Re: [sigc] function of const object as slot

[Ulrich Eckhardt <eckhardt satorlaser com>]

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. ;)

> 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.

> 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.

Uli