Re: [boost] An open typeswitch using lambdas

On 30/11/2013 09:37, Matt Calabrese wrote:

On Fri, Nov 29, 2013 at 11:47 PM, Sumant Tambe wrote:

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Well, gcc 4.9 has a name lookup bug ( http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58820)

The bug is probably just worked around by using "using operator()". When I was experimenting with that kind of thing, it worked just fine.

...

and Visual Studio 2013 Nov CTP chokes on lambda multiple inheritance.

You could wrap lambdas into proper function objects to work around this.

If you want to support type-erased objects, it's not quite that simple, though I personally am not very interested in those capabilities. At the very least, it should be possible to make the simple, overload-based one without the overhead of a more dynamic approach.

The "open" in "open typeswitch" means that it doesn't require a closed set of types. The open aspect is difficult to do properly. The issues to me with the kind of implementation used here is that it has too big a requirement on the function objects to handle the different cases, doesn't deal well with conversions, cannot use generic code etc. If we had stuff like virtual templates it would be possible to treat open and closed type switch the same way, but emulating virtual templates by hand is a bit clunky.

Anyway, regardless of the implementation, I do think that this is a worthwhile tool. Make it inherit from boost::static_visitor and I'll start using as soon as I can.

AFAIK variant doesn't require that your visitors inherit from static_visitor, just that they define the result_type typedef.