Re: [boost] [swap] End-user allowed to add overloads to boost namespace?

From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-07-26 14:22:33

AMDG

Niels Dekker - mail address until 2008-12-31 wrote:
> Steven Watanabe wrote:
>   
>> Adding overloads to namespace boost is not guaranteed to work either.
>>
>> template<class T>
>> void foo(T& t1, T&t2) {
>>     boost::swap(t1, t2);
>> }
>>
>> class X {};
>>
>> namespace boost {
>>     void swap(X&, X&); // not found by foo.
>> }
>>     
>
> Thanks, Steven! So the user should make sure that all of her custom
> boost swap overloads are declared, /before/ including any function that
> actually calls boost::swap... right?
>   

Yes, this should work, but it is hard to guarantee.

> I
> tend to think that it's "morally right" to have the end-user provide
> template specializations of boost function templates, but I'm not sure
> about overloads...  what do you think?
>   

I don't see a problem in this case.

> Unfortunately this is
> not the case for msvc-7.1: msvc-7.1 fails on both
> "specialized_in_global" and "specialized_in_other".  It just doesn't
> seem to do ADL on template arguments.  :-(
>   

msvc-7.1 is one of the compilers that does all look up in phase 2, I think.
So...

namespace users_namespace {
    class X {};
    void swap(X&, X&);
}

#if BOOST_WORKAROUND(BOOST_MSVC, == 1310)
namespace boost {
    using users_namespace::swap;
}
#endif

We have to use a #if to avoid an ODR violation for
compilers that implement two-phase name lookup correctly.

In Christ,
Steven Watanabe