adds example implementation of bind

[SVN r36961]

example/cookbook/do_the_bind.cpp

@@ -0,0 +1,224 @@
+/*=============================================================================
+    Copyright (c) 2006-2007 Tobias Schwinger
+  
+    Use modification and distribution are subject to the Boost Software 
+    License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
+    http://www.boost.org/LICENSE_1_0.txt).
+
+    Problem:
+
+    How to "do the Bind?"
+
+    This recipe shows how to implement a function binder, similar to 
+    Boost.Bind based on the Functional module of Fusion.
+
+    It works as follows:
+
+    'bind' is a global, stateless function object. It is implemented in
+    fused form (fused_binder) and transformed into a variadic function 
+    object. When called, 'bind' returns another function object, which
+    holds the arguments of the call to 'bind'. It is, again, implemented 
+    in fused form (fused_bound_function) and transformed into unfused
+    form. 
+==============================================================================*/
+
+#include <boost/fusion/functional/invoke.hpp>
+#include <boost/fusion/functional/unfused_generic.hpp>
+#include <boost/fusion/functional/unfused_rvalue_args.hpp>
+#include <boost/fusion/functional/storable_arguments.hpp>
+
+#include <boost/fusion/sequence/intrinsic/at.hpp>
+#include <boost/fusion/sequence/intrinsic/mpl.hpp>
+#include <boost/fusion/sequence/intrinsic/front.hpp>
+#include <boost/fusion/sequence/intrinsic/empty.hpp>
+#include <boost/fusion/algorithm/transformation/transform.hpp>
+#include <boost/fusion/algorithm/transformation/pop_front.hpp>
+
+#include <boost/type_traits/remove_reference.hpp>
+
+#include <boost/mpl/eval_if.hpp>
+#include <boost/mpl/identity.hpp>
+#include <boost/mpl/bool.hpp>
+#include <boost/mpl/int.hpp>
+#include <boost/mpl/count_if.hpp>
+
+#include <boost/ref.hpp>
+#include <iostream>
+#include <typeinfo>
+
+namespace impl
+{
+    namespace fusion = boost::fusion;
+    namespace result_of = boost::fusion::result_of;
+    namespace mpl = boost::mpl;
+    using mpl::placeholders::_;
+
+    // Placeholders (we inherit from mpl::int_, so we can use placeholders
+    // as indices for fusion::at, later) 
+    template <int I> struct placeholder : mpl::int_<I> { };
+
+    // A traits class to find out whether T is a placeholeder
+    template <typename T> struct is_placeholder              : mpl::false_  { };
+    template <int I> struct is_placeholder< placeholder<I> > : mpl::true_   { };
+    template <int I> struct is_placeholder< placeholder<I> & > : mpl::true_   { };
+    template <int I> struct is_placeholder< placeholder<I> const   > : mpl::true_   { };
+    template <int I> struct is_placeholder< placeholder<I> const & > : mpl::true_   { };
+
+    // This class template provides a Polymorphic Function Object to be used
+    // with fusion::transform. It is applied to the sequence of arguments that
+    // describes the binding and holds a reference to the sequence of arguments 
+    // from the final call. 
+    template<class FinalArgs> struct argument_transform
+    {
+        FinalArgs const & ref_final_args;
+    public:
+
+        explicit argument_transform(FinalArgs const & final_args)
+            : ref_final_args(final_args)
+        { }
+
+        // A placeholder? Replace it with an argument from the final call...
+        template <int Index>
+        inline typename result_of::at_c<FinalArgs const, Index>::type
+        operator()(placeholder<Index> const &) const
+        {
+            return fusion::at_c<Index>(this->ref_final_args);
+        }
+        // ...just return the bound argument, otherwise.
+        template <typename T> inline T & operator()(T & bound) const
+        {
+            return bound;
+        }
+
+        template <typename T>
+        struct result
+            : mpl::eval_if< is_placeholder<T>, 
+                result_of::at<FinalArgs,typename boost::remove_reference<T>::type>,
+                mpl::identity<T>
+            >
+        { };
+    };
+
+    // Fused implementation of the bound function, the function object 
+    // returned by bind
+    template <class BindArgs> class fused_bound_function 
+    {
+        BindArgs fsq_bind_args;
+    public:
+
+        fused_bound_function(BindArgs const & bind_args)
+          : fsq_bind_args(bind_args)
+        { }
+
+        template <class FinalArgs, bool Enable =
+        // Disable for empty sequences if we expect arguments 
+            ! (result_of::empty<FinalArgs>::value &&
+                !! mpl::count_if<BindArgs,is_placeholder<_> >::value) > 
+        struct result
+        { };
+
+        template <class FinalArgs>
+        struct result<FinalArgs,true>
+            : result_of::invoke< typename result_of::front<BindArgs>::type,
+                typename result_of::transform<
+                    typename result_of::pop_front<BindArgs>::type,
+                    argument_transform<FinalArgs> const 
+                >::type
+            >
+        { }; 
+
+        template <class FinalArgs>
+        inline typename result<FinalArgs>::type 
+        operator()(FinalArgs const & final_args) const
+        {
+            return fusion::invoke( fusion::front(this->fsq_bind_args),
+                fusion::transform( fusion::pop_front(this->fsq_bind_args),
+                    argument_transform<FinalArgs>(final_args) ) );
+        }
+        // Could add a non-const variant - omitted for readability
+
+    };
+
+    // Fused implementation of the 'bind' function
+    struct fused_binder
+    {
+        template <class BindArgs>
+        struct result
+        {
+            // We have to transform the arguments so they are held by-value
+            // in the returned function. 
+            typedef fusion::unfused_generic< fused_bound_function<
+                typename fusion::storable_arguments<BindArgs>::type > > type;
+        };
+
+        template <class BindArgs>
+        inline typename result<BindArgs>::type 
+        operator()(BindArgs & bind_args) const
+        {
+            return typename result<BindArgs>::type(bind_args);
+        }
+    };
+
+    // The binder's unfused type. We use unfused_rvalue_args to make that
+    // thing more similar to Boost.Bind. Because of that we have to use 
+    // Boost.Ref (below in the sample code)
+    typedef fusion::unfused_rvalue_args<fused_binder> binder;
+}
+
+// Placeholder globals
+impl::placeholder<0> const _1_ = impl::placeholder<0>();
+impl::placeholder<1> const _2_ = impl::placeholder<1>();
+impl::placeholder<2> const _3_ = impl::placeholder<2>();
+impl::placeholder<3> const _4_ = impl::placeholder<3>();
+
+// The bind function is a global, too
+impl::binder const bind = impl::binder();
+
+
+// OK, let's try it out:
+
+struct func
+{
+    typedef int result_type;
+
+    inline int operator()() const
+    {
+        std::cout << "operator()" << std::endl;
+        return 0;
+    }
+
+    template <typename A> 
+    inline int operator()(A const & a) const
+    {
+        std::cout << "operator()(A const & a)" << std::endl;
+        std::cout << "  a = " << a << "  A = " << typeid(A).name() << std::endl;
+        return 1;
+    }
+
+    template <typename A, typename B> 
+    inline int operator()(A const & a, B & b) const
+    {
+        std::cout << "operator()(A const & a, B & b)" << std::endl;
+        std::cout << "  a = " << a << "  A = " << typeid(A).name() << std::endl;
+        std::cout << "  b = " << b << "  B = " << typeid(B).name() << std::endl;
+        return 2;
+    }
+};
+
+int main()
+{
+    func f;
+    int value = 42;
+    using boost::ref;
+
+    int errors = 0;
+    errors += !( bind(f)() == 0);
+    errors += !( bind(f,"Hi")() == 1);
+    errors += !( bind(f,_1_)("there.") == 1);
+    errors += !( bind(f,"The answer is",_1_)(value) == 2);
+    errors += !( bind(f,_1_,ref(value))("Really?") == 2);
+    errors += !( bind(f,_1_,_2_)("Dunno. If there is an answer, it's",value) == 2);
+
+    return !! errors;
+}
+