Invoking a method from null pointer in the lambda

Question

Consider this piece of code:

struct A {
  bool f() { return true; }
};

// ...
std::vector<A*> v = {new A(), nullptr, new A()};
std::find_if(v.begin(), v.end(), [](const A* object) { return object->f(); });

If we invoke a method from nullptr object, is this an undefined behaviour? If yes and I want to resolve this issue, will it suffice to add A != nullptr in the beginning, so that the condition short-circuits before the potentially dangerous code would be executed?

How to resolve this issue depends on what you want to do for `nullptr`. — MikeCAT, Oct 07 '20 at 11:29
is it possible that you rather want a `std::vector` with two elements? Having pointers of non-polymorphic types in containers is a bit pointless when the container is supposed to own them — 463035818_is_not_an_ai, Oct 07 '20 at 11:29
why do you store pointers? The answer to this determines how to resolve the issue — 463035818_is_not_an_ai, Oct 07 '20 at 11:31
@MikeCAT Sorry, I edited the code, e.g. in this particular case find_if should return an non-end iterator if there's allocated object and f() returns true (this is an excerpt, though). — Inter Veridium, Oct 07 '20 at 11:31
@idclev463035818 Types are polymorphic, indeed -- I should have reflected it in the question. — Inter Veridium, Oct 07 '20 at 11:33

score 2 · Accepted Answer · answered Oct 07 '20 at 11:33

As per this post, yes, that would result in undefined behaviour.

If you wish to avoid the undefined behaviour, which happens if the code attempts to do something which is not defined by the standard, then yes, you will avoid UB if the code doesn't execute, which can be avoided with a condition.

Invoking a method from null pointer in the lambda

1 Answers1