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From: vandevoorde@gmail.com
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Subject: Re: replacing Koenig lookup
Date: Tue, 19 May 2009 15:38:29 CST
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On May 17, 11:50 pm, Michael Kilburn <crusader.m...@gmail.com> wrote:
> Hi,
>
> I am sure everyone here like the idea of changing core language
> feature... ;-) But has anyone ever considered getting rid or better
> replacing 'Koenig lookup'? Personally I never liked it and consider it
> a hack, another type of static polymorphism that conflicts with
> existing mechanisms and produces problems:http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1440.htm(see
> issue #226 and related)
>
> Philosophy behind my reasoning could be shortly explained as:
> - function name is a key to a semantic (obviously, that is why we give
> them meaningful names) and (via static polymorphism) to specific
> functionality (based on argument types and current scope of visibility
> (SoV))
> - 'namespace' concept is a way to separate different semantics that
> are referred using the same name
> - but Koenig lookup hacks/breaks this idea by effectively reserving
> function name regardless of current scope for the same semantic
>
> This obviously was done for a reason, e.g. you would normally expect
> the same semantic for operator+() regardless of scope. Therefore that
> thing that might replace Koenig lookup should be some kind of
> mechanism that will allow to reserve function name on a global level
> (and leave static polymorphism to function overloading) -- as an
> additional bonus it will simplify related chapters in C++
> standard. ;-)
> It does not really matter right now how this mechanism might look
> like. E.g. it could be something like this:
> - magic namespace (e.g. 'global')
> - where you are going to put all names that should have the same
> semantic regardless of current SoV
> - which will behave similar to unnamed namespace (will merge with
> current SoV)
> - therefore when container wants to provide implementation for swap
> function -- all it needs is:
> class Container { ... };
> namespace global { void swap(Container& l, Container& r) { l.swap
> (r); } }
>
> Of course various things (like nested namespaces have to be addressed
> properly and convenient things like allowing 'global' to designate the
> same namespace regardless of current nesting level) will have to be
> addressed before this idea will be mature enough...
>
> What do you think?


I don't think you'll see change in this area for the C++0x time frame.

However, you may be interested in N1691 (Dave Abrahams' "explicit
namespace" proposal) and Herb Sutter's N2103 (a proposal to limit ADL
in some ways).

       Daveed


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