Hi Terry,

How should a user/developer like me implement a custom swap so that users get the benefit of my::swap, then boost::swap, and finally std::swap without putting too much burden on my user. In the same way that swap functions are currently provided: either a specilization of std::swap or a 'swap' function in the same namespace as a UDT.

If the user does this... using namespace std; using namespace boost; using namespace my; And then somewhere in his code... HisType a = MakeA(); HisType b = MakeB(); swap(a, b); // Unqualified? Or should the user specify a namespace? Which one?

A call to std::swap will swap the two values using temporary values and assignments. A call to boost::swap will call my::swap using argument dependent lookup. A call to my::swap will just call my::swap. An unqualified call to swap will call my::swap using argument dependent lookup.

If the user has his own swap(HisType, HisType) invoke that. Otherwise, if HisType.swap() exists, then use that. Otherwise, use my::swap() if its "better" than boost::swap Otherwise, use boost::swap(), if its "better" than std::swap Otherwise, just use std::swap().

boost::swap calls my::swap() if it exists, else it calls std::swap(). HisType.swap is not considered directly.

Sorry about all the ambiguity. But that's the real problem, isn't it? Indeed - but we're using a trick to make sure that boost::swap is never ambiguous with std::swap.

So: using namespace std; using namespace boost; swap(a, b); Will call a function called swap in the namespace where 'a' is declared if it exists (adl), or std::swap otherwise. It will never call boost::swap. std::swap(a, b); Will swap 'a' and 'b' using temporaries and assignments. boost::swap(a, b); Will call a function called swap in the namespace where 'a' is declared if it exists, or std::swap otherwise. No 'using' statements or declarations are required. Joe.