Boost C++ Libraries: Ticket #3982: mpl::at documentation out of sync with code

component changed

Steven Watanabe — Tue, 09 Mar 2010 08:08:14 GMT

component Documentation → mpl

dsaritz@… — Mon, 17 May 2010 09:33:43 GMT

In my experience (with MSVC++) implementing the defaulted at as

...

{

typedef typename boost::mpl::at<Seq, Key>::type in_seq;

typedef typename boost::mpl::if_

<

boost::is_same<in_seq, boost::mpl::void_>, Default, in_seq

::type type;

}

or by using find + compare-to-end (instead of void) + deref proved to be more (compile-time) efficient (probably because of skipping the instantiation of has_key<>...)...

In any case, the defaulted at should IMO be provided out-of-the-box by MPL (both because of 'cleanness' and because the library author has the best chance to make it as efficient as possible)...

Steven Watanabe — Sat, 05 Jun 2010 20:39:37 GMT

Having a three argument at in the library is not necessarily more efficient overall, since it will increase the overhead of the two-argument at.

Gordon Woodhull — Sun, 02 Jan 2011 05:27:25 GMT

Steven, are you saying that it should probably have a different name for efficiency reasons?

at_def? at_or?

jeffrey.hellrung — Tue, 12 Jun 2012 17:46:39 GMT

So what's the correct resolution here? Update the documentation to remove the 3-argument version? Or implement the 3rd argument? Seems like the former is easier/safer. I'd be willing to do either to the best of my abilities.

Aleksey Gurtovoy — Tue, 12 Jun 2012 22:33:50 GMT

The correct resolution would be to add the default argument support. I'd be happy to review your patch.

wmamrak@… — Fri, 10 Jan 2014 13:35:17 GMT

Anyone still interested in fixing this? Why does it take so long?

anonymous — Mon, 07 Mar 2016 19:26:50 GMT

Damn... I've just wasted like 10 hours trying to feed it default value. And I'm complete newbie in C++ and it's templates. Please, please, fix it.. :)