I wish c++ did interfaces better.

[Chris Becke]

Given an interface (in the c++ sense, nothing more than a struct containing pure virtual methods)

struct Iv1 {
virtual method0()=0; };

And a class that implements the interface

class Iv1Impl : public Iv1 {
virtual method0(){...}
}

Its not unreasonable to create a version 2 interface

struct Iv2 : Iv1 {
virtual method1()=0;};

One might *try* to create an implementation class like this

class Iv2Impl : public Iv2, Iv1Impl {
virtual method1(){...}; // implement Iv2 method
};

That of course never never never works. Why not? So what that Ive introduced another method0()=0 ? Why can't it figure out it already has an implementation for that method that I'd really like to allow it to use?

[kwikius]

Re: I wish c++ did interfaces better.

Chris Becke wrote:

Given an interface (in the c++ sense, nothing more than a struct containing pure virtual methods)

<...>

That of course never never never works. Why not?
So what that Ive introduced another method0()=0 ?
Why can't it figure out it already has an implementation
for that method that I'd really like to allow it to use?

struct Iv1 {
virtual void method0()=0;
};

//And a class that implements the interface

class Iv1Impl : public virtual Iv1 {
virtual void method0(){}
};

//Its not unreasonable to create a version 2 interface

struct Iv2 : virtual Iv1 {
virtual void method1()=0;};

//One might *try* to create an implementation class like this

class Iv2Impl : public Iv2, Iv1Impl {
virtual void method1(){} // implement Iv2 method
};

int main()
{
Iv2Impl x;
}

regards
Andy Little

[Chris Becke]

Re: I wish c++ did interfaces better.

You know.
That works.
Im sure I tried virtual inheritance before and it did not work.
Perhaps it collapsed because the compiler i tried with (VC++ 8) doesn't implement virtual base classes correctly, or perhaps there is something fundamental that goes wrong when one tries to mix in overridden and inherited methods.

"kwikius" <andy@servocomm .freeserve.co.u kwrote in message news:48996a8a_1 @mk-nntp-2.news.uk.tisca li.com...

Chris Becke wrote:

>Given an interface (in the c++ sense, nothing more than a struct containing pure virtual methods)

<...>

>That of course never never never works. Why not?
So what that Ive introduced another method0()=0 ?
Why can't it figure out it already has an implementation
for that method that I'd really like to allow it to use?

struct Iv1 {
virtual void method0()=0;
};

//And a class that implements the interface

class Iv1Impl : public virtual Iv1 {
virtual void method0(){}
};

//Its not unreasonable to create a version 2 interface

struct Iv2 : virtual Iv1 {
virtual void method1()=0;};

//One might *try* to create an implementation class like this

class Iv2Impl : public Iv2, Iv1Impl {
virtual void method1(){} // implement Iv2 method
};

int main()
{
Iv2Impl x;
}

regards
Andy Little

[kwikius]

Re: I wish c++ did interfaces better.

Chris Becke wrote:

You know.
That works.
Im sure I tried virtual inheritance before and it did not work.
Perhaps it collapsed because the compiler i tried with (VC++ 8) doesn't implement virtual base classes correctly, or perhaps there is something fundamental that goes wrong when one tries to mix in overridden and inherited methods.

Well.. I think I too know the "Doh..!" feeling... ;-)

Example was tested in VC8 FWIW, maybe your version was being grumpy
today. Mine does that sometimes. Compilers... Love em hate but cant do
without em.

regards
Andy Little

[Stuart Golodetz]

Re: I wish c++ did interfaces better.

kwikius wrote:

Chris Becke wrote:

>You know.
>That works.
>Im sure I tried virtual inheritance before and it did not work.
>Perhaps it collapsed because the compiler i tried with (VC++ 8)
>doesn't implement virtual base classes correctly, or perhaps there is
>something fundamental that goes wrong when one tries to mix in
>overridden and inherited methods.

>
Well.. I think I too know the "Doh..!" feeling... ;-)
>
Example was tested in VC8 FWIW, maybe your version was being grumpy
today. Mine does that sometimes. Compilers... Love em hate but cant do
without em.
>
regards
Andy Little

Sorry to piggy-back on this thread, but it reminded me of a related
interfaces problem I had a while back actually (I found a way to
circumvent it, but it was a bit unsatisfactory) .

Suppose you're implementing an inheritance hierarchy of Cartesian
coordinate systems. You start off with interfaces:

ICoordSystem <-- IOrthogonalCoor dSystem <-- IOrthonormalCoo rdSystem

You then want to implement each of these concretely:

ICoordSystem <-- CoordSystem
IOrthogonalCoor dSystem <-- OrthogonalCoord System
IOrthonormalCoo rdSystem <-- OrthonormalCoor dSystem

The only problem is, you want to reuse the logic in CoordSystem etc., so
you also have this:

CoordSystem <-- OrthogonalCoord System <-- OrthonormalCoor dSystem

At this point, I started having fun with my compiler. It was a while
back now, so I can't quite remember what I tried any more before giving
up and doing it another way, but how should I have done it please, out
of interest?

I had come across virtual inheritance back then, but I clearly wasn't
applying it as I should have been!

Cheers,
Stu

[Stuart Golodetz]

Re: I wish c++ did interfaces better.

Stuart Golodetz wrote:

kwikius wrote:

>Chris Becke wrote:

>>You know.
>>That works.
>>Im sure I tried virtual inheritance before and it did not work.
>>Perhaps it collapsed because the compiler i tried with (VC++ 8)
>>doesn't implement virtual base classes correctly, or perhaps there is
>>something fundamental that goes wrong when one tries to mix in
>>overridden and inherited methods.

>>
>Well.. I think I too know the "Doh..!" feeling... ;-)
>>
>Example was tested in VC8 FWIW, maybe your version was being grumpy
>today. Mine does that sometimes. Compilers... Love em hate but cant do
>without em.
>>
>regards
>Andy Little

>
Sorry to piggy-back on this thread, but it reminded me of a related
interfaces problem I had a while back actually (I found a way to
circumvent it, but it was a bit unsatisfactory) .
>
Suppose you're implementing an inheritance hierarchy of Cartesian
coordinate systems. You start off with interfaces:
>
ICoordSystem <-- IOrthogonalCoor dSystem <-- IOrthonormalCoo rdSystem

(And when I say interfaces, I mean interface classes... I really am
talking about C++ here :-) Though the code was being ported from Java,
which is why I encountered this sort of problem.)

You then want to implement each of these concretely:
>
ICoordSystem <-- CoordSystem
IOrthogonalCoor dSystem <-- OrthogonalCoord System
IOrthonormalCoo rdSystem <-- OrthonormalCoor dSystem
>
The only problem is, you want to reuse the logic in CoordSystem etc., so
you also have this:
>
CoordSystem <-- OrthogonalCoord System <-- OrthonormalCoor dSystem
>
At this point, I started having fun with my compiler. It was a while
back now, so I can't quite remember what I tried any more before giving
up and doing it another way, but how should I have done it please, out
of interest?
>
I had come across virtual inheritance back then, but I clearly wasn't
applying it as I should have been!
>
Cheers,
Stu

[Stuart Golodetz]

Re: I wish c++ did interfaces better.

Stuart Golodetz wrote:

Stuart Golodetz wrote:

>kwikius wrote:

>>Chris Becke wrote:
>>>You know.
>>>That works.
>>>Im sure I tried virtual inheritance before and it did not work.
>>>Perhaps it collapsed because the compiler i tried with (VC++ 8)
>>>doesn't implement virtual base classes correctly, or perhaps there
>>>is something fundamental that goes wrong when one tries to mix in
>>>overridden and inherited methods.
>>>
>>Well.. I think I too know the "Doh..!" feeling... ;-)
>>>
>>Example was tested in VC8 FWIW, maybe your version was being grumpy
>>today. Mine does that sometimes. Compilers... Love em hate but cant
>>do without em.
>>>
>>regards
>>Andy Little

>>
>Sorry to piggy-back on this thread, but it reminded me of a related
>interfaces problem I had a while back actually (I found a way to
>circumvent it, but it was a bit unsatisfactory) .
>>
>Suppose you're implementing an inheritance hierarchy of Cartesian
>coordinate systems. You start off with interfaces:
>>
>ICoordSystem <-- IOrthogonalCoor dSystem <-- IOrthonormalCoo rdSystem

>
(And when I say interfaces, I mean interface classes... I really am
talking about C++ here :-) Though the code was being ported from Java,
which is why I encountered this sort of problem.)
>

>You then want to implement each of these concretely:
>>
>ICoordSystem <-- CoordSystem
>IOrthogonalCoo rdSystem <-- OrthogonalCoord System
>IOrthonormalCo ordSystem <-- OrthonormalCoor dSystem
>>
>The only problem is, you want to reuse the logic in CoordSystem etc.,
>so you also have this:
>>
>CoordSystem <-- OrthogonalCoord System <-- OrthonormalCoor dSystem
>>
>At this point, I started having fun with my compiler. It was a while
>back now, so I can't quite remember what I tried any more before
>giving up and doing it another way, but how should I have done it
>please, out of interest?
>>
>I had come across virtual inheritance back then, but I clearly wasn't
>applying it as I should have been!
>>
>Cheers,
>Stu

Ok, I've recreated the problem:

struct I1
{
virtual void f() = 0;
};

struct I2 : virtual I1
{
virtual void g() = 0;
};

struct I3 : virtual I2
{
virtual void h() = 0;
};

struct C1 : virtual I1
{
void f()
{
std::cout << "f()" << std::endl;
}
};

struct C2 : C1, virtual I2
{
void g()
{
std::cout << "g()" << std::endl;
}
};

struct C3 : C2, I3
{
void h()
{
std::cout << "h()" << std::endl;
}
};

When I do this, I get compiler warnings:

Warning 1 warning C4250: 'C2' : inherits 'C1::C1::f' via dominance
Warning 2 warning C4250: 'C3' : inherits 'C1::C1::f' via dominance
Warning 3 warning C4250: 'C3' : inherits 'C2::C2::g' via dominance

Please can someone explain what's going on?

Cheers!
Stu

[Pete Becker]

Re: I wish c++ did interfaces better.

On 2008-08-06 20:51:21 -0400, Stuart Golodetz
<sgolodetz@NdOi SaPlA.pMiPpLeEx A.ScEomsaid:

>
Ok, I've recreated the problem:
>
struct I1
{
virtual void f() = 0;
};
>
struct I2 : virtual I1
{
virtual void g() = 0;
};
>
struct C1 : virtual I1
{
void f()
{
std::cout << "f()" << std::endl;
}
};
>
struct C2 : C1, virtual I2
{
void g()
{
std::cout << "g()" << std::endl;
}
};

[example simplified]

>
When I do this, I get compiler warnings:
>
Warning 1 warning C4250: 'C2' : inherits 'C1::C1::f' via dominance

[additional warnings elided]

>
>
Please can someone explain what's going on?
>

The compiler is warning you that it's doing what the language
definition says it should do, and that you might not be smart enough to
understand what you've done. But you are: the code is exactly right.

Here's what's going on: C2 sees two definitons of f, one from C1 and
one from i2 (which inherits it from i1). That would be ambiguous if it
weren't for the dominance rule. Both C1 and i2 have i1 as a virtual
base, so they both see the declaration of i1::f. C1 overrides i1::f,
and i2 does not override it. C2 inherits from both, and the dominance
rule says that a call to f on a C2 object is not ambiguous and calls
C1::f. The definition of f that's nearer to C2 in the hierarchy
dominates the one that's farther away. This rule only applies when all
the inheritance paths lead to a common virtual base type that declares
the function and only one of those paths has an overriding declaration.
So if i2 also defined f, the call from C2 would be ambiguous. Got it?
<g>

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)

[Francis Litterio]

Re: I wish c++ did interfaces better.

Stuart Golodetz wrote:

Ok, I've recreated the problem:
>
struct I1
{
virtual void f() = 0;
};
>
struct I2 : virtual I1
{
virtual void g() = 0;
};
>
struct I3 : virtual I2
{
virtual void h() = 0;
};
>
struct C1 : virtual I1
{
void f()
{
std::cout << "f()" << std::endl;
}
};
>
struct C2 : C1, virtual I2
{
void g()
{
std::cout << "g()" << std::endl;
}
};
>
struct C3 : C2, I3
{
void h()
{
std::cout << "h()" << std::endl;
}
};
>
When I do this, I get compiler warnings:
>
Warning 1 warning C4250: 'C2' : inherits 'C1::C1::f' via dominance
Warning 2 warning C4250: 'C3' : inherits 'C1::C1::f' via dominance
Warning 3 warning C4250: 'C3' : inherits 'C2::C2::g' via dominance
>
Please can someone explain what's going on?

C++ dominance: http://en.wikipedia.org/wiki/Dominance_(C%2B%2B)
--
Fran

[James Kanze]

Re: I wish c++ did interfaces better.

On Aug 7, 2:35 am, Stuart Golodetz
<sgolod...@NdOi SaPlA.pMiPpLeEx A.ScEomwrote:

Sorry to piggy-back on this thread, but it reminded me of a
related interfaces problem I had a while back actually (I
found a way to circumvent it, but it was a bit
unsatisfactory) .

Suppose you're implementing an inheritance hierarchy of
Cartesian coordinate systems. You start off with interfaces:

ICoordSystem <-- IOrthogonalCoor dSystem <-- IOrthonormalCoo rdSystem

You then want to implement each of these concretely:

ICoordSystem <-- CoordSystem
IOrthogonalCoor dSystem <-- OrthogonalCoord System
IOrthonormalCoo rdSystem <-- OrthonormalCoor dSystem

The only problem is, you want to reuse the logic in
CoordSystem etc., so you also have this:

CoordSystem <-- OrthogonalCoord System <-- OrthonormalCoor dSystem

At this point, I started having fun with my compiler. It was a
while back now, so I can't quite remember what I tried any
more before giving up and doing it another way, but how should
I have done it please, out of interest?

This is a classical case; I've seen it both with interfaces (as
in your example), and with machine generated code instead of
your interfaces (the machine generated code provides a simple,
default implementation, which you can override). I call it a
"ladder hierarchy", although I don't think I've seen this term
elsewhere:

ICoordSystem <--- CoordSystem
^ ^
| |
| |
IOrthogonalCoor dSystem <--- OrthogonalCoord System
^ ^
| |
| |
IOrthonormalCoo rdSystem <--- OrthonormalCoor dSystem

(Except that in one case I encountered, it extended some six or
seven layers deep.)

Basically, anytime a class has more than one arrow leading into
it, inheritance must be virtual; the simplest solution is just
to make all heritage of the interface virtual:

class ICoordSystem { /* ... */ } ;
class IOrthogonalCoor dSystem
: public virtual ICoordSystem { /* ... */ } ;
class IOrthonormalCoo rdSystem
: public virtual IOrthogonalCoor dSystem { /* ... */ } ;
class CoordSystem
: public virtual ICoordSystem { /* ... */ } ;
class OrthogonalCoord System
: public virtual IOrthogonalCoor dSystem
: private CoordSystem { /* ... */ } ;
class OrthonormalCoor dSystem
: public virtual IOrthonormalCoo rdSystem
: private OrthogonalCoord System { /* ... */ } ;

(Note that the inheritance of the implementation is private.
This is usual as well, although not always necessary.)

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

[James Kanze]

Re: I wish c++ did interfaces better.

On Aug 7, 3:30 am, Pete Becker <p...@versatile coding.comwrote :

On 2008-08-06 20:51:21 -0400, Stuart Golodetz
<sgolod...@NdOi SaPlA.pMiPpLeEx A.ScEomsaid:

[...]

When I do this, I get compiler warnings:

Warning 1 warning C4250: 'C2' : inherits 'C1::C1::f' via dominance

Please can someone explain what's going on?

The compiler is warning you that it's doing what the language
definition says it should do, and that you might not be smart
enough to understand what you've done. But you are: the code
is exactly right.

But isn't this a stupid warning? The formal specification of
dominance might be a little complicated (but only a very
little), but in practice, it always does exactly what one would
intuitively expect and want. While I can see the compiler
warning when the standard requires it to do something you likely
don't want, or even when it isn't clear what you might want
(what happens when comparing signed and unsigned, for example),
this looks to me very close to warning that the generated code
will do an addition, because you used the + operator.

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

[Pete Becker]

Re: I wish c++ did interfaces better.

On 2008-08-07 04:41:42 -0400, James Kanze <james.kanze@gm ail.comsaid:

On Aug 7, 3:30 am, Pete Becker <p...@versatile coding.comwrote :

>On 2008-08-06 20:51:21 -0400, Stuart Golodetz
><sgolod...@NdO iSaPlA.pMiPpLeE xA.ScEomsaid:

>
[...]

>>When I do this, I get compiler warnings:

>

>>Warning 1 warning C4250: 'C2' : inherits 'C1::C1::f' via domin

ance
>

>>Please can someone explain what's going on?

>

>The compiler is warning you that it's doing what the language
>definition says it should do, and that you might not be smart
>enough to understand what you've done. But you are: the code
>is exactly right.

>
But isn't this a stupid warning?

Yes. My original draft started out "Stupid compiler warnings."

The formal specification of
dominance might be a little complicated (but only a very
little), but in practice, it always does exactly what one would
intuitively expect and want. While I can see the compiler
warning when the standard requires it to do something you likely
don't want, or even when it isn't clear what you might want
(what happens when comparing signed and unsigned, for example),
this looks to me very close to warning that the generated code
will do an addition, because you used the + operator.

Maybe it means "we worked really hard to understand and implement
dominance, so we want to make sure you notice it".

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)

[Joe Greer]

Re: I wish c++ did interfaces better.

Pete Becker <pete@versatile coding.comwrote in
news:2008080706 140516807-pete@versatilec odingcom:

>
Maybe it means "we worked really hard to understand and implement
dominance, so we want to make sure you notice it".
>

It probably means that they have gotten a lot of bug reports about this
feature and they are doing a CYA manuver. :)

joe

[Chris Becke]

Re: I wish c++ did interfaces better.

struct Iv2 : virtual Iv1 {

virtual void method1()=0;};

Actually, this is where the house of cards falls down :- In a framework that uses interfaces to facilitate communication between precompiled modules potentially written in different languages you cannot ever use 'virtual' to extend an interface. The final resulting vtables MUST be contiguous in memory and using virtual will fragment them. :-(

"kwikius" <andy@servocomm .freeserve.co.u kwrote in message news:48996a8a_1 @mk-nntp-2.news.uk.tisca li.com...

Chris Becke wrote:

>Given an interface (in the c++ sense, nothing more than a struct containing pure virtual methods)

<...>

>That of course never never never works. Why not?
So what that Ive introduced another method0()=0 ?
Why can't it figure out it already has an implementation
for that method that I'd really like to allow it to use?

struct Iv1 {
virtual void method0()=0;
};

//And a class that implements the interface

class Iv1Impl : public virtual Iv1 {
virtual void method0(){}
};

//Its not unreasonable to create a version 2 interface

struct Iv2 : virtual Iv1 {
virtual void method1()=0;};

//One might *try* to create an implementation class like this

class Iv2Impl : public Iv2, Iv1Impl {
virtual void method1(){} // implement Iv2 method
};

int main()
{
Iv2Impl x;
}

regards
Andy Little

[Pete Becker]

Re: I wish c++ did interfaces better.

On 2008-08-07 10:09:29 -0400, "Chris Becke" <chris.becke@gm ail.comsaid:

>struct Iv2 : virtual Iv1 {
>virtual void method1()=0;};

>
Actually, this is where the house of cards falls down :- In a framework
that uses interfaces to facilitate communication between precompiled
modules potentially written in different languages you cannot ever use
'virtual' to extend an interface. The final resulting vtables MUST be
contiguous in memory and using virtual will fragment them. :-(

Huh? If you don't use virtuals you won't get vtables. Unless by
"vtable" you mean some non-C++ interoperabilit y mechanism, in which
case, it's not particularly pertinent here.

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)