Question

Is there any example of the C++ object slicing effect which can cause undefined behavior, memory leak or crash in an otherwise correct set of code? For example when class A and B (inherited from A) are correct and sound, but calling a void f(A a) demonstrably causes nasty things.

It is needed for forming a test question. The goal is to know if the participant is aware of the slicing phenomenon or not, using an example code snippet whose correctness must not be a matter of opinion.

Answer 1

If A is indeed “correct and sound”, then slicing (copying the base sub-object) is well-defined and will not cause any of the problems you mention. The only problem it will cause is unexpected behaviour, if you expect the copy to behave like B.

If A is not correctly copyable, then slicing will cause whatever problems arise from copying objects of that type. For example, if it has a destructor which deletes a pointer held by object, and copying creates a new pointer to the same thing, then you will get undefined behaviour when both destructors delete the same pointer. This isn’t a problem with slicing as such, but with invalid copy semantics of the sliced object.