Delegation question...

[barcaroller]

What is the common way/design-pattern (if any) in C++ for delegating
function calls that are not handled by a certain class. Public
inheritance would be one way but not all classes are meant to inherit
from (e.g. STL).
Example:

class A
{
public:
foo();

private:
set<stringmyset ;
}

A myObj;
myObj.insert(); // compiler error of course
Is there some mechanism (direct or indirect) where a function that is
not handled by myObj gets delegated to another object (e.g. myset)?

[Daniel T.]

tedzhou <zh*******@163. comwrote:

On May 24, 5:19*pm, Ian Collins <[e-mail] mwrote:

James Kanze wrote:

On May 24, 9:47 am, Ian Collins <[e-mail] mwrote:

* * [...]
I was going to suggest that technique, but decided against it
as the OP wanted to delegate members of std::set. *I wouldn't
recommend deriving from a standard container.

Not even privately? *I have no problems with private
inheritance from a standard container; private inheritance is
part of the implementation.

Yes, you're right, I overlooked private inheritance.

Private inheritance is not suggested on standard container either.
Because the standard containers are not designed for inheritance at
all. Just think about the polymorphism and virtual destruction,
then you'll get the conclusion that inheritance from standard
containors will be dangerous.

Polymorphism and virtual destruction don't apply to private
inheritance.

[red floyd]

tedzhou wrote:

On May 24, 5:19 pm, Ian Collins <[e-mail] mwrote:

>James Kanze wrote:

>>On May 24, 9:47 am, Ian Collins <[e-mail] mwrote:
[...]
I was going to suggest that technique, but decided against it as the OP
wanted to delegate members of std::set. I wouldn't recommend deriving
from a standard container.
Not even privately? I have no problems with private inheritance
from a standard container; private inheritance is part of the
implementatio n.

Yes, you're right, I overlooked private inheritance.

Private inheritance is not suggested on standard container either.
Because the standard containers are not designed for inheritance at
all.

Not necessarily. Several of the standard adapter classes
(std::priority_ queue for example) have protected members, which implies
that they were designed to be inherited from.

[Kai-Uwe Bux]

tedzhou wrote:

On May 24, 5:19*pm, Ian Collins <[e-mail] mwrote:

>James Kanze wrote:

On May 24, 9:47 am, Ian Collins <[e-mail] mwrote:

[...]
I was going to suggest that technique, but decided against it as the
OP wanted to delegate members of std::set. *I wouldn't recommend
deriving from a standard container.

Not even privately? *I have no problems with private inheritance
from a standard container; private inheritance is part of the
implementation.

Yes, you're right, I overlooked private inheritance.

Private inheritance is not suggested on standard container either.
Because the standard containers are not designed for inheritance at
all.
Just think about the polymorphism and virtual destruction , then
you'll
get the conclusion that inheritance from standard containors will be
dangerous.

That argument is bogus for two reasons:

(a) As others have pointed out, private differs from public inheritance with
regard to destruction.

(b) Even with public inheritance, the argument does not hold as it would
apply equally well to std::unary_func tion or std::iterator, two classes
that are meant to be publicly inherited from and that lack virtual
destructors. Inheritance in C++ is not necessarily tied to polymorphism.

[Note: that you failed to point out a valid reason as to why public
inheritance from standard containers might not be such a good move, is not
to say that there are no such reasons. However, those tend to focus on
other issues rather than the lack of a virtual destructor.]
Best

Kai-Uwe Bux

[James Kanze]

On May 26, 11:00 am, Ian Collins <[e-mail] mwrote:

tedzhou wrote:

On May 24, 5:19 pm, Ian Collins <[e-mail] mwrote:

James Kanze wrote:
On May 24, 9:47 am, Ian Collins <[e-mail] mwrote:
[...]
I was going to suggest that technique, but decided against it as the OP
wanted to delegate members of std::set. I wouldn't recommend deriving
from a standard container.
Not even privately? I have no problems with private inheritance
from a standard container; private inheritance is part of the
implementation .
Yes, you're right, I overlooked private inheritance.

Private inheritance is not suggested on standard container
either. Because the standard containers are not designed
for inheritance at all. Just think about the polymorphism
and virtual destruction , then you'll get the conclusion
that inheritance from standard containors will be dangerous.

With public inheritance maybe, but you can't point a base* to
a derived object if derived uses private inheritance.

You can, and in fact, one frequent use of private inheritance
involves doing just that. But you need cooperation from the
derived class to do so: if the derived class doesn't give you
the Base*, you can't get it otherwise. (The "frequent use" is
to derive privately from something like "EventHande r": for
normal client code, this is an implementation detail, and they
can't consider the class to be an EventHander. But the class
itself may register itself as an EventHander with some
EventNotifier, who will address the class through an
EventHander*.)

Private inheritance hides the fact that a derived is a base.

Except in the explicit cases where it wants to behave as a base.

None of which is really a problem when deriving privately from a
standard container, since you, the author of the class, have
total control, and of course, you will not expose the
derivation.

--
James Kanze (GABI Software) email:ja******* **@gmail.com
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