Re: Deriving from concrete types

"Alan McKenney" <>
27 Jun 2006 07:33:20 -0400
Bernhard Jungk wrote:

      2. The only reason for derivation is to allow
           polymorphic use (treating a derived
           class as a base class.)

... Public inheritance is provided to allow polymorphic use.

      Perhaps in the OOD paradigm, this is the only reason for
      public inheritance (I'm not an expert in OOD theory.)

      However, one of C++'s strengths is that it supports
      multiple paradigms. And one real non-OOD use of
      derivation is code re-use, where there is no expectation
      of polymorphic use.

      The OP's example is, to my mind, a perfect example
      of derivation for code re-use: a new class which is
      similar to an existing class.

      By using public inheritance, the new class definition
      contains only those things which distinguish it from
      the existing class, thus making it clear to anyone
      who reads it what is going on.

      (Yes, you can use private inheritance, but then
      you have to put in a "using" declaration for every
       base class function and operator. Quick, how
       many of you can rattle off all the member functions
       and operators in std::vector without looking at
       the documentation?)

      If Stroustrup had intended to create a language
      which would force people to fit their code into
      OOD paradigms, perhaps he would have made
      it impossible to derive from a class with a non-virtual
      destructor. Or made it impossible to override
      non-virtual functions.

       Fortunately, he did not.

       And C++ is all the more useful for it.

       (Just think of all the one-paradigm languages out there --
       have *any* of them become more than niche languages?)

-- Alan McKenney

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