Re: [Boost-Users] Re: lambda library usage

From: Jaakko Jarvi (jajarvi_at_[hidden])
Date: 2002-08-27 09:15:44

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_[hidden]
-- Post Doctoral Fellow               phone: [phone]
-- Pervasive Technology Labs          fax:   [phone]
-- Indiana University, Bloomington