gecko-dev/mfbt/Maybe.h

164 строки
5.0 KiB
C
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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
/* A class for lazily constructing an object without sticking it on the heap. */
#ifndef mozilla_Maybe_h
#define mozilla_Maybe_h
#include "mozilla/Alignment.h"
#include "mozilla/Assertions.h"
// For placement new
#include <new>
namespace mozilla {
/*
* Small utility for lazily constructing objects without using dynamic storage.
* When a Maybe<T> is constructed, it is |empty()|, i.e., no value of T has
* been constructed and no T destructor will be called when the Maybe<T> is
* destroyed. Upon calling |construct|, a T object will be constructed with the
* given arguments and that object will be destroyed when the owning Maybe<T>
* is destroyed.
*
* N.B. GCC seems to miss some optimizations with Maybe and may generate extra
* branches/loads/stores. Use with caution on hot paths.
*/
template<class T>
class Maybe
{
AlignedStorage2<T> storage;
bool constructed;
T& asT() { return *storage.addr(); }
public:
Maybe() { constructed = false; }
~Maybe() { if (constructed) asT().~T(); }
bool empty() const { return !constructed; }
void construct() {
MOZ_ASSERT(!constructed);
::new (storage.addr()) T();
constructed = true;
}
template<class T1>
void construct(const T1& t1) {
MOZ_ASSERT(!constructed);
::new (storage.addr()) T(t1);
constructed = true;
}
template<class T1, class T2>
void construct(const T1& t1, const T2& t2) {
MOZ_ASSERT(!constructed);
::new (storage.addr()) T(t1, t2);
constructed = true;
}
template<class T1, class T2, class T3>
void construct(const T1& t1, const T2& t2, const T3& t3) {
MOZ_ASSERT(!constructed);
::new (storage.addr()) T(t1, t2, t3);
constructed = true;
}
template<class T1, class T2, class T3, class T4>
void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4) {
MOZ_ASSERT(!constructed);
::new (storage.addr()) T(t1, t2, t3, t4);
constructed = true;
}
template<class T1, class T2, class T3, class T4, class T5>
void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4, const T5& t5) {
MOZ_ASSERT(!constructed);
::new (storage.addr()) T(t1, t2, t3, t4, t5);
constructed = true;
}
template<class T1, class T2, class T3, class T4, class T5,
class T6>
void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4, const T5& t5,
const T6& t6) {
MOZ_ASSERT(!constructed);
::new (storage.addr()) T(t1, t2, t3, t4, t5, t6);
constructed = true;
}
template<class T1, class T2, class T3, class T4, class T5,
class T6, class T7>
void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4, const T5& t5,
const T6& t6, const T7& t7) {
MOZ_ASSERT(!constructed);
::new (storage.addr()) T(t1, t2, t3, t4, t5, t6, t7);
constructed = true;
}
template<class T1, class T2, class T3, class T4, class T5,
class T6, class T7, class T8>
void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4, const T5& t5,
const T6& t6, const T7& t7, const T8& t8) {
MOZ_ASSERT(!constructed);
::new (storage.addr()) T(t1, t2, t3, t4, t5, t6, t7, t8);
constructed = true;
}
template<class T1, class T2, class T3, class T4, class T5,
class T6, class T7, class T8, class T9>
void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4, const T5& t5,
const T6& t6, const T7& t7, const T8& t8, const T9& t9) {
MOZ_ASSERT(!constructed);
::new (storage.addr()) T(t1, t2, t3, t4, t5, t6, t7, t8, t9);
constructed = true;
}
template<class T1, class T2, class T3, class T4, class T5,
class T6, class T7, class T8, class T9, class T10>
void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4, const T5& t5,
const T6& t6, const T7& t7, const T8& t8, const T9& t9, const T10& t10) {
MOZ_ASSERT(!constructed);
::new (storage.addr()) T(t1, t2, t3, t4, t5, t6, t7, t8, t9, t10);
constructed = true;
}
T* addr() {
MOZ_ASSERT(constructed);
return &asT();
}
T& ref() {
MOZ_ASSERT(constructed);
return asT();
}
const T& ref() const {
MOZ_ASSERT(constructed);
return const_cast<Maybe*>(this)->asT();
}
void destroy() {
ref().~T();
constructed = false;
}
void destroyIfConstructed() {
if (!empty())
destroy();
}
private:
Maybe(const Maybe& other) MOZ_DELETE;
const Maybe& operator=(const Maybe& other) MOZ_DELETE;
};
} // namespace mozilla
#endif /* mozilla_Maybe_h */