mbed-os/platform/NonCopyable.h

/* Copyright (c) 2017 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 MBED_NONCOPYABLE_H_
#define MBED_NONCOPYABLE_H_

#if (!defined(MBED_DEBUG) && (MBED_CONF_PLATFORM_FORCE_NON_COPYABLE_ERROR == 0))
#include "mbed_toolchain.h"
#include "mbed_debug.h"
#endif

namespace mbed {

/**
 * Inheriting from this class autogeneration of copy construction and copy
 * assignment operations.
 *
 * Classes which are not value type should inherit privately from this class
 * to avoid generation of invalid copy constructor or copy assignment operator
 * which can lead to unnoticeable programming errors.
 *
 * As an example consider the following signature:
 *
 * @code
 * class Resource;
 *
 * class Foo {
 * public:
 *   Foo() : _resource(new Resource()) { }
 *   ~Foo() { delete _resource; }
 * private:
 *   Resource* _resource;
 * }
 *
 * Foo get_foo();
 *
 * Foo foo = get_foo();
 * @endcode
 *
 * There is a bug in this function, it returns a temporary value which will be
 * byte copied into foo then destroyed. Unfortunately, internally the Foo class
 * manage a pointer to a Resource object. This pointer will be released when the
 * temporary is destroyed and foo will manage a pointer to an already released
 * Resource.
 *
 * Two issues has to be fixed in the example above:
 *   - Function signature has to be changed to reflect the fact that Foo
 *     instances cannot be copied. In that case accessor should return a
 *     reference to give access to objects already existing and managed.
 *     Generator on the other hand should return a pointer to the created object.
 *
 * @code
 * // return a reference to an already managed Foo instance
 * Foo& get_foo();
 * Foo& foo = get_foo();
 *
 * // create a new Foo instance
 * Foo* make_foo();
 * Foo* m = make_foo();
 * @endcode
 *
 *   - Copy constructor and copy assignment operator has to be made private
 *     in the Foo class. It prevents unwanted copy of Foo objects. This can be
 *     done by declaring copy constructor and copy assignment in the private
 *     section of the Foo class.
 *
 * @code
 * class Foo {
 * public:
 *   Foo() : _resource(new Resource()) { }
 *   ~Foo() { delete _resource; }
 * private:
 *   // disallow copy operations
 *   Foo(const Foo&);
 *   Foo& operator=(const Foo&);
 *   // data members
 *   Resource* _resource;
 * }
 * @endcode
 *
 * Another solution is to inherit privately from the NonCopyable class.
 * It reduces the boiler plate needed to avoid copy operations but more
 * importantly it clarifies the programmer intent and the object semantic.
 *
 * class Foo : private NonCopyable<Foo> {
 * public:
 *   Foo() : _resource(new Resource()) { }
 *   ~Foo() { delete _resource; }
 * private:
 *   Resource* _resource;
 * }
 *
 * @tparam T The type that should be made non copyable. It prevent cases where
 * the empty base optimization cannot be applied and therefore ensure that the
 * cost of this semantic sugar is null.
 *
 * As an example, the empty base optimization is prohibited if one of the empty
 * base class is also a base type of the first non static 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 shall 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 makes 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
 *
 * @note Compile time errors are disabled if the develop or the release profile
 * is used. To override this behavior and force compile time errors in all profile
 * set the configuration parameter "platform.force-non-copyable-error" to true.
 */
template<typename T>
class NonCopyable {
protected:
    /**
     * Disallow construction of NonCopyable objects from outside of its hierarchy.
     */
    NonCopyable() { }
    /**
     * Disallow destruction of NonCopyable objects from outside of its hierarchy.
     */
    ~NonCopyable() { }

#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 non copyable happens.
     *
     * If you see this warning, your code is probably doing something unspecified.
     * Copy of non copyable 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 non copyable happens.
     *
     * If you see this warning, your code is probably doing something unspecified.
     * Copy of non copyable 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
};

} // namespace mbed

#endif /* MBED_NONCOPYABLE_H_ */