Why is it undefined behavior to delete[] an array of derived objects via a base pointer?

Question

I found the following snippet in the C++03 Standard under 5.3.5 [expr.delete] p3:

In the first alternative (delete object), if the static type of the object to be deleted is different from its dynamic type, the static type shall be a base class of the operand’s dynamic type and the static type shall have a virtual destructor or the behavior is undefined. In the second alternative (delete array) if the dynamic type of the object to be deleted differs from its static type, the behavior is undefined.

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Quick review on static and dynamic types:

struct B{ virtual ~B(){} }; struct D : B{};  B* p = new D(); 

Static type of p is B*, while the dynamic type of *p is D, 1.3.7 [defns.dynamic.type]:

[Example: if a pointer p whose static type is “pointer to class B” is pointing to an object of class D, derived from B, the dynamic type of the expression *p is “D.”]

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Now, looking at the quote at the top again, this would mean that the follwing code invokes undefined behaviour if I got that right, regardless of the presence of a virtual destructor:

struct B{ virtual ~B(){} }; struct D : B{};  B* p = new D[20]; delete [] p; // undefined behaviour here 

Did I misunderstand the wording in the standard somehow? Did I overlook something? Why does the standard specify this as undefined behaviour?

Answer 1

Base* p = new Base[n] creates an n-sized array of Base elements, of which p then points to the first element. Base* p = new Derived[n] however, creates an n-sized array of Derived elements. p then points to the Base subobject of the first element. p does not however refer to the first element of the array, which is what a valid delete[] p expression requires.

Of course it would be possible to mandate (and then implement) that delete [] p Does The Right Thing™ in this case. But what would it take? An implementation would have to take care to somehow retrieve the element type of the array, and then morally dynamic_cast p to this type. Then it's a matter of doing a plain delete[] like we already do.

The problem with that is that this would be needed every time an array of polymorphic element type, regardless of whether the polymorphism is used on not. In my opinion, this doesn't fit with the C++ philosophy of not paying for what you don't use. But worse: a polymorphic-enabled delete[] p is simply useless because p is almost useless in your question. p is a pointer to a subobject of an element and no more; it's otherwise completely unrelated to the array. You certainly can't do p[i] (for i > 0) with it. So it's not unreasonable that delete[] p doesn't work.

To sum up:

• arrays already have plenty of legitimate uses. By not allowing arrays to behave polymorphically (either as a whole or only for delete[]) this means that arrays with a polymorphic element type are not penalized for those legitimate uses, which is in line with the philosophy of C++.

• if on the other hand an array with polymorphic behaviour is needed, it's possible to implement one in terms of what we have already.

Answer 2

It's wrong to treat an array-of-derived as an array-of-base, not only when deleting items. For example even just accessing the elements will usually cause disaster:

B *b = new D[10]; b[5].foo(); 

b[5] will use the size of B to calculate which memory location to access, and if B and D have different sizes, this will not lead to the intended results.

Just like a std::vector<D> can't be converted to a std::vector<B>, a pointer to D[] shouldn't be convertible to a B*, but for historic reasons it compiles anyway. If a std::vector would be used instead, it would produce a compile time error.

This is also explained in the C++ FAQ Lite answer on this topic.

So delete causes undefined behavior in this case because it's already wrong to treat an array in this way, even though the type system can't catch the error.