Mbed OS Reference | NonCopyable.h Source File

 /* Copyright (c) 2017-2019 ARM Limited
  * SPDX-License-Identifier: Apache-2.0
  *
  * 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 MBED_NONCOPYABLE_H_
 #define MBED_NONCOPYABLE_H_
 
 #if (!defined(MBED_DEBUG) && (MBED_CONF_PLATFORM_FORCE_NON_COPYABLE_ERROR == 0))
 #include "platform/mbed_toolchain.h"
 #include "platform/mbed_debug.h"
 #endif
 
 namespace mbed {
 
 /** \addtogroup platform-public-api */
 /** @{*/
 
 /**
  * \defgroup platform_NonCopyable NonCopyable class
  * @{
  */
 
 /**
  * Prevents generation of copy constructor and copy assignment operator in
  * derived classes.
  *
  * @par Usage
  *
  * To prevent generation of copy constructor and copy assignment operator,
  * inherit privately from the NonCopyable class.
  *
  * @code
  * class Resource : NonCopyable<Resource> { };
  *
  * Resource r;
  * // generates compile time error:
  * Resource r2 = r;
  * @endcode
  *
  * @par Background information
  *
  * Instances of polymorphic classes are not meant to be copied. The
  * C++ standards generate a default copy constructor and copy assignment
  * function if these functions have not been defined in the class.
  *
  * Consider the following example:
  *
  * @code
  * // base class representing a connection
  * struct Connection {
  *     Connection();
  *     virtual ~Connection();
  *     virtual void open() = 0;
  * }
  *
  * class SerialConnection : public Connection {
  * public:
  *     SerialConnection(Serial*);
  *
  * private:
  *     Serial* _serial;
  * };
  *
  * Connection& get_connection() {
  *     static SerialConnection serial_connection;
  *     return serial_connection;
  * }
  *
  * Connection connection = get_connection();
  * @endcode
  *
  * There is a subtle bug in this code, the function get_connection returns a
  * reference to a Connection which is captured by value instead of reference.
  *
  * When `get_connection` returns a reference to serial_connection it is copied into
  * the local variable connection. The vtable and others members defined in Connection
  * are copied, but members defined in SerialConnection are left apart. This can cause
  * severe crashes or bugs if the virtual functions captured use members not present
  * in the base declaration.
  *
  * To solve that problem, the copy constructor and assignment operator have to
  * be declared (but don't need to be defined) in the private section of the
  * Connection class:
  *
  * @code
  * struct Connection {
  * private:
  *     Connection(const Connection&);
  *     Connection& operator=(const Connection&);
  * }
  * @endcode
  *
  * Although manually declaring private copy constructor and assignment functions
  * works, it is not ideal. These declarations are usually easy to forget,
  * not immediately visible, and may be obscure to uninformed programmers.
  *
  * Using the NonCopyable class reduces the boilerplate required and expresses
  * the intent because class inheritance appears right after the class name
  * declaration.
  *
  * @code
  * struct Connection : private NonCopyable<Connection> {
  *      // regular declarations
  * }
  * @endcode
  *
  *
  * @par Implementation details
  *
  * Using a template type prevents cases where the empty base optimization cannot
  * be applied and therefore ensures that the cost of the NonCopyable semantic
  * sugar is null.
  *
  * As an example, the empty base optimization is prohibited if one of the empty
  * base classes is also a base type of the first nonstatic data member:
  *
  * @code
  * struct A { };
  * struct B : A {
  *    int foo;
  * };
  * // thanks to empty base optimization, sizeof(B) == sizeof(int)
  *
  * struct C : A {
  *   B b;
  * };
  *
  * // empty base optimization cannot be applied here because A from C and A from
  * // B have a different address. In that case, with the alignment
  * // sizeof(C) == 2* sizeof(int)
  * @endcode
  *
  * The solution to that problem is to templatize the empty class to make it
  * unique to the type it is applied to:
  *
  * @code
  * template<typename T>
  * struct A<T> { };
  * struct B : A<B> {
  *    int foo;
  * };
  * struct C : A<C> {
  *   B b;
  * };
  *
  * // empty base optimization can be applied B and C does not refer to the same
  * // kind of A. sizeof(C) == sizeof(B) == sizeof(int).
  * @endcode
  *
  * @tparam T The type that should be made noncopyable.
  *
  * @note Compile time errors are disabled if you use the develop or release profile.
  * To override this behavior and force compile time errors in all profiles,
  * set the configuration parameter "platform.force-non-copyable-error" to true.
  */
 template<typename T>
 class NonCopyable {
 #ifndef DOXYGEN_ONLY
 protected:
     /**
      * Disallow construction of NonCopyable objects from outside of its hierarchy.
      */
     NonCopyable() = default;
     /**
      * Disallow destruction of NonCopyable objects from outside of its hierarchy.
      */
     ~NonCopyable() = default;
 
 #if (!defined(MBED_DEBUG) && (MBED_CONF_PLATFORM_FORCE_NON_COPYABLE_ERROR == 0))
     /**
      * NonCopyable copy constructor.
      *
      * A compile time warning is issued when this function is used, and a runtime
      * warning is printed when the copy construction of the noncopyable happens.
      *
      * If you see this warning, your code is probably doing something unspecified.
      * Copying of noncopyable resources can lead to resource leak and random error.
      */
     MBED_DEPRECATED("Invalid copy construction of a NonCopyable resource.")
     NonCopyable(const NonCopyable &)
     {
         debug("Invalid copy construction of a NonCopyable resource: %s\r\n", MBED_PRETTY_FUNCTION);
     }
 
     /**
      * NonCopyable copy assignment operator.
      *
      * A compile time warning is issued when this function is used, and a runtime
      * warning is printed when the copy construction of the noncopyable happens.
      *
      * If you see this warning, your code is probably doing something unspecified.
      * Copying of noncopyable resources can lead to resource leak and random error.
      */
     MBED_DEPRECATED("Invalid copy assignment of a NonCopyable resource.")
     NonCopyable &operator=(const NonCopyable &)
     {
         debug("Invalid copy assignment of a NonCopyable resource: %s\r\n", MBED_PRETTY_FUNCTION);
         return *this;
     }
 
 #else
 private:
     /**
      * Declare copy constructor as private. Any attempt to copy construct
      * a NonCopyable will fail at compile time.
      */
     NonCopyable(const NonCopyable &);
 
     /**
      * Declare copy assignment operator as private. Any attempt to copy assign
      * a NonCopyable will fail at compile time.
      */
     NonCopyable &operator=(const NonCopyable &);
 #endif
 #endif
 };
 
 /**@}*/
 
 /**@}*/
 
 } // namespace mbed
 
 #endif /* MBED_NONCOPYABLE_H_ */
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