Why use virtual functions in derived classes?

[robwhess]

This is part question, part issue. I don't really understand the design decision to make functions in derived classes virtual. For example, each layer class derived from Layer has all of its functions (which override the ones in Layer) declared as virtual as well. Why are things done this way? Is the polymorphism here used anywhere beyond the initially derived classes themselves? I think there's typically a (very) slight effect on performance with virtual functions, so it might make sense to use virtual functions only when we know the polymorphism is going to be used.

[longjon]

I have wondered about this myself, so I did a little searching.

Virtual methods are implicitly virtual in derived classes; there is no "additional" penalty from using the optional keyword virtual when overriding a method. The use of the keyword is a stylistic choice; following the Google C++ style guide, we include it so that all virtual methods are explicitly noted as such.

See, e.g.,
http://stackoverflow.com/questions/2963965/why-is-virtual-optional-for-overridden-methods-in-derived-classes
http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml?showone=Inheritance#Inheritance

[robwhess]

Oh, huh. I didn't know overridden virtual methods were implicitly virtual. Thanks @longjon. Everything make sense now.