Having at least one virtual method in a C++ class (or any of its parent classes) means that the class will have a virtual table, and every instance will have a virtual pointer.
So the memory cost is quite clear. The most important is the memory cost on the instances (especially if the instances are small, for example if they are just meant to contain an integer: in this case having a virtual pointer in every instance might double the size of the instances. As for the memory space used up by the virtual tables, I guess it is usually negligible compared to the space used up by the actual method code.
This brings me to my question: is there a measurable performance cost (i.e. speed impact) for making a method virtual? There will be a lookup in the virtual table at runtime, upon every method call, so if there are very frequent calls to this method, and if this method is very short, then there might be a measurable performance hit? I guess it depends on the platform, but has anyone run some benchmarks?
The reason I am asking is that I came across a bug that happened to be due to a programmer forgetting to define a method virtual. This is not the first time I see this kind of mistake. And I thought: why do we add the virtual keyword when needed instead of removing the virtual keyword when we are absolutely sure that it is not needed? If the performance cost is low, I think I will simply recommend the following in my team: simply make every method virtual by default, including the destructor, in every class, and only remove it when you need to. Does that sound crazy to you?
