Re: lambda library usage

From: khuroth (dansilva_at_[hidden])
Date: 2002-08-27 09:31:04

Yeah, I'm not sure why I used _1 in the example, but ultimately my 
goal is to delay the call to the DoSomething method of Controller.

something like this would feel kind of natural:

boost::function0<Controller&> delayed_controller = (this ->* 
&View::GetController);

bind(_1 ->* &Controller::DoSomething, delayed_controller );

but delayed_controller is a function instead of a Controller pointer 
and its return type isn't a Controller pointer either, it's a 
Controller reference.

--- In Boost-Users_at_y..., Jaakko Jarvi <jajarvi_at_c...> wrote:
> I'm not exactly sure what you are trying to do, but here's some
> explanation on bind and ->*
> 
> First, The dot operator cannot be overloaded, so you do this:
> 
> (_1 ->* &A::GetController).DoSomething
> 
> // DoSomething is not defined for (_1 ->* &A::GetController)
> 
> 
> 
> The semantics of ->* in general is rather tricky, and the Lambda 
Library
> mimics that:
>   
> Here's a snippet of the LL docs:
> 
> For a built-in call like this, the result is kind of a delayed 
> member function call. Such an expression must be followed by a 
function 
> argument list, with which the delayed member function call is 
performed. 
> For example:
> 
> struct B { int foo(int); };
> B* b = new B();
>   ...
> (b ->* &B::foo)         // returns a delayed call to b->foo
>                         // a function argument list must follow
> (b ->* &B::foo)(1)      // ok, calls b->foo(1)
> 
> (_1 ->* &B::foo)(b);    // returns a delayed call to b->foo, 
>                         // no effect as such
> (_1 ->* &B::foo)(b)(1); // calls b->foo(1)
> 
> 
> The LL can figure out the return type when binding a pointer to 
member 
> function so you do no have to say bind<Controller&>, just bind is 
enugh.
> 
> If you just try to bind a member function call, leaving the object 
open, 
> you would write:
> 
> bind(&View::GetController, _1); 
> 
> Could be called as:
> 
> View w;
> bind(&View::GetController, _1)(v); 
> 
> Cheers, Jaakko
> 
> On Tue, 27 Aug 2002, khuroth wrote:
> 
> > whoops, sorry about the typos.  any A:: here is really View::
> > 
> > --- In Boost-Users_at_y..., "khuroth" <dansilva_at_l...> wrote:
> > > I have a View class that has some widgets, and a Controller 
class
> > > that controls the view, so the View class has a GetController()
> > > member function that returns a reference to its controller.
> > > When the View's constructor is called, GetController() would
> > return
> > > a bad ref, but it would return a valid reference by the time a
> > > button is clicked... so what I'm trying to do is, inside the 
View
> > > constructor:
> > >
> > > my_button.on_click = bind( (_1 ->* 
&A::GetController).DoSomething
> > (),
> > > this );
> > >
> > > but that obviously doesn't work... I'm even having trouble just
> > > saving the first part:
> > >
> > > my_button.on_click = bind<Controller&>(_1 ->* 
&A::GetController,
> > > this);
> > >
> > > also doesn't work...
> > > I tried defining the on_click member of the button as a
> > > boost::function<void> or a boost::function0<void> in the first
> > > example, and a boost::function<Controller&> or
> > > boost::function0<Controller&> in the second snip... I get 
immense
> > > compiler errors, so I'm not sure where I messed up.
> > >
> > > What am I doing wrong?
> > 
> > 
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> -- 
> --
> -- Jaakko Järvi                       email: jajarvi_at_c...
> -- Post Doctoral Fellow               phone: [phone]
> -- Pervasive Technology Labs          fax:   [phone]
> -- Indiana University, Bloomington