On Sat, Feb 23, 2013 at 3:56 PM, Jeffrey Lee Hellrung, Jr. < jeffrey.hellrung@gmail.com> wrote:
On Sat, Feb 23, 2013 at 2:32 AM, Niitsuma Hirotaka < hirotaka.niitsuma@gmail.com> wrote:
I would like to use mpl::lambda in nest.
In this case works fine. --------- typedef boost::mpl::lambda<boost::mpl::max<boost::mpl::_1,boost::mpl::_2>
my_max; typedef boost::mpl::fold< boost::mpl::vector_c<int,1,7,0,-2,5,-1> , boost::mpl::int_<6> //, boost::mpl::max<boost::mpl::_1,boost::mpl::_2> , my_max >::type r2;
BOOST_MPL_ASSERT(( boost::is_same< r2, boost::mpl::integral_c<int, 7> > )); ---------
However
---------
typedef boost::mpl::lambda< boost::mpl::fold< boost::mpl::_1 ,boost::mpl::front<boost::mpl::_1> ,my_max >::type >::type my_max_types;
Also: remove the ::type from the fold<> expression. Works for me after that (and the fix with adding the ::type on the definition of my_max). I would think you would've gotten a compiler error at this point but, meh. typedef my_max_types::apply<boost::mpl::vector_c<int,1,7,0,-2,5,-1>
::type r3;
BOOST_MPL_ASSERT(( boost::is_same< r3, boost::mpl::integral_c<int, 7> > )); ///error => becomes boost::mpl::integral_c<int, 1> ---------
How to define such nest lambda in mpl?
Uh, I see a red flag in that you don't grab the nested type typedef from your mpl::lambda invocations, i.e., I think you should have
typedef mpl::lambda< mpl::max< mpl::_1, mpl::_2 > >::type my_max;
Incidentally, you could also do
typedef mpl::quote2< mpl::max > my_max
(Well, assuming mpl::max doesn't have some hidden default parameters...)
Without grabbing the type typedef in your mpl::lambda invocation, your use of mpl::_1 and mpl::_2 are "exposed" to premature substitution.
- Jeff