Re: Warning

James Kanze <>
Fri, 5 Mar 2010 10:32:41 -0800 (PST)
On 5 Mar, 09:26, Michael Doubez <> wrote:

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 Gamma&GoF).
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

Which doesn't really address the question: C++ does support
those three fundamental features, so it is object-oriented.
(But it's not only object-oriented; it supports other things as
well.) And those three features say nothing about deriving from
a concrete class---the template method pattern was frequent long
before the GoF named it; as far as I know, it was always
considered good OO practice. And if the base class in the
template method pattern provides a default implementation, it is

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 inheritance).

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

As it is in many modern OO languages. Static typing isn't
without advantages.

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

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.

Yes. Separation of concerns is a good principle, but there are
times that it can be taken too far. (Actually, which of the
above is preferrable depends on the context. What if it were:

    class TemplateBase
        virtual void customizationFunction() = 0;

    class DefaultTemplate : public TemplateBase
        virtual void customizationFunction()
            // default implementation...


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.

Also, maybe. But my rule holds: deriving from a class which was
not designed to be used as a base will generally cause problems
in the long run, and should generall be avoided. (There are
exceptions, and I have no problems with people deriving in order
to provide a few special constructors; e.g. to create a fully
filled const vector.)

James Kanze

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