Re: [sigc] function of const object as slot

[Roel Vanhout <roel riks nl>]

Ulrich Eckhardt wrote:
> Roel Vanhout wrote:
>  > The program below doesn't compile (at least on MSVC, the error is:
> Naming a compiler without a version is almost meaningless. Also it would be
> nice to know that you're using sigc++ 1.x. ;)

Yeah you're right of course, the compiler was msvc .net 2003. And it was 
indeed 1.5, I didn't mention because it didn't even occur to me that 
that might make a difference ;)

>  > error C2440:
>  > 'initializing' : cannot convert from 'const Foo' to 'Foo &'
>  >          Conversion loses qualifiers
>  > see reference to
>  > function template instantiation
>  > 'SigC::Slot0<R>
>  >  SigC::slot<std::string,const Foo,Foo>(O1 &,R (__thiscall Foo::* )(void)
>  > const)' being compiled
>  >          with
>  >          [
>  >              R=std::string,
>  >              O1=const Foo
>  >          ]
>  > )
>  >
>  > How can I connect to a slot of a const object?
> You can't. The point is that connecting a  SigC::Object is a mutating
> operation on the SigC::Object underneath - your connection is registered
> there, so it can be automatically disconnected when the SigC::Object is
> destroyed.

Aha yes indeed I see now.

>  > I've tried putting
>  > 'const' just about everywhere I could imagine but no change apart from
>  > other error messages :) It compiles ok when I don't make the object 'f'
>  > const. Is it possible at all?
> Ideas:
> - Use const_cast, but that invokes UB when the object is a constant, 
> i.e. if
> it isn't just that you only have a const reference to it.
> - Use SigC::class_slot, but then you loose the automatic disconnecting and
> need to implement that manually - sometimes the automatic one doesn't work
> that good anyway so it might not be a problem.
> - Don't use a const object.

Well to be honest I don't need it myself, I've never bothered with 
const-correctness (that may explain why I didn't see what the problem 
was in the first place), I got this question as a reply to my article on 
codeproject on libsigc++ 
(http://www.codeproject.com/cpp/sigcart.asp?msg=1201497#xx1201497xx) and 
I couldn't figure it out myself. I'll post a pointer to this message in 
response to that question :) Thanks (thanks to Arnaud for replying as 
well of course).

cheers,

roel