mpl::at documentation out of sync with code

[manfred.doudar@…]

Documentation of mpl::at indicates that it can take either 2 or 3 template parameters, but in fact only takes just 2. The documentation indicates a third template parameter to forward a default if lookup fails to find a match. This option does not exist.

Code or Documentation should be amended. If there will be no 3 template parameter option, the following would be nice to have in the documentation, as how to achieve a default when no match is found:

typename boost::mpl::eval_if<

typename boost::mpl::has_key< Map, Key >::type

, boost::mpl::at< Map, Key > , boost::mpl::identity< Default >

::type

[dsaritz@…]

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]

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]

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

at_def? at_or?

[jeffrey.hellrung]

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]

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

[wmamrak@…]

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

[anonymous]

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.. :)