When should I use a vector of objects instead of a vector of pointers?

Question

I have a collection of polymorphic objects, all derived from my Animal class: Cat, Dog, and MonkeyFish.

My usual mode of operation is to store these objects in a vec

Answer 1

In this case, storing a vector of Animal would not work for you, as your animals have different sizes, and you wouldn't be able to store the derived objects in the spaces intended to hold the base class. (And even if they're the same size, you won't get the intended polymorphic effect, as base class methods will be executed - the virtualness of a method doesn't come into play unless you access it through a pointer or reference.)

If you want to avoid the annoyance of managing the memory yourself, you could consider storing a smart pointer such as a shared_ptr (note that auto_ptr doesn't work with STL containers, according to Max Lybbert), or some variant thereof. That way you can still use your polymorphic class, but it's a little less work for you.

There's no real hard and fast rules about when to use objects and pointers, although it's worth noting that in some cases, like yours, objects just aren't going to work for you. I tend to use objects whenever nothing precludes it though, although you do have to be concerned about expensive copy operations as you note (although sometimes they can be ameliorated by passing containers by reference).