Prefer empty abstract classes as interfaces to class hierarchies

Reason

Abstract classes that are empty (have no non-static member data) are more likely to be stable than base classes with state.

Example, bad

You just knew that Shape would turn up somewhere :-)

class Shape {  // bad: interface class loaded with data
public:
    Point center() const { return c; }
    virtual void draw() const;
    virtual void rotate(int);
    // ...
private:
    Point c;
    vector<Point> outline;
    Color col;
};

This will force every derived class to compute a center -- even if that's non-trivial and the center is never used. Similarly, not every Shape has a Color, and many Shapes are best represented without an outline defined as a sequence of Points. Using an abstract class is better:

class Shape {    // better: Shape is a pure interface
public:
    virtual Point center() const = 0;   // pure virtual functions
    virtual void draw() const = 0;
    virtual void rotate(int) = 0;
    // ...
    // ... no data members ...
    // ...
    virtual ~Shape() = default;
};

Enforcement

(Simple) Warn if a pointer/reference to a class C is assigned to a pointer/reference to a base of C and the base class contains data members.