1/*
2Copyright 2018 Glen Joseph Fernandes
3(glenjofe@gmail.com)
4
5Distributed under the Boost Software License, Version 1.0.
6(http://www.boost.org/LICENSE_1_0.txt)
7*/
8#ifndef BOOST_CORE_EMPTY_VALUE_HPP
9#define BOOST_CORE_EMPTY_VALUE_HPP
10
11#include <boost/config.hpp>
12#if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES)
13#include <utility>
14#endif
15
16#if defined(BOOST_GCC_VERSION) && (BOOST_GCC_VERSION >= 40700)
17#define BOOST_DETAIL_EMPTY_VALUE_BASE
18#elif defined(BOOST_INTEL) && defined(_MSC_VER) && (_MSC_VER >= 1800)
19#define BOOST_DETAIL_EMPTY_VALUE_BASE
20#elif defined(BOOST_MSVC) && (BOOST_MSVC >= 1800)
21#define BOOST_DETAIL_EMPTY_VALUE_BASE
22#elif defined(BOOST_CLANG) && !defined(__CUDACC__)
23#if __has_feature(is_empty) && __has_feature(is_final)
24#define BOOST_DETAIL_EMPTY_VALUE_BASE
25#endif
26#endif
27
28namespace boost {
29
30template<class T>
31struct use_empty_value_base {
32 enum {
33#if defined(BOOST_DETAIL_EMPTY_VALUE_BASE)
34 value = __is_empty(T) && !__is_final(T)
35#else
36 value = false
37#endif
38 };
39};
40
41struct empty_init_t { };
42
43namespace empty_ {
44
45template<class T, unsigned N = 0,
46 bool E = boost::use_empty_value_base<T>::value>
47class empty_value {
48public:
49 typedef T type;
50
51#if !defined(BOOST_NO_CXX11_DEFAULTED_FUNCTIONS)
52 empty_value() = default;
53#else
54 empty_value() { }
55#endif
56
57 empty_value(boost::empty_init_t)
58 : value_() { }
59
60#if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES)
61#if !defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES)
62 template<class U, class... Args>
63 empty_value(boost::empty_init_t, U&& value, Args&&... args)
64 : value_(std::forward<U>(value), std::forward<Args>(args)...) { }
65#else
66 template<class U>
67 empty_value(boost::empty_init_t, U&& value)
68 : value_(std::forward<U>(value)) { }
69#endif
70#else
71 template<class U>
72 empty_value(boost::empty_init_t, const U& value)
73 : value_(value) { }
74
75 template<class U>
76 empty_value(boost::empty_init_t, U& value)
77 : value_(value) { }
78#endif
79
80 const T& get() const BOOST_NOEXCEPT {
81 return value_;
82 }
83
84 T& get() BOOST_NOEXCEPT {
85 return value_;
86 }
87
88private:
89 T value_;
90};
91
92#if !defined(BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION)
93template<class T, unsigned N>
94class empty_value<T, N, true>
95 : T {
96public:
97 typedef T type;
98
99#if !defined(BOOST_NO_CXX11_DEFAULTED_FUNCTIONS)
100 empty_value() = default;
101#else
102 empty_value() { }
103#endif
104
105 empty_value(boost::empty_init_t)
106 : T() { }
107
108#if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES)
109#if !defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES)
110 template<class U, class... Args>
111 empty_value(boost::empty_init_t, U&& value, Args&&... args)
112 : T(std::forward<U>(value), std::forward<Args>(args)...) { }
113#else
114 template<class U>
115 empty_value(boost::empty_init_t, U&& value)
116 : T(std::forward<U>(value)) { }
117#endif
118#else
119 template<class U>
120 empty_value(boost::empty_init_t, const U& value)
121 : T(value) { }
122
123 template<class U>
124 empty_value(boost::empty_init_t, U& value)
125 : T(value) { }
126#endif
127
128 const T& get() const BOOST_NOEXCEPT {
129 return *this;
130 }
131
132 T& get() BOOST_NOEXCEPT {
133 return *this;
134 }
135};
136#endif
137
138} /* empty_ */
139
140using empty_::empty_value;
141
142BOOST_INLINE_CONSTEXPR empty_init_t empty_init = empty_init_t();
143
144} /* boost */
145
146#endif
147

source code of include/boost/core/empty_value.hpp