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Lazy evaluation in lambdas

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[SVN r18318]
This commit is contained in:
Dave Abrahams
2003-04-27 11:44:15 +00:00
parent a17353cd8f
commit 4b05246d3d
1 changed files with 78 additions and 0 deletions
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78
include/boost/mpl/if.hpp
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@@ -73,6 +73,84 @@ struct if_
BOOST_MPL_AUX_LAMBDA_SUPPORT(3,if_,(C,T1,T2)) BOOST_MPL_AUX_LAMBDA_SUPPORT(3,if_,(C,T1,T2))
}; };
#ifndef BOOST_MPL_NO_FULL_LAMBDA_SUPPORT
// Aleksey, check it out: lazy if_ evaluation in lambdas!
// I think this doesn't handle the case of
//
// _1<foo<_2>, bar<_2>, baz<_2> >
//
// (or however it is that you express that... when the ordinary bind3
// computes the function based on the actual arguments). That leads me
// to think that some kind of true currying might be a better
// approach, e.g.:
//
//
// boost::mpl::bind3<
// boost::mpl::quote3<boost::mpl::if_>
// , boost::mpl::bind1<boost::mpl::quote1<boost::is_reference>, boost::mpl::arg<1> >
// , boost::mpl::arg<1>
// , boost::mpl::bind1<boost::mpl::quote1<add_ptr>, boost::mpl::arg<1> >
// >::apply<...>
//
// becomes:
//
// boost::mpl::bind<
// boost::mpl::quote3<boost::mpl::if_>
// >::bind<
// , boost::mpl::bind1<boost::mpl::quote1<boost::is_reference>,
// boost::mpl::arg<1> >
// >::bind<
// boost::mpl::arg<1>
// >::bind<
// boost::mpl::bind1<boost::mpl::quote1<add_ptr>, boost::mpl::arg<1> >
// >::apply<...>
//
// so that after the 2nd bind we have a different function depending
// on the result of is_reference.
template <class T1, class T2, class T3, class T4> struct bind3;
template <template <class T1, class T2, class T3> class> struct quote3;
template<
typename T1, typename T2, typename T3
>
struct bind3<quote3<if_>, T1, T2, T3>
{
template<
typename U1 = void_, typename U2 = void_, typename U3 = void_
, typename U4 = void_, typename U5 = void_
>
struct apply
{
private:
typedef aux::replace_unnamed_arg<quote3<if_>, mpl::arg< 1> > r0;
typedef typename r0::type a0;
typedef typename r0::next_arg n1;
typedef aux::replace_unnamed_arg< T1,n1 > r1;
typedef typename r1::type a1;
typedef typename r1::next_arg n2;
typedef typename aux::resolve_bind_arg< a1,U1,U2,U3,U4,U5 >::type t1;
typedef aux::replace_unnamed_arg< T2,n2 > r2;
typedef typename r2::type a2;
typedef typename r2::next_arg n3;
typedef typename aux::resolve_bind_arg< a2,U1,U2,U3,U4,U5 > f2;
typedef aux::replace_unnamed_arg< T3,n3 > r3;
typedef typename r3::type a3;
typedef typename r3::next_arg n4;
typedef typename aux::resolve_bind_arg< a3,U1,U2,U3,U4,U5 > f3;
typedef typename if_<t1,f2,f3>::type f_;
public:
typedef typename f_::type type;
};
};
#endif
#elif defined(BOOST_MSVC) && (BOOST_MSVC <= 1300) #elif defined(BOOST_MSVC) && (BOOST_MSVC <= 1300)
// MSVC6.5-specific version // MSVC6.5-specific version