Re: [boost] Re: Unnamed namespaces in headers, was Boost.Threads proposed change: lock constructors

From: Peter Dimov (pdimov_at_[hidden])
Date: 2004-06-30 07:28:13

David Abrahams wrote:
> "Peter Dimov" <pdimov_at_[hidden]> writes:
> 
>> David Abrahams wrote:
>> 
>> I'm not sure how the bind placeholders can cause ODR violations;
>> could you elaborate?
> 
> Simple:
> 
>   // f.hpp
>   template <class S, class T>
>   bool f(S const& s, T const& x)
>   {
>      return std::find_if(
>           s.begin(), s.end()
>         , boost::bind(std::less<T>(), _1, x)) != s.end();
>   }
> 
>   // t1.cpp
>   #include "f.hpp"
>   #include <vector>
>   bool f1(std::vector<int> const& s, int x) { return f(s,x); }
> 
>   // t2.cpp
>   #include "f.hpp"
>   #include <vector>
>   bool f2(std::vector<int> const& s, int x) { return f(s,x); }
> 
> 
> f<std::vector<int>,int> refers to a different object when it names
> "_1" in each translation unit.

Thanks.

>> Anyway, I guess we'll have to migrate to TR1 style placeholders one
>> day, but this will break existing code.
> 
> I'm not sure it has to:
> 
>   namespace bind {
>     extern arg<1> _1;
>   }

#1: extern implies .obj. Currently bind doesn't need a build/install phase.

>   namespace { // if you like
>     using bind::_1;
>   }

#2: using declarations of this form have a lot of potential. :-)

I had in mind something like

namespace boost
{

namespace placeholders // also sagt TR1
{

inline boost::arg<1> _1() { return boost::arg<1>(); }

} // placeholders

} // boost

#ifndef BOOST_BIND_NO_GLOBAL_PLACEHOLDERS

    using namespace boost::placeholders;

#endif

But not for 1.32, I think. First thing post-branch, maybe. ;-)