Don't make a function `virtual` without reason

Reason

Redundant virtual increases run-time and object-code size. A virtual function can be overridden and is thus open to mistakes in a derived class. A virtual function ensures code replication in a templated hierarchy.

Example, bad

template<class T>
class Vector {
public:
    // ...
    virtual int size() const { return sz; }   // bad: what good could a derived class do?
private:
    T* elem;   // the elements
    int sz;    // number of elements
};

This kind of "vector" isn't meant to be used as a base class at all.

Enforcement

• Flag a class with virtual functions but no derived classes.
• Flag a class where all member functions are virtual and have implementations.