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 namespace SafeBool_ {
00023 /**
00024  * @brief Base class for all intances of SafeBool, 
00025  * This base class reduce instantiation of trueTag function
00026  */
00027 class base {
00028   template<typename>
00029   friend class SafeBool;
00030 
00031 protected:
00032     //the bool type is a pointer to method which can be used in boolean context
00033   typedef void (base::*BoolType_t)() const;
00034 
00035   // non implemented call, use to disallow conversion between unrelated types 
00036   void invalidTag() const;
00037 
00038   // member function which indicate true value
00039   void trueTag() const {}
00040 };
00041 
00042 
00043 }
00044 
00045 /**
00046  * @brief template class SafeBool use CRTP to made boolean conversion easy and correct.
00047  * Derived class should implement the function bool toBool() const to make this work. Inheritance 
00048  * should be public.
00049  *
00050  * @tparam T Type of the derived class
00051  * 
00052  * \code 
00053  * 
00054  * class A : public SafeBool<A> { 
00055  * public:
00056  * 
00057  *      // boolean conversion
00058  *      bool toBool() { 
00059  *      
00060  *      }  
00061  * };
00062  * 
00063  * class B : public SafeBool<B> { 
00064  * public:
00065  * 
00066  *      // boolean conversion
00067  *      bool toBool() const { 
00068  *      
00069  *      }  
00070  * };
00071  * 
00072  * A a;
00073  * B b;
00074  * 
00075  * // will compile 
00076  * if(a) { 
00077  * 
00078  * }
00079  * 
00080  * // compilation error 
00081  * if(a == b) { 
00082  * 
00083  * }
00084  * 
00085  * 
00086  * \endcode
00087  */
00088 template <typename T> 
00089 class SafeBool : public SafeBool_::base {
00090 public:
00091   /** 
00092    * bool operator implementation, derived class has to provide bool toBool() const function.
00093    */
00094   operator BoolType_t() const {
00095     return (static_cast<const T*>(this))->toBool()
00096       ? &SafeBool<T>::trueTag : 0;
00097   }
00098 };
00099 
00100 //Avoid conversion to bool between different classes
00101 template <typename T, typename U>
00102 void operator==(const SafeBool<T>& lhs,const SafeBool<U>& rhs) {
00103     lhs.invalidTag();
00104 //    return false;
00105 }
00106 
00107 //Avoid conversion to bool between different classes
00108 template <typename T,typename U>
00109 void operator!=(const SafeBool<T>& lhs,const SafeBool<U>& rhs) {
00110   lhs.invalidTag();
00111 //  return false;
00112 }
00113 
00114 #endif /* BLE_API_SAFE_BOOL_H_ */