Re: Deriving from concrete types
Holger Sebert wrote:
I know that is in general a bad idea to derive from concrete types.
Oh, you *know* that, don't you? As if it's true...
However, I wonder if it is o.k. if the derived class does not add any
data members but only extends the interface.
Only if it truly *extends* it.
Deriving from a concrete type if perfectly fine. People do it all the
time and live to talk about it. There are no language problems in it.
The problems are of the OOD kind.
For example:
class SpecialIntVector : public std::vector< int > {
public:
// ... int vector specific member functions go here ...
Like what?
};
class SpecialFloatVector : public std::vector< float > {
public:
// ... float vector specific member functions go here ...
Again, like what?
};
In this example, one automatically obtains STL-compatible vector
types (except for the constructors, of course) and has a
syntactically clean extension of the interface:
SpecialIntVector v;
v.resize( 1000 );
v.doSomethingSpecial();
Special? Don't you mean, "SomethingEvenMoreGeneric"?
Where are the problems with this approach?
You mean, besides that you don't really describe what kind of
"extension" you're talking about?
This has been discussed so many times (try reading archives of the
'comp.object' newsgroup), I don't want to beat the dead horse. One
of the most often given examples is "extending" a string to model
a phone number. Or, "extending" a rectangle to model a square
(because we know that a square "is-a" rectangle, don't we?)
Again, there is no C++ language problem when it comes to inheriting
from a concrete class. A better place to ask would be comp.object,
since with public inheritance you may be violating the basic OOD
principle of substitutability. Of course, *private inheritance* is
a totally different bag of kittens, and is a very well-known and
widely used idiom, called "implement in terms of" (and ont "is-a").
V
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