File content as of revision 949:1bb402105289:

/* mbed Microcontroller Library
 * Copyright (c) 2006-2013 ARM Limited
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#ifndef BLE_API_SAFE_BOOL_H_
#define BLE_API_SAFE_BOOL_H_

//safe bool idiom, see : http://www.artima.com/cppsource/safebool.html

namespace SafeBool_ {
/**
 * @brief Base class for all intances of SafeBool, 
 * This base class reduce instantiation of trueTag function
 */
class base {
  template<typename>
  friend class SafeBool;

protected:
    //the bool type is a pointer to method which can be used in boolean context
  typedef void (base::*BoolType_t)() const;

  // non implemented call, use to disallow conversion between unrelated types 
  void invalidTag() const;

  // member function which indicate true value
  void trueTag() const {}
};


}

/**
 * @brief template class SafeBool use CRTP to made boolean conversion easy and correct.
 * Derived class should implement the function bool toBool() const to make this work. Inheritance 
 * should be public.
 *
 * @tparam T Type of the derived class
 * 
 * \code 
 * 
 * class A : public SafeBool<A> { 
 * public:
 * 
 *      // boolean conversion
 *      bool toBool() { 
 *      
 *      }  
 * };
 * 
 * class B : public SafeBool<B> { 
 * public:
 * 
 *      // boolean conversion
 *      bool toBool() const { 
 *      
 *      }  
 * };
 * 
 * A a;
 * B b;
 * 
 * // will compile 
 * if(a) { 
 * 
 * }
 * 
 * // compilation error 
 * if(a == b) { 
 * 
 * }
 * 
 * 
 * \endcode
 */
template <typename T> 
class SafeBool : public SafeBool_::base {
public:
  /** 
   * bool operator implementation, derived class has to provide bool toBool() const function.
   */
  operator BoolType_t() const {
    return (static_cast<const T*>(this))->toBool()
      ? &SafeBool<T>::trueTag : 0;
  }
};

//Avoid conversion to bool between different classes
template <typename T, typename U>
void operator==(const SafeBool<T>& lhs,const SafeBool<U>& rhs) {
    lhs.invalidTag();
//    return false;
}

//Avoid conversion to bool between different classes
template <typename T,typename U>
void operator!=(const SafeBool<T>& lhs,const SafeBool<U>& rhs) {
  lhs.invalidTag();
//  return false;
}

#endif /* BLE_API_SAFE_BOOL_H_ */