1// Copyright (c) 2012 The Chromium Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5// ManualConstructor statically-allocates space in which to store some
6// object, but does not initialize it.  You can then call the constructor
7// and destructor for the object yourself as you see fit.  This is useful
8// for memory management optimizations, where you want to initialize and
9// destroy an object multiple times but only allocate it once.
10//
11// (When I say ManualConstructor statically allocates space, I mean that
12// the ManualConstructor object itself is forced to be the right size.)
13//
14// For example usage, check out base/containers/small_map.h.
15
16#ifndef BASE_MEMORY_MANUAL_CONSTRUCTOR_H_
17#define BASE_MEMORY_MANUAL_CONSTRUCTOR_H_
18
19#include <stddef.h>
20
21#include "base/compiler_specific.h"
22#include "base/memory/aligned_memory.h"
23
24namespace base {
25
26template <typename Type>
27class ManualConstructor {
28 public:
29  // No constructor or destructor because one of the most useful uses of
30  // this class is as part of a union, and members of a union cannot have
31  // constructors or destructors.  And, anyway, the whole point of this
32  // class is to bypass these.
33
34  // Support users creating arrays of ManualConstructor<>s.  This ensures that
35  // the array itself has the correct alignment.
36  static void* operator new[](size_t size) {
37    return AlignedAlloc(size, ALIGNOF(Type));
38  }
39  static void operator delete[](void* mem) {
40    AlignedFree(mem);
41  }
42
43  inline Type* get() {
44    return space_.template data_as<Type>();
45  }
46  inline const Type* get() const  {
47    return space_.template data_as<Type>();
48  }
49
50  inline Type* operator->() { return get(); }
51  inline const Type* operator->() const { return get(); }
52
53  inline Type& operator*() { return *get(); }
54  inline const Type& operator*() const { return *get(); }
55
56  template <typename... Ts>
57  inline void Init(Ts&&... params) {
58    new(space_.void_data()) Type(std::forward<Ts>(params)...);
59  }
60
61  inline void InitFromMove(ManualConstructor<Type>&& o) {
62    Init(std::move(*o));
63  }
64
65  inline void Destroy() {
66    get()->~Type();
67  }
68
69 private:
70  AlignedMemory<sizeof(Type), ALIGNOF(Type)> space_;
71};
72
73}  // namespace base
74
75#endif  // BASE_MEMORY_MANUAL_CONSTRUCTOR_H_
76