Do ALL virtual functions need to be implemented in derived classes?

Question

This may seem like a simple question, but I can\'t find the answer anywhere else.

Suppose I have the following:

class Abstract {
public:
    virtual          


        

Answer 1

Only the pure virtual methods have to be implemented in derived classes, but you still need a definition (and not just a declaration) of the other virtual methods. If you don't supply one, the linker might very well complain.

So, just putting {} after your optional virtual method gives you an empty default implementation:

class Abstract {
public:
    virtual void foo() = 0; // pure virtual must be overridden
    virtual void bar() {}   // virtual with empty default implementation
};

class Derived : Abstract {
public:
    virtual void foo();
};

A more involved default implementation would go into a separate source file though.