1 /*
2  * Copyright (C) 2013-2017 Max Kellermann <max.kellermann@gmail.com>
3  *
4  * Redistribution and use in source and binary forms, with or without
5  * modification, are permitted provided that the following conditions
6  * are met:
7  *
8  * - Redistributions of source code must retain the above copyright
9  * notice, this list of conditions and the following disclaimer.
10  *
11  * - Redistributions in binary form must reproduce the above copyright
12  * notice, this list of conditions and the following disclaimer in the
13  * documentation and/or other materials provided with the
14  * distribution.
15  *
16  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
17  * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
18  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
19  * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
20  * FOUNDATION OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
21  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
22  * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
23  * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
24  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
25  * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
26  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
27  * OF THE POSSIBILITY OF SUCH DAMAGE.
28  */
29 
30 #ifndef MANUAL_HXX
31 #define MANUAL_HXX
32 
33 #include <cassert>
34 #include <new>
35 #include <utility>
36 
37 #if defined(__GNUC__) || defined(__clang__)
38 #pragma GCC diagnostic push
39 #pragma GCC diagnostic ignored "-Wstrict-aliasing"
40 #endif
41 
42 /**
43  * Container for an object that gets constructed and destructed
44  * manually.  The object is constructed in-place, and therefore
45  * without allocation overhead.  It can be constructed and destructed
46  * repeatedly.
47  */
48 template<class T>
49 class Manual {
50 	alignas(T)
51 	char data[sizeof(T)];
52 
53 #ifndef NDEBUG
54 	bool initialized = false;
55 #endif
56 
57 public:
58 #ifndef NDEBUG
~Manual()59 	~Manual() {
60 		assert(!initialized);
61 	}
62 #endif
63 
64 	/**
65 	 * Cast a value reference to the containing Manual instance.
66 	 */
Cast(T & value)67 	static constexpr Manual<T> &Cast(T &value) {
68 		return reinterpret_cast<Manual<T> &>(value);
69 	}
70 
71 	template<typename... Args>
Construct(Args &&...args)72 	void Construct(Args&&... args) {
73 		assert(!initialized);
74 
75 		void *p = data;
76 		new(p) T(std::forward<Args>(args)...);
77 
78 #ifndef NDEBUG
79 		initialized = true;
80 #endif
81 	}
82 
Destruct()83 	void Destruct() {
84 		assert(initialized);
85 
86 		T &t = Get();
87 		t.T::~T();
88 
89 #ifndef NDEBUG
90 		initialized = false;
91 #endif
92 	}
93 
Get()94 	T &Get() {
95 		assert(initialized);
96 
97 		void *p = static_cast<void *>(data);
98 		return *static_cast<T *>(p);
99 	}
100 
Get() const101 	const T &Get() const {
102 		assert(initialized);
103 
104 		const void *p = static_cast<const void *>(data);
105 		return *static_cast<const T *>(p);
106 	}
107 
operator T&()108 	operator T &() {
109 		return Get();
110 	}
111 
operator const T&() const112 	operator const T &() const {
113 		return Get();
114 	}
115 
operator ->()116 	T *operator->() {
117 		return &Get();
118 	}
119 
operator ->() const120 	const T *operator->() const {
121 		return &Get();
122 	}
123 };
124 
125 #if defined(__GNUC__) || defined(__clang__)
126 #pragma GCC diagnostic pop
127 #endif
128 
129 #endif
130