SafeBool.h

00001 /* mbed Microcontroller Library
00002  * Copyright (c) 2006-2013 ARM Limited
00003  *
00004  * Licensed under the Apache License, Version 2.0 (the "License");
00005  * you may not use this file except in compliance with the License.
00006  * You may obtain a copy of the License at
00007  *
00008  *     http://www.apache.org/licenses/LICENSE-2.0
00009  *
00010  * Unless required by applicable law or agreed to in writing, software
00011  * distributed under the License is distributed on an "AS IS" BASIS,
00012  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
00013  * See the License for the specific language governing permissions and
00014  * limitations under the License.
00015  */
00016 
00017 #ifndef BLE_API_SAFE_BOOL_H_
00018 #define BLE_API_SAFE_BOOL_H_
00019 
00020 /* Safe bool idiom, see: http://www.artima.com/cppsource/safebool.html */
00021 
00022 /**
00023  * @file
00024  * @addtogroup ble
00025  * @{
00026  * @addtogroup common
00027  * @{
00028  */
00029 
00030 /**
00031  *  Private namespace used to host details of the SafeBool implementation.
00032  */
00033 namespace SafeBool_ {
00034 /**
00035  * Base class of all SafeBool instances.
00036  *
00037  * This nontemplate base class exists to reduce the number of instantiation of
00038  * the trueTag function.
00039  */
00040 class base {
00041   template<typename>
00042   friend class SafeBool;
00043 
00044 protected:
00045     /**
00046      * The bool type is a pointer to method that can be used in boolean context.
00047      */
00048     typedef void (base::*BoolType_t)() const;
00049 
00050     /**
00051      * Nonimplemented call, use to disallow conversion between unrelated types.
00052      */
00053     void invalidTag() const;
00054 
00055     /**
00056      * Special member function that indicates a true value.
00057      */
00058     void trueTag() const {}
00059 };
00060 
00061 
00062 }
00063 
00064 /**
00065  * Safe conversion of objects in boolean context.
00066  *
00067  * Classes wanting evaluation of their instances in boolean context must derive
00068  * publicly from this class rather than implementing the easy to misuse
00069  * operator bool().
00070  *
00071  * Descendant classes must implement the function bool toBool() const to enable
00072  * the safe conversion in boolean context.
00073  *
00074  * @tparam T Type of the derived class
00075  *
00076  * @code
00077  *
00078  * class A : public SafeBool<A> {
00079  * public:
00080  *
00081  *      // boolean conversion
00082  *      bool toBool() const {
00083  *
00084  *      }
00085  * };
00086  *
00087  * class B : public SafeBool<B> {
00088  * public:
00089  *
00090  *      // boolean conversion
00091  *      bool toBool() const {
00092  *
00093  *      }
00094  * };
00095  *
00096  * A a;
00097  * B b;
00098  *
00099  * // will compile
00100  * if(a) {
00101  *
00102  * }
00103  *
00104  * // compilation error
00105  * if(a == b) {
00106  *
00107  * }
00108  * @endcode
00109  */
00110 template <typename T>
00111 class SafeBool : public SafeBool_::base {
00112 public:
00113     /**
00114      * Bool operator implementation, derived class must provide a bool
00115      * toBool() const function.
00116      */
00117     operator BoolType_t() const
00118     {
00119         return (static_cast<const T*>(this))->toBool()
00120             ? &SafeBool<T>::trueTag : 0;
00121     }
00122 };
00123 
00124 /**
00125  * Avoid conversion to bool between different classes.
00126  *
00127  * @important Will generate a compile time error if instantiated.
00128  */
00129 template <typename T, typename U>
00130 void operator==(const SafeBool<T>& lhs,const SafeBool<U>& rhs)
00131 {
00132     lhs.invalidTag();
00133     // return false;
00134 }
00135 
00136 /**
00137  * Avoid conversion to bool between different classes.
00138  *
00139  * @important Will generate a compile time error if instantiated.
00140  */
00141 template <typename T,typename U>
00142 void operator!=(const SafeBool<T>& lhs,const SafeBool<U>& rhs)
00143 {
00144     lhs.invalidTag();
00145     // return false;
00146 }
00147 
00148 /**
00149  * @}
00150  * @}
00151  */
00152 
00153 
00154 #endif /* BLE_API_SAFE_BOOL_H_ */