Re: [boost] boost::result_of and C++03

From: shunsuke (pstade.mb_at_[hidden])
Date: 2008-04-05 22:52:50

Eric Niebler wrote:
> Correct. However, I can do this:
> 
> struct make_plus : function_adaptor<make_plus>
> {
>    // same as before
> };
> 
> where function_adaptor implements the TR1 function object protocol and 
> makes some guess about const-ref arguments -- probably that they are 
> rvalues. It could even implement the reference_wrapper gunk to treat 
> reference_wrapper<X> arguments as if they were X&.
> 
> This is the route Proto is taking. Proto can query a function object F 
> if it uses this protocol and use it if it's available, getting correct 
> lvalue/rvalue handling. If it's not available, it uses result_of, 
> stripping references from argument types first.
> 
> I think I got the idea from reading through Egg's documentation. It 
> seems like if this sort of thing doesn't exist there, it probably 
> should. Shunsuke?

In C++03, it is (practically speaking) impossible to overload on "rvalueness".
So, Egg doesn't tell underlying objects(called LittleFunction in Egg)
whether an argument is an lvalue or rvalue.
A LittleFunction always regards arguments as lvalues.
It might be possible to offer some protocol for overload on "rvalueness"
(e.g. rvalue_wrapper) which will be turned on in C++0x, though.

Regards,

-- 
Shunsuke Sogame