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From: Niels Dekker - mail address until 2008-12-31 (nd_mail_address_valid_until_2008-12-31_at_[hidden])
Date: 2008-08-15 17:01:17

Isaac Dupree wrote:
> (please include some explanatory comments!:-)

Yes, I think you're right, <boost/utility/swap.hpp> deserves some more
lines of comment.

> // various workarounds to avoid compiler bugs that make the compiler
> // find boost::swap before std::swap make us
> // - put this outside the boost namespace,
> // - use a using-directive rather than a using-declaration,
> // - and make boost::swap have two template arguments.

Thanks for your input! Quite nice actually, but not yet entirely
correct: the using-directive is there to work around some other compiler
bugs! Both MSVC 7.1 and Borland didn't do argument-dependent lookup
when ::boost_swap_impl::swap_impl had a using-declaration ("using
std::swap"). While now they do, due to Joseph's commit [47967] :-)
See also the thread, "[boost] [swap] Workaround for ADL failures of MSVC
7.1 and Borland okay?",

So I've just edited your comments a little bit:

// Note: the implementation of the boost::swap utility contains
// various workarounds:
// - swap_impl is put outside the boost namespace, to avoid infinite
// recursion (stack overflow) when swapping objects of a primitive type.
// - swap_impl has a using-directive, rather than a using-declaration,
// because some compilers (including MSVC 7.1 and Borland 5.9.3) don't
// do argument-dependent lookup when it has a using-declaration instead.
// - boost::swap has two template arguments, instead of one, to
// avoid ambiguity when swapping objects of a Boost type that does
// not have its own boost::swap overload.

Okay? I wouldn't mind adding it to utility/swap.hpp, and committing it
to the trunk.

> Although I couldn't say how strong the use-case is; probably
> it's rarely if ever a good idea to specify *that* template
> argument explicitly.

Okay. I'm not sure if my swap<animal_base_class>(cat,dog) use-case is
very strong either. So as long as nobody really feels the need to have
direct access to ::boost_swap_impl::swap_impl, I wouldn't mind leaving
it outside of the boost namespace.

> The function name already confuses me a little though: why
> should a function named boost::swap do ADL when the nearly
> equivalent name std::swap doesn't?

Because of its name, the boost::swap utility is overloaded by swap
functions from other Boost libraries. And of course, when boost::swap is
called for a type that has its own boost::swap overload, this overload
will be called directly. Isn't that nice? :-)

By the way, boost::exception is also quite different from
std::exception. Even more so!

> The name swap_using_adl, besides that not everyone knows
> the acronym "adl", makes it less obvious whether it
> can find the generic std::swap implementation
> (through 'using' not ADL).

Indeed, you're right. I was actually considering to spell it out:
swap_using_argument_dependent_lookup(T&,T&). But do you have a better
suggestion? What about just renaming the function to
"call_swap(T&,T&)"? Because that's what it does: it just calls the most
appropriate swap function. Anyway, its name is not that important, as
long as the helper function remains an "implementation detail".

> But maybe the main function has to be named boost::swap to convince
> clueless people to use it instead of std::swap...

Hope you're convinced already...

Kind regards, Niels

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