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Adam
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 Posted: Tue May 23, 2006 10:21 pm Post subject: Multiply inherit from classes with conflicting function name |
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I have an unfortunate case where a single class wants to derive from two
existing classes:
struct A { virtual long fun() = 0; };
struct B { virtual bool fun() = 0; };
struct Unfortunate : public A, public B { ??? };
Is it possible to fill in the ??? here with legal code?
I need two different function bodies; A::fun and B::fun do unrelated
things.
More or less the same question with a twist: if A::fun and B::fun both
returned the same type, would it be possible to implement two functions
in C such that
C().A::fun()
and
C().B::fun()
would execute two different functions?
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Alf P. Steinbach
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| Posted: Wed May 24, 2006 4:21 pm Post subject: Multi-posting clc++ and clc++m (was: Multiply inherit from c |
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* Adam, in clc++m:
| Quote: | I have an unfortunate case where a single class wants to derive from two
existing classes:
struct A { virtual long fun() = 0; };
struct B { virtual bool fun() = 0; };
struct Unfortunate : public A, public B { ??? };
|
I already answered this one (the exact same message text), apparently to
Adam's satisfaction, in [clc++].
Could people please /stop/ multi-posting (at least not without noting in
the message that it's multi-posted)?
IMO it's now become a nuisance.
Note: this message has been /cross-posted/ to clc++ and clc++m.
For the difference between cross-posting and multi-posting, see Google.
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
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Carl Barron
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| Posted: Wed May 24, 2006 4:21 pm Post subject: Re: Multiply inherit from classes with conflicting function |
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In article <15-dnUdwgJlQB-_ZRVn-pQ (AT) speakeasy (DOT) net>, Adam
<usenet-filter (AT) aulick (DOT) net> wrote:
| Quote: | I have an unfortunate case where a single class wants to derive from two
existing classes:
struct A { virtual long fun() = 0; };
struct B { virtual bool fun() = 0; };
struct Unfortunate : public A, public B { ??? };
give them new names by adding mid_A,mid_B to the inheritance? |
struct mid_A:A {virtual long fun();long fun_A(){return fun();}};
struct mid_B:B {virtual bool fun(); bool fun_B(){return fun();}};
struct Unfortunate:mid_A,Mid_B {}
if these are really structs the above fun()'s can access
Unfortunates' data via a static_cast<Unfortunate *>(this);
if Unfortunate is a class then also make mid_A,mid_B friends of
Unfortunate.
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Guest
|
| Posted: Wed May 24, 2006 4:21 pm Post subject: Re: Multiply inherit from classes with conflicting function |
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Adam wrote:
| Quote: | I have an unfortunate case where a single class wants to derive from two
existing classes:
struct A { virtual long fun() = 0; };
struct B { virtual bool fun() = 0; };
struct Unfortunate : public A, public B { ??? };
Is it possible to fill in the ??? here with legal code?
|
You can introduce trivial intermediate classes to "rename" the
functions.
| Quote: | I need two different function bodies; A::fun and B::fun do unrelated
things.
More or less the same question with a twist: if A::fun and B::fun both
returned the same type, would it be possible to implement two functions
in C such that
C().A::fun()
and
C().B::fun()
would execute two different functions?
|
The intermediate classes would still work, but with a different syntax:
C().A_fun();
Here is a complete test:
#include <iostream>
struct A { virtual long fun() = 0; };
struct B { virtual bool fun() = 0; };
struct A_impl : public A
{
virtual long A_fun() = 0;
virtual long fun()
{
return A_fun();
}
};
struct B_impl : public B
{
virtual bool B_fun() = 0;
virtual bool fun()
{
return B_fun();
}
};
struct Unfortunate : public A_impl, public B_impl
{
virtual long A_fun()
{
std::cout << "long\n";
return 0;
}
virtual bool B_fun()
{
std::cout << "bool\n";
return false;
}
};
int main()
{
Unfortunate u;
// Ambiguous to call fun()
u.A_fun();
u.B_fun();
// No ambiguity through the base interfaces
A & a = u;
a.fun();
B & b = u;
b.fun();
}
Ali
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Nicola Musatti
Guest
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| Posted: Thu May 25, 2006 2:21 pm Post subject: Re: Multiply inherit from classes with conflicting function |
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Adam wrote:
| Quote: | I have an unfortunate case where a single class wants to derive
from two
existing classes:
struct A { virtual long fun() = 0; };
struct B { virtual bool fun() = 0; };
struct Unfortunate : public A, public B { ??? };
Is it possible to fill in the ??? here with legal code?
I need two different function bodies; A::fun and B::fun do unrelated
things.
|
The usual technique is the following:
struct AdaptA : A {
virtual long funA() = 0;
long fun() { return funA(); }
};
struct AdaptB : B {
virtual long funB() = 0;
long fun() { return funB(); }
};
struct Unfortunate : AdaptA, AdaptB {
long funA() { return 42L }
bool funB() { return false; }
};
| Quote: | More or less the same question with a twist: if A::fun and B::fun
both
returned the same type, would it be possible to implement two
functions
in C such that
C().A::fun()
and
C().B::fun()
would execute two different functions?
|
I don't think the return type makes any difference.
Cheers,
Nicola Musatti
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kanze
Guest
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| Posted: Thu May 25, 2006 2:21 pm Post subject: Re: Multiply inherit from classes with conflicting function |
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Adam wrote:
| Quote: | I have an unfortunate case where a single class wants to
derive from two existing classes:
struct A { virtual long fun() = 0; };
struct B { virtual bool fun() = 0; };
struct Unfortunate : public A, public B { ??? };
Is it possible to fill in the ??? here with legal code?
|
Legal code, certainly. But nothing that would stop the class
from being abstract -- there's no way you can override fun().
| Quote: | I need two different function bodies; A::fun and B::fun do
unrelated things.
|
The classical solution is to create intermediate classes:
struct Abis { virtual long funA() = 0 ;
virtual long fun() { return funA() ; } } ;
struct Bbis { virtual bool funB() = 0 ;
virtual bool fun() { return funB() ; } } ;
struct Unfortunate : A, B
{
virtual long funA() { ... }
virtual long funB() { ... }
} ;
| Quote: | More or less the same question with a twist: if A::fun and
B::fun both returned the same type, would it be possible to
implement two functions in C such that
C().A::fun()
and
C().B::fun()
would execute two different functions?
|
Same solution as above.
--
James Kanze GABI Software
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
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Maxim Yegorushkin
Guest
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| Posted: Thu May 25, 2006 2:21 pm Post subject: Re: Multiply inherit from classes with conflicting function |
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Adam wrote:
| Quote: | I have an unfortunate case where a single class wants to derive
from two
existing classes:
struct A { virtual long fun() = 0; };
struct B { virtual bool fun() = 0; };
struct Unfortunate : public A, public B { ??? };
Is it possible to fill in the ??? here with legal code?
I need two different function bodies; A::fun and B::fun do unrelated
things.
|
Yes, you need an intermediate class here.
struct A_tag {} const a_tag = {};
struct B_tag {} const b_tag = {};
// zero overhead shim
template<class derived>
struct A_shim : A
{
long fun() { return static_cast<derived*>(this)->fun(a_tag); }
};
// virtual call overhead shim
struct B_shim : B
{
bool fun() { this->fun(b_tag); }
virtual bool fun(B_tag) = 0;
};
struct fortunate : A_shim<fortunate>, B_shim
{
long fun(A_tag);
bool fun(B_tag);
};
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Adam Aulick
Guest
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| Posted: Fri May 26, 2006 7:58 am Post subject: Re: Multi-posting clc++ and clc++m |
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Alf P. Steinbach wrote:
| Quote: | Could people please /stop/ multi-posting (at least not without
noting in
the message that it's multi-posted)?
IMO it's now become a nuisance.
|
For the record, posting to the moderated group was an accident in this
case. Embarassing since I've been impatient with multi-posters
elsewhere in the past, but there's karma for you. My apologies.
~Adam
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