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.