Re: [boost] boost::function error

From: Douglas Gregor (gregod_at_[hidden])
Date: 2002-04-01 23:33:29

On Monday 01 April 2002 08:17 pm, you wrote:
[snip]
> The first argument has changed from OpaqueDataT * to
> boost::any. That does not work.
>
>  //replacing the arg from OpaqueDataT * to boost::any
> gives error.
>  boost::function<void, boost::any> f2 = yieldcallback;
> //error
>
> VC++ 6.0 issues the error:
> c:\virtualsilicon\vs\vre\debuggers\shared\boost\function\function_template.
>h pp(94) : error C2664: 'void (struct OpaqueDataT *)' :
> cannot convert parameter 1 from 'class boost::any' to
> 'struct OpaqueDataT *'
>
> What is the right way to handle this?
>
> Thanks,
> -Ani

The problem is that boost::any cannot be implicitly converted to OpaqueData*, 
so you'll need to perform the conversion explicitly. You can use boost::bind 
to do this:

 boost::function<void, boost::any> f2 =
   boost::bind(yieldcallback,
               boost::bind<OpaqueDataT*>(&boost::any_cast<OpaqueDataT*>, _1));

This means that a call 'f2(cbdata)' becomes a call like 
'yieldcallback(boost::any_cast<OpaqueDataT*>(boost::any(cbdata)))'

However, I think that this is not really what you want. I'm going to guess 
that the argument that is send back to the callback (that, in this case, has 
type 'OpaqueDataT*') is provided by the user along with the callback. In this 
case boost::bind is the better solution, used like this:

boost::function<void> f3 = boost::bind(yieldcallback, cbdata);

Notice that f3 doesn't even take an argument, because the argument is a part 
of what f3 will call. Also, you need not worry about casting from a 
boost::any back to the appropriate callback data type, because cbdata is 
already of the correct data type. A call like 'f3()' then becomes a call 
like'yieldcallback(cbdata)'.

        Doug