Re: Why is 'Variant' used in so many contexts?
On Jul 25, 2:24 am, Eugene Gershnik <gersh...@hotmail.com> wrote:
On Jul 21, 8:49 pm, Mathias Gaunard <loufo...@gmail.com> wrote:
boost::variant uses switch, which is by the way, way more efficient
than virtual functions unless your optimizer is capable of virtual
function inlining.
The repeated reference to switch made me curious so I spent some time
looking over the boost::variant code.
It is indeed a[n extremely ugly] switch that relies on preprocessor
tricks of all things. I was indeed deeply ignorant that such
preprocessor things are even possible. I wouldn't pass such code in a
code review or use it but that's just my personal opinion.
In any event now I am confident it is a *runtime* dispatch rather than
compile time thing as was claimed by Kirit.
What I was talking about was type safety. Clearly there must be a
mechanism to determine what is in the variant at runtime and execute
the correct function -- I would never claim that the dispatch is done
by the compiler as that would clearly be an impossible feat.
I only said that the dispatch wasn't a virtual. I haven't examined the
Boost variant code to see what is in there, but I can tell from how it
works that it couldn't use a virtual.
The point I was making has to do with type safety, NOT dispatch
mechanisms and when the type check is done.
A COM variant (especially one that arrives through a COM interface)
could contain literally anything. This means that there is always the
possibility of a type check failure at runtime. With the Boost
variant, because an incompatible type cannot be placed in it (a check
enforced by the compiler) it is also impossible to pull an
incompatible type out of it (again enforced by the compiler).
So, although the Boost variant must also work out which code to
execute at runtime, because of the way it works with the compiler it
can guarantee that there will never be a type failure at runtime.
K
--
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]