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1 | // Copyright Louis Dionne 2013-2016 |
2 | // Distributed under the Boost Software License, Version 1.0. | |
3 | // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) | |
4 | ||
5 | #include <boost/hana/assert.hpp> | |
6 | #include <boost/hana/core/tag_of.hpp> | |
7 | #include <boost/hana/integral_constant.hpp> | |
8 | #include <boost/hana/minus.hpp> | |
9 | #include <boost/hana/not_equal.hpp> | |
10 | #include <boost/hana/tuple.hpp> | |
11 | ||
12 | #include <cstddef> | |
13 | #include <iostream> | |
14 | #include <sstream> | |
15 | namespace hana = boost::hana; | |
16 | ||
17 | ||
18 | //! [setup] | |
19 | template <typename Tag> | |
20 | struct print_impl { | |
21 | template <typename X> | |
22 | static void apply(std::ostream&, X const&) { | |
23 | // possibly some default implementation | |
24 | } | |
25 | }; | |
26 | ||
27 | template <typename X> | |
28 | void print(std::ostream& os, X x) { | |
29 | using Tag = typename hana::tag_of<X>::type; | |
30 | print_impl<Tag>::apply(os, x); | |
31 | } | |
32 | //! [setup] | |
33 | ||
34 | //! [vector] | |
35 | struct vector_tag; | |
36 | ||
37 | struct vector0 { | |
38 | using hana_tag = vector_tag; | |
39 | static constexpr std::size_t size = 0; | |
40 | }; | |
41 | ||
42 | template <typename T1> | |
43 | struct vector1 { | |
44 | T1 t1; | |
45 | using hana_tag = vector_tag; | |
46 | static constexpr std::size_t size = 1; | |
47 | ||
48 | template <typename Index> | |
49 | auto const& operator[](Index i) const { | |
50 | static_assert(i == 0u, "index out of bounds"); | |
51 | return t1; | |
52 | } | |
53 | }; | |
54 | ||
55 | template <typename T1, typename T2> | |
56 | struct vector2 { | |
57 | T1 t1; T2 t2; | |
58 | using hana_tag = vector_tag; | |
59 | static constexpr std::size_t size = 2; | |
60 | ||
61 | // Using Hana as a backend to simplify the example. | |
62 | template <typename Index> | |
63 | auto const& operator[](Index i) const { | |
64 | return *hana::make_tuple(&t1, &t2)[i]; | |
65 | } | |
66 | }; | |
67 | ||
68 | // and so on... | |
69 | //! [vector] | |
70 | ||
71 | //! [customize] | |
72 | template <> | |
73 | struct print_impl<vector_tag> { | |
74 | template <typename vectorN> | |
75 | static void apply(std::ostream& os, vectorN xs) { | |
76 | constexpr auto N = hana::size_c<vectorN::size>; | |
77 | ||
78 | os << "["; | |
79 | N.times.with_index([&](auto i) { | |
80 | os << xs[i]; | |
81 | if (i != N - hana::size_c<1>) os << ", "; | |
82 | }); | |
83 | os << "]"; | |
84 | } | |
85 | }; | |
86 | //! [customize] | |
87 | ||
88 | #if 0 | |
89 | //! [customize-when] | |
90 | template <typename Tag> | |
91 | struct print_impl<Tag, hana::when<Tag represents some kind of sequence>> { | |
92 | template <typename Seq> | |
93 | static void apply(std::ostream& os, Seq xs) { | |
94 | // Some implementation for any sequence | |
95 | } | |
96 | }; | |
97 | //! [customize-when] | |
98 | #endif | |
99 | ||
100 | int main() { | |
101 | { | |
102 | std::stringstream ss; | |
103 | vector0 v0; | |
104 | print(ss, v0); | |
105 | BOOST_HANA_RUNTIME_CHECK(ss.str() == "[]"); | |
106 | } | |
107 | ||
108 | { | |
109 | std::stringstream ss; | |
110 | vector1<int> v1{1}; | |
111 | print(ss, v1); | |
112 | BOOST_HANA_RUNTIME_CHECK(ss.str() == "[1]"); | |
113 | } | |
114 | ||
115 | { | |
116 | std::stringstream ss; | |
117 | vector2<int, char> v2{1, 'x'}; | |
118 | print(ss, v2); | |
119 | BOOST_HANA_RUNTIME_CHECK(ss.str() == "[1, x]"); | |
120 | } | |
121 | } | |
122 | ||
123 | namespace old_way { | |
124 | //! [old_way] | |
125 | void print(std::ostream& os, vector0) | |
126 | { os << "[]"; } | |
127 | ||
128 | template <typename T1> | |
129 | void print(std::ostream& os, vector1<T1> v) | |
130 | { os << "[" << v.t1 << "]"; } | |
131 | ||
132 | template <typename T1, typename T2> | |
133 | void print(std::ostream& os, vector2<T1, T2> v) | |
134 | { os << "[" << v.t1 << ", " << v.t2 << "]"; } | |
135 | ||
136 | // and so on... | |
137 | //! [old_way] | |
138 | } | |
139 | ||
140 | namespace preconditions { | |
141 | //! [preconditions] | |
142 | template <typename X> | |
143 | void print(std::ostream& os, X x) { | |
144 | // **** check some precondition **** | |
145 | ||
146 | // The precondition only has to be checked here; implementations | |
147 | // can assume their arguments to always be sane. | |
148 | ||
149 | using Tag = typename hana::tag_of<X>::type; | |
150 | print_impl<Tag>::apply(os, x); | |
151 | } | |
152 | //! [preconditions] | |
153 | } | |
154 | ||
155 | namespace function_objects { | |
156 | //! [function_objects] | |
157 | // Defining a function object is only needed once and implementations do not | |
158 | // have to worry about static initialization and other painful tricks. | |
159 | struct print_t { | |
160 | template <typename X> | |
161 | void operator()(std::ostream& os, X x) const { | |
162 | using Tag = typename hana::tag_of<X>::type; | |
163 | print_impl<Tag>::apply(os, x); | |
164 | } | |
165 | }; | |
166 | constexpr print_t print{}; | |
167 | //! [function_objects] | |
168 | ||
169 | static_assert(sizeof(print) || true, "remove unused variable print warning"); | |
170 | } |