C++ tips - Callbacks and multiple inheritance, part II

[ben2ong2]

This example is based upon "Subobject Members", Stephen Dewhurst,
C++ Report, V5 N3. This is part 2 of a multipart example.

In this example, assume that the engine-button example of the previous
tip are extended two include two buttons. One will be for starting the
engine, one for stopping. Using the design technique of the previous
example, one would have:

.................................................................

class StartButtonCallback : public ButtonCallback {
  public:
    void callback () { start (); }
    virtual void start () = 0;
};

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class StopButtonCallback : public ButtonCallback {
  public:
    void callback () { stop (); }
    virtual void stop () = 0;
};

.................................................................

class Engine : public StartButtonCallback,
          public StopButtonCallback {
    // ...
  public:
    void start ();
    void stop  ();
};

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The first drawback with this approach is the global namespace
pollution introduced by the two callback classes. In addition,
these classes are unlikely to be reused except with engines.