pointer (in the case of a null pointer,
delete produces no effect).

From http://msdn.microsoft.com/en-us/library/h6227113.aspx

Using delete on a pointer to an object not allocated with new gives
unpredictable results. You can, however, use delete on a pointer with the
value 0. This provision means that, when new returns 0 on failure, deleting
the result of a failed new operation is harmless.