[swap] ruminations

[Jeffrey Hellrung]

1) Would it be a good idea (aside from breaking backwards compatibility)
to insulate boost::swap from being found via ADL?  Consider another
library, tsoob, which also has a swap intended to be used in the same
manner as boost::swap (i.e., it finds a free function swap via ADL,
falling back to std::swap or maybe some custom swap or whatever).  Now
tsoob::swap and boost::swap cannot be used in the same program
(ambiguous call to overloaded function swap) if the associated
namespaces of the type being swapped includes both boost and tsoob
(certainly possible).

What I'm thinking about is the possibility of creating a namespace
boost::adl which contains (what would have previously been called)
boost::swap, as well as any other similar free functions.  So the only
way to invoke boost::adl::swap is to make it explicit; it will never be
found via ADL.  Plus it helps document the underlying mechanism (ADL) of
boost::swap.

2) Is there some way for boost::swap and tsoob::swap to coexist
peacefully while retaining backwards compatibility for both?

- Jeff

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[Joseph Gauterin]

I Googled for tsoob, but couldnt' find any references to it. It would
be helpful to have a look at tsoob::swap to see what's causing the
ambiguity and how/if it could be resolved.

Also, I always intended for boost::swap to be called explicitly,
rather than though ADL - it's supposed to hise the use of ADL from the
user (no need for 'using std::swap' etc). If you use a fully qualified
call, is there still ambiguity between boost and tsoob?
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[Jeffrey Hellrung]

Joseph Gauterin wrote:
> I Googled for tsoob, but couldnt' find any references to it. It would
> be helpful to have a look at tsoob::swap to see what's causing the
> ambiguity and how/if it could be resolved.
>
> Also, I always intended for boost::swap to be called explicitly,
> rather than though ADL - it's supposed to hise the use of ADL from the
> user (no need for 'using std::swap' etc). If you use a fully qualified
> call, is there still ambiguity between boost and tsoob?

Of course, tsoob is just a made-up name...

Upon further reflection, this is probably something that shouldn't
happen, as it depends on the user wanting to use their own fallback swap
function rather than std::swap.  I was (however rightly or wrongly)
concerned with code that does something like the following:

-------- BEGIN CODE --------
#include <algorithm> // for std::swap

#include <boost/utility/swap.hpp>

namespace std { struct X; }
namespace boost { struct Y; }

namespace tsoob { namespace no_adl {
#if 0 // Enabling this one gives ambiguities with std::swap
template< class T >
void swap(T& x1, T& x2) { /* fallback custom generic swap */ throw 0; }
#endif
#if 0 // Enabling this one gives ambiguities with boost::swap
template< class T1, class T2 >
void swap(T1& x1, T2& x2) { /* fallback custom generic swap */ throw 0; }
#endif
} } // namespace no_adl / namespace tsoob

namespace tsoob_swap_impl {
template< class T >
void swap_impl(T& x1, T& x2)
{
     using namespace ::tsoob::no_adl;
     swap(x1, x2);
}
} // namespace tsoob_swap_impl

namespace tsoob { namespace adl {
template< class T >
void swap(T& x1, T& x2) { ::tsoob_swap_impl::swap_impl(x1, x2); }
} // namespace adl
template< class T > struct Z { };
} // namespace tsoob

int main()
{
     tsoob::Z< std::X > zx1, zx2;
     tsoob::adl::swap(zx1, zx2);
     tsoob::Z< boost::Y > zy1, zy2;
     tsoob::adl::swap(zy1, zy2);
}
-------- END CODE --------

At the end of the day, this is probably just not a Good Idea.

However, I still think it might've been a good idea to insulate
boost::swap from being found via ADL at all by inserting another
namespace in there (so it's boost::adl::swap) rather than declaring
boost::swap to have 2 template parameters to make it less preferred than
std::swap.  Evidently I can't think of a good example where this is a
problem, though :/

Apologies for the noise,

- Jeff
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[Joseph Gauterin]

Of course tsoob is a made up name - I'm slightly embarrassed not to
have noticed!
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