Why should I mark all methods virtual in C++? Is there a trade-off?

Question

I know why and how virtual methods work, and most of the time people tell me I should always mark a method virtual, but, I don't understand why if I'm not going to override it. And I also know there's a tiny memory issue.

Please explain me why I should mark all methods virtual and what's the trade-off.

Code example:

class Point
{
   int x, y;
   public:
      virtual void setX(int i);
      virtual void setY(int i);
};

(That question is not equal to Should I mark all methods virtual? because I want to know the trade-off and because the programming language in the case is C++, not C#)

OBS: I'm sorry if there's any grammar error, English is not my native language.

Answer 1

No, you should not "mark all methods as virtual".

If you want the method to be virtual, then mark it as such. If not, leave the keyword out.

There is an overhead for virtual methods compared to regular ones. If you want to read more about this, check out the Wikipedia side about VTables.