Re: [boost] [SIGNAL/BIND] Regressions with GCC 4.1 RC.

From: Douglas Gregor (doug.gregor_at_[hidden])
Date: 2006-02-28 09:45:28

On Feb 28, 2006, at 7:49 AM, Peter Dimov wrote:
> Douglas Gregor wrote:
>> The Boost bug is an interaction between the storage optimizations
>> that went in to Bind (to make the storage optimizations of Function
>> actually useful) and the visit_each mechanism.
>
> You're mixing these up. The bug in this thread is present in 1.33.1  
> which doesn't have the storage optimization. It only shows up on  
> proper two-phase lookup compilers because the primary declaration  
> of visit_each wasn't #included.

Ah, okay. In that case, I was fixing the other Signal/Bind bug that  
shows up in Boost CVS.

> The patch seems good to me. There are several things that I don't  
> like in the test, though; (a) it doesn't actually test anything,  
> (b) it uses boost/test instead of lightweight_test.hpp, (c) it  
> binds std::plus<> with one argument, which is invalid and can be  
> rejected by a conforming bind.

You're right, of course. Thanks for the improved test case.

> I tried to fix it, but an interesting question has come up. The  
> primary visit_each is defined as:
>
>    template<typename Visitor, typename T>
>    inline void visit_each(Visitor& visitor, const T& t);
>
> By forcing a const qualification on the visited object, this  
> interface prevents a visitor from mutating it (in addition to  
> breaking when a function is being passed for t.) Is this deliberate  
> or accidental?

I think it was deliberate when I originally implemented visit_each,  
because I wasn't imagining that it would be used for mutation.  
However, now I think it was the wrong decision... taking a non-const  
T& even has the side benefit of causing a failure if someone  
accidentally tries to stick a temporary into visit_each.

> Anyway. Could you please look at the attached test case and confirm  
> that it passes with your patch applied?

Yes, it passes on both GCC 4.0.1 and GCC 3.3.

        Doug