1.1 --- /dev/null Thu Jan 01 00:00:00 1970 +0000
1.2 +++ b/mfbt/Maybe.h Wed Dec 31 06:09:35 2014 +0100
1.3 @@ -0,0 +1,163 @@
1.4 +/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
1.5 +/* vim: set ts=8 sts=2 et sw=2 tw=80: */
1.6 +/* This Source Code Form is subject to the terms of the Mozilla Public
1.7 + * License, v. 2.0. If a copy of the MPL was not distributed with this
1.8 + * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
1.9 +
1.10 +/* A class for lazily constructing an object without sticking it on the heap. */
1.11 +
1.12 +#ifndef mozilla_Maybe_h
1.13 +#define mozilla_Maybe_h
1.14 +
1.15 +#include "mozilla/Alignment.h"
1.16 +#include "mozilla/Assertions.h"
1.17 +
1.18 +// For placement new
1.19 +#include <new>
1.20 +
1.21 +namespace mozilla {
1.22 +
1.23 +/*
1.24 + * Small utility for lazily constructing objects without using dynamic storage.
1.25 + * When a Maybe<T> is constructed, it is |empty()|, i.e., no value of T has
1.26 + * been constructed and no T destructor will be called when the Maybe<T> is
1.27 + * destroyed. Upon calling |construct|, a T object will be constructed with the
1.28 + * given arguments and that object will be destroyed when the owning Maybe<T>
1.29 + * is destroyed.
1.30 + *
1.31 + * N.B. GCC seems to miss some optimizations with Maybe and may generate extra
1.32 + * branches/loads/stores. Use with caution on hot paths.
1.33 + */
1.34 +template<class T>
1.35 +class Maybe
1.36 +{
1.37 + AlignedStorage2<T> storage;
1.38 + bool constructed;
1.39 +
1.40 + T& asT() { return *storage.addr(); }
1.41 +
1.42 + public:
1.43 + Maybe() { constructed = false; }
1.44 + ~Maybe() { if (constructed) asT().~T(); }
1.45 +
1.46 + bool empty() const { return !constructed; }
1.47 +
1.48 + void construct() {
1.49 + MOZ_ASSERT(!constructed);
1.50 + ::new (storage.addr()) T();
1.51 + constructed = true;
1.52 + }
1.53 +
1.54 + template<class T1>
1.55 + void construct(const T1& t1) {
1.56 + MOZ_ASSERT(!constructed);
1.57 + ::new (storage.addr()) T(t1);
1.58 + constructed = true;
1.59 + }
1.60 +
1.61 + template<class T1, class T2>
1.62 + void construct(const T1& t1, const T2& t2) {
1.63 + MOZ_ASSERT(!constructed);
1.64 + ::new (storage.addr()) T(t1, t2);
1.65 + constructed = true;
1.66 + }
1.67 +
1.68 + template<class T1, class T2, class T3>
1.69 + void construct(const T1& t1, const T2& t2, const T3& t3) {
1.70 + MOZ_ASSERT(!constructed);
1.71 + ::new (storage.addr()) T(t1, t2, t3);
1.72 + constructed = true;
1.73 + }
1.74 +
1.75 + template<class T1, class T2, class T3, class T4>
1.76 + void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4) {
1.77 + MOZ_ASSERT(!constructed);
1.78 + ::new (storage.addr()) T(t1, t2, t3, t4);
1.79 + constructed = true;
1.80 + }
1.81 +
1.82 + template<class T1, class T2, class T3, class T4, class T5>
1.83 + void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4, const T5& t5) {
1.84 + MOZ_ASSERT(!constructed);
1.85 + ::new (storage.addr()) T(t1, t2, t3, t4, t5);
1.86 + constructed = true;
1.87 + }
1.88 +
1.89 + template<class T1, class T2, class T3, class T4, class T5,
1.90 + class T6>
1.91 + void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4, const T5& t5,
1.92 + const T6& t6) {
1.93 + MOZ_ASSERT(!constructed);
1.94 + ::new (storage.addr()) T(t1, t2, t3, t4, t5, t6);
1.95 + constructed = true;
1.96 + }
1.97 +
1.98 + template<class T1, class T2, class T3, class T4, class T5,
1.99 + class T6, class T7>
1.100 + void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4, const T5& t5,
1.101 + const T6& t6, const T7& t7) {
1.102 + MOZ_ASSERT(!constructed);
1.103 + ::new (storage.addr()) T(t1, t2, t3, t4, t5, t6, t7);
1.104 + constructed = true;
1.105 + }
1.106 +
1.107 + template<class T1, class T2, class T3, class T4, class T5,
1.108 + class T6, class T7, class T8>
1.109 + void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4, const T5& t5,
1.110 + const T6& t6, const T7& t7, const T8& t8) {
1.111 + MOZ_ASSERT(!constructed);
1.112 + ::new (storage.addr()) T(t1, t2, t3, t4, t5, t6, t7, t8);
1.113 + constructed = true;
1.114 + }
1.115 +
1.116 + template<class T1, class T2, class T3, class T4, class T5,
1.117 + class T6, class T7, class T8, class T9>
1.118 + void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4, const T5& t5,
1.119 + const T6& t6, const T7& t7, const T8& t8, const T9& t9) {
1.120 + MOZ_ASSERT(!constructed);
1.121 + ::new (storage.addr()) T(t1, t2, t3, t4, t5, t6, t7, t8, t9);
1.122 + constructed = true;
1.123 + }
1.124 +
1.125 + template<class T1, class T2, class T3, class T4, class T5,
1.126 + class T6, class T7, class T8, class T9, class T10>
1.127 + void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4, const T5& t5,
1.128 + const T6& t6, const T7& t7, const T8& t8, const T9& t9, const T10& t10) {
1.129 + MOZ_ASSERT(!constructed);
1.130 + ::new (storage.addr()) T(t1, t2, t3, t4, t5, t6, t7, t8, t9, t10);
1.131 + constructed = true;
1.132 + }
1.133 +
1.134 + T* addr() {
1.135 + MOZ_ASSERT(constructed);
1.136 + return &asT();
1.137 + }
1.138 +
1.139 + T& ref() {
1.140 + MOZ_ASSERT(constructed);
1.141 + return asT();
1.142 + }
1.143 +
1.144 + const T& ref() const {
1.145 + MOZ_ASSERT(constructed);
1.146 + return const_cast<Maybe*>(this)->asT();
1.147 + }
1.148 +
1.149 + void destroy() {
1.150 + ref().~T();
1.151 + constructed = false;
1.152 + }
1.153 +
1.154 + void destroyIfConstructed() {
1.155 + if (!empty())
1.156 + destroy();
1.157 + }
1.158 +
1.159 + private:
1.160 + Maybe(const Maybe& other) MOZ_DELETE;
1.161 + const Maybe& operator=(const Maybe& other) MOZ_DELETE;
1.162 +};
1.163 +
1.164 +} // namespace mozilla
1.165 +
1.166 +#endif /* mozilla_Maybe_h */
