When is it appropriate to use virtual methods?-Collection of common programming errors

I guess one possible way to determine quickly would be to consider if you are going to deal with a bunch of similar classes that you are going to use to perform the same tasks, with the change being the way you go about doing those tasks.

One trivial example would be the problem of computing areas for various geometric figures. You need the areas of squares, circles, rectangles, triangles etc. and the only thing that changes here is the math formulae (the way) you use to compute the area. Therefore it would be a good decision to have each of these shapes inherit from a common base class and to add a virtual method in the base class that returns the area (which you can then implement in each of the child with the respective math formula).

Making everything virtual “just in case” will make your objects take up more memory. Additionally, there is a small (but non-zero) overhead when calling virtual functions. So, IMHO, making everything virtual “just in case” would be bad idea when performance/memory constraints are important (which basically means in always every real-world program that you write).

However, this again is debatable based on how clearly the requirements are spelled out and how often code-changes expected. For example, in a quick-and-dirty tool or an initial prototype where a few extra bytes of memory and a few milliseconds of lost time do not mean much, it would be OK to have a bunch (unnecessarily) virtual functions for the sake of flexibility.