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common/SafeBool.h
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1 /* mbed Microcontroller Library
2  * Copyright (c) 2006-2020 ARM Limited
3  *
4  * SPDX-License-Identifier: Apache-2.0
5  *
6  * Licensed under the Apache License, Version 2.0 (the "License");
7  * you may not use this file except in compliance with the License.
8  * You may obtain a copy of the License at
9  *
10  * http://www.apache.org/licenses/LICENSE-2.0
11  *
12  * Unless required by applicable law or agreed to in writing, software
13  * distributed under the License is distributed on an "AS IS" BASIS,
14  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
15  * See the License for the specific language governing permissions and
16  * limitations under the License.
17  */
18 
19 #ifndef BLE_API_SAFE_BOOL_H_
20 #define BLE_API_SAFE_BOOL_H_
21 
22 /* Safe bool idiom, see: http://www.artima.com/cppsource/safebool.html */
23 
24 /**
25  * @file
26  * @addtogroup ble
27  * @{
28  * @addtogroup common
29  * @{
30  */
31 
32 /**
33  * Private namespace used to host details of the SafeBool implementation.
34  */
35 namespace SafeBool_ {
36 /**
37  * Base class of all SafeBool instances.
38  *
39  * This nontemplate base class exists to reduce the number of instantiation of
40  * the trueTag function.
41  */
42 class base {
43  template<typename>
44  friend class SafeBool;
45 
46 protected:
47  /**
48  * The bool type is a pointer to method that can be used in boolean context.
49  */
50  typedef void (base::*BoolType_t)() const;
51 
52  /**
53  * Nonimplemented call, use to disallow conversion between unrelated types.
54  */
55  void invalidTag() const;
56 
57  /**
58  * Special member function that indicates a true value.
59  */
60  void trueTag() const {}
61 };
62 
63 
64 }
65 
66 /**
67  * Safe conversion of objects in boolean context.
68  *
69  * Classes wanting evaluation of their instances in boolean context must derive
70  * publicly from this class rather than implementing the easy to misuse
71  * operator bool().
72  *
73  * Descendant classes must implement the function bool toBool() const to enable
74  * the safe conversion in boolean context.
75  *
76  * @tparam T Type of the derived class
77  *
78  * @code
79  *
80  * class A : public SafeBool<A> {
81  * public:
82  *
83  * // boolean conversion
84  * bool toBool() const {
85  *
86  * }
87  * };
88  *
89  * class B : public SafeBool<B> {
90  * public:
91  *
92  * // boolean conversion
93  * bool toBool() const {
94  *
95  * }
96  * };
97  *
98  * A a;
99  * B b;
100  *
101  * // will compile
102  * if(a) {
103  *
104  * }
105  *
106  * // compilation error
107  * if(a == b) {
108  *
109  * }
110  * @endcode
111  */
112 template <typename T>
113 class SafeBool : public SafeBool_::base {
114 public:
115  /**
116  * Bool operator implementation, derived class must provide a bool
117  * toBool() const function.
118  */
119  operator BoolType_t() const
120  {
121  return (static_cast<const T*>(this))->toBool()
122  ? &SafeBool<T>::trueTag : 0;
123  }
124 };
125 
126 /**
127  * Avoid conversion to bool between different classes.
128  *
129  * @attention Will generate a compile time error if instantiated.
130  */
131 template <typename T, typename U>
132 void operator==(const SafeBool<T>& lhs,const SafeBool<U>& rhs)
133 {
134  lhs.invalidTag();
135  // return false;
136 }
137 
138 /**
139  * Avoid conversion to bool between different classes.
140  *
141  * @attention Will generate a compile time error if instantiated.
142  */
143 template <typename T,typename U>
144 void operator!=(const SafeBool<T>& lhs,const SafeBool<U>& rhs)
145 {
146  lhs.invalidTag();
147  // return false;
148 }
149 
150 /**
151  * @}
152  * @}
153  */
154 
155 
156 #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: common/SafeBool.h:50
SafeBool_::base::invalidTag
void invalidTag() const
Nonimplemented call, use to disallow conversion between unrelated types.
SafeBool_::base
Base class of all SafeBool instances.
Definition: common/SafeBool.h:42
SafeBool_
Private namespace used to host details of the SafeBool implementation.
Definition: common/SafeBool.h:35
operator!=
void operator!=(const SafeBool< T > &lhs, const SafeBool< U > &rhs)
Avoid conversion to bool between different classes.
Definition: common/SafeBool.h:144
operator==
void operator==(const SafeBool< T > &lhs, const SafeBool< U > &rhs)
Avoid conversion to bool between different classes.
Definition: common/SafeBool.h:132
SafeBool
Safe conversion of objects in boolean context.
Definition: common/SafeBool.h:113
SafeBool_::base::trueTag
void trueTag() const
Special member function that indicates a true value.
Definition: common/SafeBool.h:60
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