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The official Mbed 2 C/C++ SDK provides the software platform and libraries to build your applications.

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mbed 2

This is the mbed 2 library. If you'd like to learn about Mbed OS please see the mbed-os docs.

platform/NonCopyable.h@147:a97add6d7e64, 2017-07-19 (annotated)

Committer:
Kojto
Date:
Wed Jul 19 16:46:19 2017 +0100
Revision:
147:a97add6d7e64
Parent:
146:22da6e220af6
Child:
160:5571c4ff569f
Release 147 of the mbed library.

Who changed what in which revision?

UserRevisionLine numberNew contents of line
AnnaBridge 146:22da6e220af6 1 /* Copyright (c) 2017 ARM Limited
AnnaBridge 146:22da6e220af6 2 *
AnnaBridge 146:22da6e220af6 3 * Licensed under the Apache License, Version 2.0 (the "License");
AnnaBridge 146:22da6e220af6 4 * you may not use this file except in compliance with the License.
AnnaBridge 146:22da6e220af6 5 * You may obtain a copy of the License at
AnnaBridge 146:22da6e220af6 6 *
AnnaBridge 146:22da6e220af6 7 * http://www.apache.org/licenses/LICENSE-2.0
AnnaBridge 146:22da6e220af6 8 *
AnnaBridge 146:22da6e220af6 9 * Unless required by applicable law or agreed to in writing, software
AnnaBridge 146:22da6e220af6 10 * distributed under the License is distributed on an "AS IS" BASIS,
AnnaBridge 146:22da6e220af6 11 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
AnnaBridge 146:22da6e220af6 12 * See the License for the specific language governing permissions and
AnnaBridge 146:22da6e220af6 13 * limitations under the License.
AnnaBridge 146:22da6e220af6 14 */
AnnaBridge 146:22da6e220af6 15
AnnaBridge 146:22da6e220af6 16 #ifndef MBED_NONCOPYABLE_H_
AnnaBridge 146:22da6e220af6 17 #define MBED_NONCOPYABLE_H_
AnnaBridge 146:22da6e220af6 18
AnnaBridge 146:22da6e220af6 19 namespace mbed {
AnnaBridge 146:22da6e220af6 20
AnnaBridge 146:22da6e220af6 21 /**
AnnaBridge 146:22da6e220af6 22 * Inheriting from this class autogeneration of copy construction and copy
AnnaBridge 146:22da6e220af6 23 * assignement operations.
AnnaBridge 146:22da6e220af6 24 *
AnnaBridge 146:22da6e220af6 25 * Classes which are not value type should inherit privately from this class
AnnaBridge 146:22da6e220af6 26 * to avoid generation of invalid copy constructor or copy assignement operator
AnnaBridge 146:22da6e220af6 27 * which can lead to unoticeable programming errors.
AnnaBridge 146:22da6e220af6 28 *
AnnaBridge 146:22da6e220af6 29 * As an example consider the following signature:
AnnaBridge 146:22da6e220af6 30 *
AnnaBridge 146:22da6e220af6 31 * @code
AnnaBridge 146:22da6e220af6 32 * class Resource;
AnnaBridge 146:22da6e220af6 33 *
AnnaBridge 146:22da6e220af6 34 * class Foo {
AnnaBridge 146:22da6e220af6 35 * public:
AnnaBridge 146:22da6e220af6 36 * Foo() : _resource(new Resource()) { }
AnnaBridge 146:22da6e220af6 37 * ~Foo() { delete _resource; }
AnnaBridge 146:22da6e220af6 38 * private:
AnnaBridge 146:22da6e220af6 39 * Resource* _resource;
AnnaBridge 146:22da6e220af6 40 * }
AnnaBridge 146:22da6e220af6 41 *
AnnaBridge 146:22da6e220af6 42 * Foo get_foo();
AnnaBridge 146:22da6e220af6 43 *
AnnaBridge 146:22da6e220af6 44 * Foo foo = get_foo();
AnnaBridge 146:22da6e220af6 45 * @endcode
AnnaBridge 146:22da6e220af6 46 *
AnnaBridge 146:22da6e220af6 47 * There is a bug in this function, it returns a temporary value which will be
AnnaBridge 146:22da6e220af6 48 * byte copied into foo then destroyed. Unfortunately, internaly the Foo class
AnnaBridge 146:22da6e220af6 49 * manage a pointer to a Resource object. This pointer will be released when the
AnnaBridge 146:22da6e220af6 50 * temporary is destroyed and foo will manage a pointer to an already released
AnnaBridge 146:22da6e220af6 51 * Resource.
AnnaBridge 146:22da6e220af6 52 *
AnnaBridge 146:22da6e220af6 53 * Two issues has to be fixed in the example above:
AnnaBridge 146:22da6e220af6 54 * - Function signature has to be changed to reflect the fact that Foo
AnnaBridge 146:22da6e220af6 55 * instances cannot be copied. In that case accessor should return a
AnnaBridge 146:22da6e220af6 56 * reference to give access to objects already existing and managed.
AnnaBridge 146:22da6e220af6 57 * Generator on the other hand should return a pointer to the created object.
AnnaBridge 146:22da6e220af6 58 *
AnnaBridge 146:22da6e220af6 59 * @code
AnnaBridge 146:22da6e220af6 60 * // return a reference to an already managed Foo instance
AnnaBridge 146:22da6e220af6 61 * Foo& get_foo();
AnnaBridge 146:22da6e220af6 62 * Foo& foo = get_foo();
AnnaBridge 146:22da6e220af6 63 *
AnnaBridge 146:22da6e220af6 64 * // create a new Foo instance
AnnaBridge 146:22da6e220af6 65 * Foo* make_foo();
AnnaBridge 146:22da6e220af6 66 * Foo* m = make_foo();
AnnaBridge 146:22da6e220af6 67 * @endcode
AnnaBridge 146:22da6e220af6 68 *
AnnaBridge 146:22da6e220af6 69 * - Copy constructor and copy assignement operator has to be made private
AnnaBridge 146:22da6e220af6 70 * in the Foo class. It prevents unwanted copy of Foo objects. This can be
AnnaBridge 146:22da6e220af6 71 * done by declaring copy constructor and copy assignement in the private
AnnaBridge 146:22da6e220af6 72 * section of the Foo class.
AnnaBridge 146:22da6e220af6 73 *
AnnaBridge 146:22da6e220af6 74 * @code
AnnaBridge 146:22da6e220af6 75 * class Foo {
AnnaBridge 146:22da6e220af6 76 * public:
AnnaBridge 146:22da6e220af6 77 * Foo() : _resource(new Resource()) { }
AnnaBridge 146:22da6e220af6 78 * ~Foo() { delete _resource; }
AnnaBridge 146:22da6e220af6 79 * private:
AnnaBridge 146:22da6e220af6 80 * // disallow copy operations
AnnaBridge 146:22da6e220af6 81 * Foo(const Foo&);
AnnaBridge 146:22da6e220af6 82 * Foo& operator=(const Foo&);
AnnaBridge 146:22da6e220af6 83 * // data members
AnnaBridge 146:22da6e220af6 84 * Resource* _resource;
AnnaBridge 146:22da6e220af6 85 * }
AnnaBridge 146:22da6e220af6 86 * @endcode
AnnaBridge 146:22da6e220af6 87 *
AnnaBridge 146:22da6e220af6 88 * Another solution is to inherit privately from the NonCopyable class.
AnnaBridge 146:22da6e220af6 89 * It reduces the boiler plate needed to avoid copy operations but more
AnnaBridge 146:22da6e220af6 90 * importantly it clarifies the programer intent and the object semantic.
AnnaBridge 146:22da6e220af6 91 *
AnnaBridge 146:22da6e220af6 92 * class Foo : private NonCopyable<Foo> {
AnnaBridge 146:22da6e220af6 93 * public:
AnnaBridge 146:22da6e220af6 94 * Foo() : _resource(new Resource()) { }
AnnaBridge 146:22da6e220af6 95 * ~Foo() { delete _resource; }
AnnaBridge 146:22da6e220af6 96 * private:
AnnaBridge 146:22da6e220af6 97 * Resource* _resource;
AnnaBridge 146:22da6e220af6 98 * }
AnnaBridge 146:22da6e220af6 99 *
AnnaBridge 146:22da6e220af6 100 * @tparam T The type that should be made non copyable. It prevent cases where
AnnaBridge 146:22da6e220af6 101 * the empty base optimization cannot be applied and therefore ensure that the
AnnaBridge 146:22da6e220af6 102 * cost of this semantic sugar is null.
AnnaBridge 146:22da6e220af6 103 *
AnnaBridge 146:22da6e220af6 104 * As an example, the empty base optimization is prohibited if one of the empty
AnnaBridge 146:22da6e220af6 105 * base class is also a base type of the first non static data member:
AnnaBridge 146:22da6e220af6 106 *
AnnaBridge 146:22da6e220af6 107 * @code
AnnaBridge 146:22da6e220af6 108 * struct A { };
AnnaBridge 146:22da6e220af6 109 * struct B : A {
AnnaBridge 146:22da6e220af6 110 * int foo;
AnnaBridge 146:22da6e220af6 111 * };
AnnaBridge 146:22da6e220af6 112 * // thanks to empty base optimization, sizeof(B) == sizeof(int)
AnnaBridge 146:22da6e220af6 113 *
AnnaBridge 146:22da6e220af6 114 * struct C : A {
AnnaBridge 146:22da6e220af6 115 * B b;
AnnaBridge 146:22da6e220af6 116 * };
AnnaBridge 146:22da6e220af6 117 *
AnnaBridge 146:22da6e220af6 118 * // empty base optimization cannot be applied here because A from C and A from
AnnaBridge 146:22da6e220af6 119 * // B shall have a different address. In that case, with the alignement
AnnaBridge 146:22da6e220af6 120 * // sizeof(C) == 2* sizeof(int)
AnnaBridge 146:22da6e220af6 121 * @endcode
AnnaBridge 146:22da6e220af6 122 *
AnnaBridge 146:22da6e220af6 123 * The solution to that problem is to templatize the empty class to makes it
AnnaBridge 146:22da6e220af6 124 * unique to the type it is applied to:
AnnaBridge 146:22da6e220af6 125 *
AnnaBridge 146:22da6e220af6 126 * @code
AnnaBridge 146:22da6e220af6 127 * template<typename T>
AnnaBridge 146:22da6e220af6 128 * struct A<T> { };
AnnaBridge 146:22da6e220af6 129 * struct B : A<B> {
AnnaBridge 146:22da6e220af6 130 * int foo;
AnnaBridge 146:22da6e220af6 131 * };
AnnaBridge 146:22da6e220af6 132 * struct C : A<C> {
AnnaBridge 146:22da6e220af6 133 * B b;
AnnaBridge 146:22da6e220af6 134 * };
AnnaBridge 146:22da6e220af6 135 *
AnnaBridge 146:22da6e220af6 136 * // empty base optimization can be applied B and C does not refer to the same
AnnaBridge 146:22da6e220af6 137 * // kind of A. sizeof(C) == sizeof(B) == sizeof(int).
AnnaBridge 146:22da6e220af6 138 * @endcode
AnnaBridge 146:22da6e220af6 139 */
AnnaBridge 146:22da6e220af6 140 template<typename T>
AnnaBridge 146:22da6e220af6 141 class NonCopyable {
AnnaBridge 146:22da6e220af6 142 protected:
AnnaBridge 146:22da6e220af6 143 /**
AnnaBridge 146:22da6e220af6 144 * Disalow construction of NonCopyable objects from outside of its hierarchy.
AnnaBridge 146:22da6e220af6 145 */
AnnaBridge 146:22da6e220af6 146 NonCopyable() { }
AnnaBridge 146:22da6e220af6 147 /**
AnnaBridge 146:22da6e220af6 148 * Disalow destruction of NonCopyable objects from outside of its hierarchy.
AnnaBridge 146:22da6e220af6 149 */
AnnaBridge 146:22da6e220af6 150 ~NonCopyable() { }
AnnaBridge 146:22da6e220af6 151
AnnaBridge 146:22da6e220af6 152 private:
AnnaBridge 146:22da6e220af6 153 /**
AnnaBridge 146:22da6e220af6 154 * Declare copy constructor as private, any attempt to copy construct
AnnaBridge 146:22da6e220af6 155 * a NonCopyable will fail at compile time.
AnnaBridge 146:22da6e220af6 156 */
AnnaBridge 146:22da6e220af6 157 NonCopyable(const NonCopyable&);
AnnaBridge 146:22da6e220af6 158
AnnaBridge 146:22da6e220af6 159 /**
AnnaBridge 146:22da6e220af6 160 * Declare copy assignement operator as private, any attempt to copy assign
AnnaBridge 146:22da6e220af6 161 * a NonCopyable will fail at compile time.
AnnaBridge 146:22da6e220af6 162 */
AnnaBridge 146:22da6e220af6 163 NonCopyable& operator=(const NonCopyable&);
AnnaBridge 146:22da6e220af6 164 };
AnnaBridge 146:22da6e220af6 165
AnnaBridge 146:22da6e220af6 166 } // namespace mbed
AnnaBridge 146:22da6e220af6 167
AnnaBridge 146:22da6e220af6 168 #endif /* MBED_NONCOPYABLE_H_ */