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SafeBool.h
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1 /* mbed Microcontroller Library
2  * Copyright (c) 2006-2013 ARM Limited
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 BLE_API_SAFE_BOOL_H_
18 #define BLE_API_SAFE_BOOL_H_
19 
20 /* Safe bool idiom, see: http://www.artima.com/cppsource/safebool.html */
21 
22 /**
23  * @file
24  * @addtogroup ble
25  * @{
26  * @addtogroup common
27  * @{
28  */
29 
30 /**
31  * Private namespace used to host details of the SafeBool implementation.
32  */
33 namespace SafeBool_ {
34 /**
35  * Base class of all SafeBool instances.
36  *
37  * This nontemplate base class exists to reduce the number of instantiation of
38  * the trueTag function.
39  */
40 class base {
41  template<typename>
42  friend class SafeBool;
43 
44 protected:
45  /**
46  * The bool type is a pointer to method that can be used in boolean context.
47  */
48  typedef void (base::*BoolType_t)() const;
49 
50  /**
51  * Nonimplemented call, use to disallow conversion between unrelated types.
52  */
53  void invalidTag() const;
54 
55  /**
56  * Special member function that indicates a true value.
57  */
58  void trueTag() const {}
59 };
60 
61 
62 }
63 
64 /**
65  * Safe conversion of objects in boolean context.
66  *
67  * Classes wanting evaluation of their instances in boolean context must derive
68  * publicly from this class rather than implementing the easy to misuse
69  * operator bool().
70  *
71  * Descendant classes must implement the function bool toBool() const to enable
72  * the safe conversion in boolean context.
73  *
74  * @tparam T Type of the derived class
75  *
76  * @code
77  *
78  * class A : public SafeBool<A> {
79  * public:
80  *
81  * // boolean conversion
82  * bool toBool() const {
83  *
84  * }
85  * };
86  *
87  * class B : public SafeBool<B> {
88  * public:
89  *
90  * // boolean conversion
91  * bool toBool() const {
92  *
93  * }
94  * };
95  *
96  * A a;
97  * B b;
98  *
99  * // will compile
100  * if(a) {
101  *
102  * }
103  *
104  * // compilation error
105  * if(a == b) {
106  *
107  * }
108  * @endcode
109  */
110 template <typename T>
111 class SafeBool : public SafeBool_::base {
112 public:
113  /**
114  * Bool operator implementation, derived class must provide a bool
115  * toBool() const function.
116  */
117  operator BoolType_t() const
118  {
119  return (static_cast<const T*>(this))->toBool()
120  ? &SafeBool<T>::trueTag : 0;
121  }
122 };
123 
124 /**
125  * Avoid conversion to bool between different classes.
126  *
127  * @attention Will generate a compile time error if instantiated.
128  */
129 template <typename T, typename U>
130 void operator==(const SafeBool<T>& lhs,const SafeBool<U>& rhs)
131 {
132  lhs.invalidTag();
133  // return false;
134 }
135 
136 /**
137  * Avoid conversion to bool between different classes.
138  *
139  * @attention Will generate a compile time error if instantiated.
140  */
141 template <typename T,typename U>
142 void operator!=(const SafeBool<T>& lhs,const SafeBool<U>& rhs)
143 {
144  lhs.invalidTag();
145  // return false;
146 }
147 
148 /**
149  * @}
150  * @}
151  */
152 
153 
154 #endif /* BLE_API_SAFE_BOOL_H_ */
SafeBool_::base::BoolType_t
void(base::* BoolType_t)() const
The bool type is a pointer to method that can be used in boolean context.
Definition: SafeBool.h:48
SafeBool_::base::invalidTag
void invalidTag() const
Nonimplemented call, use to disallow conversion between unrelated types.
SafeBool_::base
Base class of all SafeBool instances.
Definition: SafeBool.h:40
SafeBool_
Private namespace used to host details of the SafeBool implementation.
Definition: SafeBool.h:33
operator!=
void operator!=(const SafeBool< T > &lhs, const SafeBool< U > &rhs)
Avoid conversion to bool between different classes.
Definition: SafeBool.h:142
operator==
void operator==(const SafeBool< T > &lhs, const SafeBool< U > &rhs)
Avoid conversion to bool between different classes.
Definition: SafeBool.h:130
SafeBool
Safe conversion of objects in boolean context.
Definition: SafeBool.h:111
SafeBool_::base::trueTag
void trueTag() const
Special member function that indicates a true value.
Definition: SafeBool.h:58
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