[boost] Re: once again: boost.bind namespace conflicts and how tosolvethem

From: David Abrahams (dave_at_[hidden])
Date: 2004-11-29 20:32:51

Peter Dimov wrote:
> David Abrahams wrote:
> 
>> Peter, I've been expecting for some time that you were going to put
>> the placeholders in a sub-namespace of boost, if only to avoid the ODR
>> violations it is currently causing.  No?
> 
> 
> Correct me if I'm wrong, but the ODR violations are caused by the use of 
> an unnamed namespace; 

True.

> this is not related to whether the placeholders 
> are in a subnamespace of boost. 

True again.  That's a separate issue; anything we put outside boost is 
subject to unresolvable conflicts.  <unnamed>:: is especially vulnerable.

> It seems to me that Lambda's 
> placeholders should be susceptible to the same sort of ODR violations. 

I don't know what lambda does.  I know mpl doesn't do that.

> (I know that I asked you for an ODR/_1 example once and you provided it, 
> but I don't remember what it was and can't find it at the moment. Sorry.)

   int x(int);

   template <class T>
   void f()
   {
     boost::bind(x, _1);
   }

Now all you need is for two translation units to instantiate f<X> for 
some X, the same in each TU.

>> The fact that I have to write mpl::_1 just to avoid colliding with
>> <unnamed>::_1 is disconcerting at best.  If another library came along
>> and did what boost/bind/placeholders.hpp does, it would cause a clash.
> 
> First mover advantage, you know. (Kidding.)
> 
>> My suggestion, assuming you care about backwards compatibility, is:
>>
>> boost/bind/placeholders.hpp
>>
>>   declares placeholders in namespace boost::bind::placeholders.
> 
> 
> But does not _define_ them? 

You could define them as constants.  Maybe enums would work:

   enum _1t { _1 };
   enum _2t { _2 };

??

> (I had boost::placeholders in mind for a 
> namespace, consistent with std::tr1::placeholders; when you use 'boost' 
> as a substitute for 'std::tr1', boost::bind::placeholders won't work.)

Okay, as long as you cooperate with phoenix and lambda, as far as I'm 
concerned.

>> boost/bind/bind.hpp
>>
>>   has the current contents of boost/bind.hpp, except that
>>   placeholders are not declared in the <unnamed>::
>>
>> boost/bind.hpp
>>
>>   #include <boost/bind/bind.hpp>
>>   #include <boost/bind/placeholders.hpp>
>>   namespace { using namespace boost::bind::placeholders; }
> 
> 
> Hm. What is the difference between this directive and the simple
> 
>    using namespace boost::bind::placeholders;
> 
> ? I can't think of any.

Honestly, I'm not sure there is any.

> This, of course, will still require you to use mpl::_1. 

Yes, I expect that.  Anyone conscientious will have to use qualification 
or local using directives to access placeholders.

> I assume that 
> you intend to fix all your includes to belong to... err, point to the 
> using-directive-less version?

You bet.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com