Re: [boost] Streaming Lambda functions

From: David Abrahams (dave_at_[hidden])
Date: 2008-07-11 14:26:13

on Fri Jul 11 2008, "Robert Jones" <robertgbjones-AT-gmail.com> wrote:

> On Fri, Jul 11, 2008 at 7:08 PM, Steven Watanabe <watanabesj_at_[hidden]>
> wrote:
>
>>
>> Robert Jones wrote:
>>
>>> I'm sorry, I'm not quite following! I understand ADL in general, but not
>>> how
>>> it applies
>>> specifically here. What overloading is being found/not found by ADL in
>>> this
>>> context?
>>>
>>>
>>
>> The overload of operator<< for std::pair is not found by Boost.Lambda.
>
> This much I have discovered painfully by trial and error - the bit I
> don't understand is why! 

Because was not in scope at the point where the lambda's operator() is
defined, and it isn't defined in a namespace associated with any of the
arguments you passed to it (ostream and std::pair<...>).  Those are the
lookup rules for unqualified calls in templates.

    namespace bll
    {
       template <class T>
       void f(T x)
       {
          // looks backwards from this point for op<< defined in 
          // namespaces :: and ::bll::.  Looks backwards from the
          // point of instantiation for op<< defined in any 
          // namespaces associated with T.
          x << x;
       }
    };

I think I got that right.

> Also, it would be very handy if you can suggest a method of achieving
> the effect I'm looking for.

Depends what effect that is; it may or may not be achievable.

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com