Re: [boost] Re: Re: reference binding

From: Jaakko (jajarvi_at_[hidden])
Date: 2004-04-15 21:39:21

On Apr 15, 2004, at 3:21 PM, Dill, John wrote:

>> Hello John,
>>
>> There's a big technical reason:
>>
>> If bind would have reference semantics, you could not do this:
>>
>> bind(f, 1, _1)
>>
>> Bind would have to take its arguments as
>>
>> template <class F, class A, class B, ...>
>> bind( F, A&, B&, ...)
>>
>> 1 is a non-const rvalue, which cannot be bound to non-const reference.
>
> I think I almost got it now, but I'm still a little confused about 
> bind_template.hpp where the list template parameters are A1&, A2&, 
> A3&, ...
>
> list1 for example stores a template parameter A1 as "A1 a1;".  So this 
> ends up not actually being a reference to a normal type T, but to the 
> value<T> wrapper class?  If not, what type is actually A1& in the 
> listN declarations in bind_template.hpp?
>
In bind_template.hpp you are looking at the parameter passing 
conventions for the arguments to
a function object that bind has created. Those are passed by reference.
They are temporarily stored into the listN structures, or references to 
those arguments are,

in

   bind(f, _1, a)(b)

bind(f, _1, a) creates a function object which stores a copy of a, 
which defines a unary
function call operator that takes its argument, b in this case, by 
reference.

So e.g. list1 is always instantiated with a reference type, and thus it 
also stores the arguments
as references.

Hope this helps,

   Jaakko