CanBeCalled.h
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1 /*!
2  *
3  *
4  * \brief Template class checking whether for a functor F and Argument U, F(U) can be called.
5  *
6  * \par Implementation is based on
7  * http://www.boost.org/doc/libs/1_52_0/doc/html/proto/appendices.html
8  *
9  *
10  *
11  * \author O. Krause
12  * \date 2013
13  *
14  *
15  * \par Copyright 1995-2017 Shark Development Team
16  *
17  * <BR><HR>
18  * This file is part of Shark.
19  * <http://shark-ml.org/>
20  *
21  * Shark is free software: you can redistribute it and/or modify
22  * it under the terms of the GNU Lesser General Public License as published
23  * by the Free Software Foundation, either version 3 of the License, or
24  * (at your option) any later version.
25  *
26  * Shark is distributed in the hope that it will be useful,
27  * but WITHOUT ANY WARRANTY; without even the implied warranty of
28  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
29  * GNU Lesser General Public License for more details.
30  *
31  * You should have received a copy of the GNU Lesser General Public License
32  * along with Shark. If not, see <http://www.gnu.org/licenses/>.
33  *
34  */
35 
36 #include <type_traits>
37 
38  namespace shark{
39 
40  ///\brief detects whether Functor(Argument) can be called.
41  ///
42  ///Suppose you have a general Functor. It might take several arguments,
43  ///some more suited for your computation than others. In this case one may
44  ///want to check, which input argument may work.
45  ///The exact implementation and inner workings are outlined in
46  ///http://www.boost.org/doc/libs/1_52_0/doc/html/proto/appendices.html
47  ///the trick is to generate a functor F2 from our supplied functor, such that
48  /// F2(arg) is always a valid expression, but at the same time F2(arg) and F(arg)
49  /// have clearly distinguishable return types. There are a few tricks needed for the
50  /// case that F(arg) has return type void.
51  template<class Functor, class Argument>
52  struct CanBeCalled{
53  private:
54  struct dont_care
55  {
56  dont_care(...);
57  };
58  struct private_type
59  {
60  private_type const &operator,(int) const;
61  };
62 
63  typedef char yes_type; // sizeof(yes_type) == 1
64  typedef char (&no_type)[2]; // sizeof(no_type) == 2
65 
66  template<typename T>
67  static no_type is_private_type(T const &);
68 
69  static yes_type is_private_type(private_type const &);
70 
71  template<typename Fun>
72  struct funwrap : Fun
73  {
74  funwrap();
75  typedef private_type const &(*pointer_to_function)(dont_care);
76  operator pointer_to_function() const;
77  };
78 
79  static funwrap<Functor> & m_fun;
80  static Argument & m_arg;
81  public:
82  static bool const value = (
83  sizeof(no_type) == sizeof(is_private_type( (m_fun(m_arg), 0) ))
84  );
85 
86  typedef std::integral_constant<bool, value> type;
87  };
88 
89  template<class R, class T, class Argument>
90  struct CanBeCalled<R(T), Argument>{
91  typedef std::integral_constant<bool, std::is_convertible<T, Argument>::value > type;
92  static bool const value = type::value;
93  };
94  template<class R, class T, class Argument>
95  struct CanBeCalled<R(*)(T), Argument>{
96  typedef std::integral_constant<bool, std::is_convertible<T, Argument>::value > type;
97  static bool const value = type::value;
98  };
99 }