269 lines
8.9 KiB
C++
269 lines
8.9 KiB
C++
/*=============================================================================
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Copyright (c) 2006-2007 Tobias Schwinger
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Use modification and distribution are subject to the Boost Software
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License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt).
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Problem:
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How to "do the Bind?"
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This recipe shows how to implement a function binder, similar to
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Boost.Bind based on the Functional module of Fusion.
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It works as follows:
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'bind' is a global, stateless function object. It is implemented in
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fused form (fused_binder) and transformed into a variadic function
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object. When called, 'bind' returns another function object, which
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holds the arguments of the call to 'bind'. It is, again, implemented
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in fused form (fused_bound_function) and transformed into unfused
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form.
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==============================================================================*/
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#include <boost/fusion/functional/invocation/invoke.hpp>
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#include <boost/fusion/functional/adapter/unfused.hpp>
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#include <boost/fusion/support/deduce_sequence.hpp>
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#include <boost/fusion/sequence/intrinsic/at.hpp>
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#include <boost/fusion/sequence/intrinsic/front.hpp>
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#include <boost/fusion/sequence/intrinsic/size.hpp>
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#include <boost/fusion/algorithm/transformation/transform.hpp>
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#include <boost/fusion/algorithm/transformation/pop_front.hpp>
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#include <boost/fusion/algorithm/iteration/fold.hpp>
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#include <boost/fusion/view/filter_view.hpp>
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#include <boost/functional/forward_adapter.hpp>
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#include <boost/functional/lightweight_forward_adapter.hpp>
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#include <boost/type_traits/remove_reference.hpp>
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#include <boost/mpl/eval_if.hpp>
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#include <boost/mpl/identity.hpp>
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#include <boost/mpl/int.hpp>
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#include <boost/mpl/max.hpp>
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#include <boost/mpl/next.hpp>
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#include <boost/ref.hpp>
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#include <iostream>
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#include <typeinfo>
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namespace impl
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{
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namespace fusion = boost::fusion;
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namespace traits = boost::fusion::traits;
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namespace result_of = boost::fusion::result_of;
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namespace mpl = boost::mpl;
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using mpl::placeholders::_;
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// Placeholders (we inherit from mpl::int_, so we can use placeholders
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// as indices for fusion::at, later)
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template <int I> struct placeholder : mpl::int_<I> { };
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// A traits class to find out whether T is a placeholeder
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template <typename T> struct is_placeholder : mpl::false_ { };
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template <int I> struct is_placeholder< placeholder<I> > : mpl::true_ { };
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template <int I> struct is_placeholder< placeholder<I> & > : mpl::true_ { };
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template <int I> struct is_placeholder< placeholder<I> const > : mpl::true_ { };
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template <int I> struct is_placeholder< placeholder<I> const & > : mpl::true_ { };
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// This class template provides a Polymorphic Function Object to be used
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// with fusion::transform. It is applied to the sequence of arguments that
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// describes the binding and holds a reference to the sequence of arguments
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// from the final call.
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template<class FinalArgs> struct argument_transform
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{
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FinalArgs const & ref_final_args;
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public:
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explicit argument_transform(FinalArgs const & final_args)
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: ref_final_args(final_args)
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{ }
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// A placeholder? Replace it with an argument from the final call...
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template <int Index>
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inline typename result_of::at_c<FinalArgs const, Index>::type
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operator()(placeholder<Index> const &) const
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{
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return fusion::at_c<Index>(this->ref_final_args);
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}
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// ...just return the bound argument, otherwise.
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template <typename T> inline T & operator()(T & bound) const
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{
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return bound;
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}
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template <typename Signature>
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struct result;
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template <class Self, typename T>
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struct result< Self (T) >
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: mpl::eval_if< is_placeholder<T>,
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result_of::at<FinalArgs,typename boost::remove_reference<T>::type>,
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mpl::identity<T>
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>
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{ };
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};
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// Fused implementation of the bound function, the function object
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// returned by bind
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template <class BindArgs> class fused_bound_function
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{
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// Transform arguments to be held by value
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typedef typename traits::deduce_sequence<BindArgs>::type bound_args;
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bound_args fsq_bind_args;
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public:
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fused_bound_function(BindArgs const & bind_args)
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: fsq_bind_args(bind_args)
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{ }
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template <typename Signature>
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struct result;
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template <class FinalArgs>
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struct result_impl
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: result_of::invoke< typename result_of::front<bound_args>::type,
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typename result_of::transform<
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typename result_of::pop_front<bound_args>::type,
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argument_transform<FinalArgs> const
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>::type
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>
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{ };
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template <class Self, class FinalArgs>
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struct result< Self (FinalArgs) >
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: result_impl< typename boost::remove_reference<FinalArgs>::type >
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{ };
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template <class FinalArgs>
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inline typename result_impl<FinalArgs>::type
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operator()(FinalArgs const & final_args) const
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{
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return fusion::invoke( fusion::front(this->fsq_bind_args),
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fusion::transform( fusion::pop_front(this->fsq_bind_args),
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argument_transform<FinalArgs>(final_args) ) );
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}
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// Could add a non-const variant - omitted for readability
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};
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// Find the number of placeholders in use
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struct n_placeholders
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{
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struct fold_op
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{
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template <typename Sig> struct result;
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template <class S, class A, class B> struct result< S(A &,B &) >
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: mpl::max<A,B> { };
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};
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struct filter_pred
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{
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template <class X> struct apply : is_placeholder<X> { };
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};
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template <typename Seq>
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struct apply
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: mpl::next< typename result_of::fold<
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fusion::filter_view<Seq,filter_pred>, mpl::int_<-1>, fold_op
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>::type>::type
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{ };
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};
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// Fused implementation of the 'bind' function
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struct fused_binder
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{
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template <class Signature>
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struct result;
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template <class BindArgs,
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int Placeholders = n_placeholders::apply<BindArgs>::value>
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struct result_impl
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{
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typedef boost::forward_adapter<fusion::unfused<
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fused_bound_function<BindArgs>,!Placeholders>,Placeholders> type;
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};
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template <class Self, class BindArgs>
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struct result< Self (BindArgs) >
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: result_impl< typename boost::remove_reference<BindArgs>::type >
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{ };
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template <class BindArgs>
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inline typename result_impl< BindArgs >::type
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operator()(BindArgs & bind_args) const
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{
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return typename result< void(BindArgs) >::type(
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fusion::unfused< fused_bound_function<BindArgs>,
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! n_placeholders::apply<BindArgs>::value >(bind_args) );
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}
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};
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// The binder's unfused type. We use lightweght_forward_adapter to make
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// that thing more similar to Boost.Bind. Because of that we have to use
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// Boost.Ref (below in the sample code)
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typedef boost::lightweight_forward_adapter< fusion::unfused<fused_binder> > binder;
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}
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// Placeholder globals
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impl::placeholder<0> const _1_ = impl::placeholder<0>();
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impl::placeholder<1> const _2_ = impl::placeholder<1>();
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impl::placeholder<2> const _3_ = impl::placeholder<2>();
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impl::placeholder<3> const _4_ = impl::placeholder<3>();
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// The bind function is a global, too
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impl::binder const bind = impl::binder();
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// OK, let's try it out:
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struct func
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{
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typedef int result_type;
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inline int operator()() const
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{
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std::cout << "operator()" << std::endl;
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return 0;
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}
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template <typename A>
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inline int operator()(A const & a) const
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{
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std::cout << "operator()(A const & a)" << std::endl;
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std::cout << " a = " << a << " A = " << typeid(A).name() << std::endl;
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return 1;
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}
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template <typename A, typename B>
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inline int operator()(A const & a, B & b) const
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{
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std::cout << "operator()(A const & a, B & b)" << std::endl;
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std::cout << " a = " << a << " A = " << typeid(A).name() << std::endl;
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std::cout << " b = " << b << " B = " << typeid(B).name() << std::endl;
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return 2;
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}
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};
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int main()
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{
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func f;
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int value = 42;
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using boost::ref;
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int errors = 0;
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errors += !( bind(f)() == 0);
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errors += !( bind(f,"Hi")() == 1);
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errors += !( bind(f,_1_)("there.") == 1);
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errors += !( bind(f,"The answer is",_1_)(12) == 2);
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errors += !( bind(f,_1_,ref(value))("Really?") == 2);
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errors += !( bind(f,_1_,_2_)("Dunno. If there is an answer, it's",value) == 2);
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return !! errors;
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}
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