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 
24 namespace base {
25 
26 template <typename Type>
27 class 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 
get()43   inline Type* get() {
44     return space_.template data_as<Type>();
45   }
get()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>
Init(const Ts &...params)57   inline void Init(const Ts&... params) {
58     new(space_.void_data()) Type(params...);
59   }
60 
Destroy()61   inline void Destroy() {
62     get()->~Type();
63   }
64 
65  private:
66   AlignedMemory<sizeof(Type), ALIGNOF(Type)> space_;
67 };
68 
69 }  // namespace base
70 
71 #endif  // BASE_MEMORY_MANUAL_CONSTRUCTOR_H_
72