Re: [Boost-users] [mpl] Trivial metafunctions and placeholders don't mix ?

AMDG jean-louis.a.leroy@fortis.com wrote:

I'm trying to use a pair<> to pass values inside a fold<> :

template<typename Seq> struct remove_duplicates // even if not consecutive; also keep order { typedef typename mpl::second< mpl::fold< Seq, mpl::pair< mpl::set<>, mpl::vector<> >, typedef typename mpl::if_<

typedef?

mpl::has_keympl::_1, mpl::_2>, mpl::_1, mpl::pair< typename mpl::insert< mpl::firstmpl::_1, void, mpl::_2 >::type, // insert

Leave off the ::type. It forces insert to be evaluated immediately, but you want insert to be evaluated lazily.

mpl::push_back< mpl::secondmpl::_1, _2 >::type

Same thing here.

> // pair > >::type

And yet again.

...

type; };

It looks like first<> and second<> don't work on placeholders. My compiler - Visual Studio 2008 - complains that "'first' : is not a member of 'boost::mpl::arg<1>". When I look at the definition of first<> I see a "BOOST_MPL_AUX_LAMBDA_SUPPORT(1,first,(P))" that should take care of placeholders I guess. But when I look at the definition of the macro, alas :

#if !defined(BOOST_MPL_CFG_NO_FULL_LAMBDA_SUPPORT)

# define BOOST_MPL_AUX_LAMBDA_SUPPORT_SPEC(i, name, params) /**/ # define BOOST_MPL_AUX_LAMBDA_SUPPORT(i,name,params) /**/

What's the story here? My compiler is not good enough? Could be...

Visual Studio 2008 doesn't have problems with lambda.

While we're at it, should I put a BOOST_MPL_AUX_LAMBDA_SUPPORT(1,remove_duplicates,(P)) at the bottom of remove_duplicates<> ? Would it make my metafunction work with lambda placeholders?

It should have no effect. In Christ, Steven Watanabe