[Boost-users] [BLL] bind(data_member_ptr,_1) functor won't accept temporaries as arguments

From: Jaakko Jarvi (jajarvi_at_[hidden])
Date: 2003-10-13 11:52:51

In our last exciting episode Eelis van der Weegen wrote:
> Lines: 28
> User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US;
> 	rv:1.5b) Gecko/20030901 Thunderbird/0.2

> When Lambda's bind() is used to create a functor for dereferencing a 
> stored data member pointer given an instance pointer, it seems it is not 
> possible to pass the resulting functor a temporary instance pointer. For 
> example:

>    #include <boost/lambda/bind.hpp>
>    #include <boost/lambda/lambda.hpp>

>    struct S { int i; };

>    S * f ();

>    void g (S * p)
>    {
>      using namespace boost::lambda;

>      bind(&S::i, _1)(p); // ok
>      bind(&S::i, _1)(p + 0); // not ok, "no match for call ..."
>      bind(&S::i, _1)(f()); // not ok, "no match for call ..."
>    }

> Does this restriction have a purpose? If not, is this something that 
> needs to be (and can be) fixed?

Lambda functors cannot take a non-const rvalue as an argument.
There isn't a pretty fix for it at the momemnt, but there is some 
work going on in the standards committee regarding the so called
"forwarding problem" which would allow a fix. So in a few years... :)

Now, there are several workarounds:

Use make_const helper function:

bind(&S::i, _1)(make_const(p + 0));

or do:

S* p2 = p + 0;
bind(&S::i, _1)(p2);

and more, check 

5.9.2. Rvalues as actual arguments to lambda functors

in lambda docs.

  Cheers, Jaakko