Re: Warning

"Leigh Johnston" <>
Fri, 5 Mar 2010 10:25:36 -0000
"Michael Doubez" <> wrote in message

On 4 mar, 19:37, James Kanze <> wrote:

On Mar 4, 8:32 am, Michael Doubez <> wrote:

On 3 mar, 21:10, "Alf P. Steinbach" <> wrote:

* Leigh Johnston:

IMHO "don't derive from concrete class" is a poor wording. I prefer
"inherit an interface, not an implementation" (IIRC adapted from
IMNSHO inheriting from a concrete class is perfectly valid
when you only want to add to the interface of an object -
after all, it is part of the 3 fundamental OOP rules.

Before arguing against the rule, it would be interesting to see
who is proposing it, and why.

I cannot remember who stated them, it dates to my academics:
"A computer language is object-oriented if they support the three
fundamental features: polymorphism, inheritance, and encapsulation."

Inheritance in the OOP sense, i.e. (arguably) to reuse a class
implementation in order to *add* to its interface (not modifying its
internals, that's polymorphism).

 To date, the only thing I've seen
recently is a mention that Herb Sutter cites it. Until I've
seen why it's being recommended, I can't really argue one way or
the other. (I haven't actuall seen anyone propose it since
about 1995, but I haven't seen everything. Still, I would like
to know what is wrong with something like:

        class TemplateBase
                virtual void customizationFunction();
                                        // with a default implemenation

A class using the template method pattern for customization,
but which provides defaults for all of the customization, seems
like a classical example of a case where a concrete class is
actuall designed to be used as a base class (with public

IMHO, part of the misunderstanding stems from the fact that in C++,
polymorphism is /usually/ performed through inheritance.

As a consequence, in C++ a class can inherit from another for both
reasons: reusing code (of the base classe) and create a polymorphic

Whether it is a good thing or not is IMO what is (should be) debated
here; should a virtual function from a base class be implemented ?
Separating the concern, with your example we could write:

class TemplateBaseIsPolymorphic
   virtual void customizationFunction()=0;

class TemplateBase: TemplateBaseIsPolymorphic
virtual void customizationFunction()
 // default implementation

From the C++ language point of view, this doesn't add a lot and
personally I prefer the original example.

Deriving from classes which weren't designed to be bases (such
as the standard containers) is generally a bad idea.

That's because, IMO the standard containers usually have a complete
interface and there is no need to.

The examples I gave where for the case where an interface needs to be
augmented but you use the word "usually" which is fine: "usually" is not

But the
key to whether you should derive from a class or not isn't
whether it is concrete or not; it's whether it was designed to
be used as a base class or not.

My point exactly.


A class does not have to be abstract to be usable as a base class.


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