1 /*
2  * Copyright (C) 2016 The Android Open Source Project
3  *
4  * Licensed under the Apache License, Version 2.0 (the "License");
5  * you may not use this file except in compliance with the License.
6  * You may obtain a copy of the License at
7  *
8  *      http://www.apache.org/licenses/LICENSE-2.0
9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 
17 #ifndef CHRE_UTIL_SINGLETON_H_
18 #define CHRE_UTIL_SINGLETON_H_
19 
20 #include <type_traits>
21 
22 #include "chre/util/non_copyable.h"
23 
24 namespace chre {
25 
26 /**
27  * The Singleton template provides static storage for one instance of the
28  * provided type. Initialization does not happen automatically which allows
29  * users of this API to control the order of initialization.
30  *
31  * Caution is recommended when using this class to avoid Singleton hell. In
32  * many cases there is a better solution than using a Singleton in your design.
33  * One good use of this class is for a class that owns the state of your
34  * application (the "root" of a tree of object ownership).
35  */
36 template<typename ObjectType>
37 class Singleton : public NonCopyable {
38  public:
39   /**
40    * Constructs the object in the space provided by this container. If the
41    * object is already constructed, no operation is performed. Use the
42    * isInitialized method to determine if construction is required.
43    *
44    * @param args The constructor arguments to pass to the singleton instance.
45    */
46   template<typename... Args>
47   static void init(Args&&... args);
48 
49   /**
50    * Invokes the destructor on the underlying object if it has been constructed
51    * already.
52    */
53   static void deinit();
54 
55   /**
56    * Returns whether or not this singleton instance has been constructed.
57    *
58    * @return Returns true if the object has already been constructed.
59    */
60   static bool isInitialized();
61 
62   /**
63    * Returns a pointer to the underlying object. The singleton object must be
64    * constructed prior to using get to interact with it. If the object is not
65    * initialized, nullptr is returned.
66    *
67    * @return A pointer to the singleton instance.
68    */
69   static ObjectType *get();
70 
71  private:
72   //! Static storage for the type of this singleton.
73   static typename std::aligned_storage<
74       sizeof(ObjectType), alignof(ObjectType)>::type sObject;
75 
76   //! Static storage for the initialized state of this singleton.
77   static bool sIsInitialized;
78 };
79 
80 }  // namespace chre
81 
82 #include "chre/util/singleton_impl.h"
83 
84 #endif  // CHRE_UTIL_SINGLETON_H_
85