Re: [Boost-users] boost & c-style calback issue

From: Michael Bradley Jr (mbradley.jr_at_[hidden])
Date: 2008-06-02 10:27:32

Hi Nikolai,

thanks for your speedy reply.

Nikolai N Fetissov wrote:
>> Hi,
>>
>> i'm facing this problem may be someone on the list do have an elegant
>> solution
>>
>>
>> class A;
>> class B;
>>
>> class C {
>> int operator () (A*,B*)
>> };
>>
>> typedef int (*foobar)(A*,B*);
>> void do_something(foobar cb);
>>
>> C obj;
>> do_something(obj);	*ERROR*
>
> Object of type C is not convertible to function
> pointer type foobar.
>
>> did try boost::function<int(A*,B*)>  ftor = obj;
>> do_something(ftor);	*ERROR*
>>
>> compiler generates in both cases the following error messages
>> cannot convert parameter 2 from 'boost::function<Signature>' to 'int
>> (__cdecl
>> *)(A *,B *)'
>>
>>    with
>> 1>         [
>> 1>             Signature=int (A *, B *)
>> 1>         ]
>> 1>         No user-defined-conversion operator available that can perform
>> this
>> conversion, or the operator cannot be called
>>
>>
>> I can't make the corresponding class B member function static due to
>> side-effects.
>> Any clue how to get rid of this?
>
> You got it backwards - raw function pointer is convertible
> to boost::function, not the other way around (compiler is
> trying to tell you exactly that.)
> Make 'do_something' take boost::function instead.
>

The problem is that "do_something" is a legacy code written in plain C and it 
expect to get a function pointer with the signature

typedef int (*foobar)(A*,B*);
where A and B are struct.

Making the member function that has that signature static helps solve the 
problem but then we do have undesirable side-effect since this operation should 
be instance based and not object-bases.
May be someone can come out with a better approach?!

Thanks

Cheers
Mike