Diff: platform/NonCopyable.h

Revision:

168:9672193075cf

Child:

180:96ed750bd169

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/platform/NonCopyable.h	Thu Jul 06 15:42:05 2017 +0100
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+/* 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_
+
+namespace mbed { 
+
+/**
+ * Inheriting from this class autogeneration of copy construction and copy 
+ * assignement operations. 
+ * 
+ * Classes which are not value type should inherit privately from this class 
+ * to avoid generation of invalid copy constructor or copy assignement operator
+ * which can lead to unoticeable 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, internaly 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 assignement 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 assignement 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 programer 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 alignement 
+ * // 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
+ */
+template<typename T>
+class NonCopyable { 
+protected:
+    /** 
+     * Disalow construction of NonCopyable objects from outside of its hierarchy.
+     */
+    NonCopyable() { }
+    /** 
+     * Disalow destruction of NonCopyable objects from outside of its hierarchy.
+     */
+    ~NonCopyable() { }
+
+private: 
+    /**
+     * Declare copy constructor as private, any attempt to copy construct 
+     * a NonCopyable will fail at compile time.
+     */
+    NonCopyable(const NonCopyable&);
+
+    /**
+     * Declare copy assignement operator as private, any attempt to copy assign 
+     * a NonCopyable will fail at compile time.
+     */
+    NonCopyable& operator=(const NonCopyable&);
+};
+
+} // namespace mbed 
+
+#endif /* MBED_NONCOPYABLE_H_ */