C++: The Rule of Three/Five

整理自 C++ Primer, 5th Edition

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There are 5 (only 3 before C++11) basic operations to control copies of class objects:

1. destructor $\Rightarrow$ ~X()
2. $\mathtt{copy}$ constructor $\Rightarrow$ X(const X&)
3. $\mathtt{move}$ constructor $\Rightarrow$ X(X&&) (Since C++11)
4. $\mathtt{copy}$ assignment $\Rightarrow$ X& operator=(const X&)
5. $\mathtt{move}$ assignment $\Rightarrow$ X& operator=(X&&) (Since C++11)

Ordinarily these 5 operations should be thought of as a unit. In general, it is unusual to need one without needing to define them all.

Rule of thumb No.1: Decide first whether the class needs a destructor, because often, the need for a destructor is more obvious than the need for the $\mathtt{copy}$ constructor or assignment operator.

Rule of thumb No.2: If the class needs a destructor, it almost surely needs a $\mathtt{copy}$ constructor and $\mathtt{copy}$-assignment operator as well.

• Virtual destructors are an important exception because a base class almost always needs a destructor.
  • 我们说 “need a destructor” 是指确实需要 free 掉某些资源；virtual destructor 是多态的要求，算是语言特性，在这个场合下要注意区分。

Rule of thumb No.3: Classes that need $\mathtt{copy}$ need assignment, and vice versa.

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For some classes, the compiler defines these synthesized members as =delete (参 C++: =default & =delete):

• The synthesized destructor will be =delete
  • if the class has a member whose own destructor is =delete or is inaccessible (e.g., private).
• The synthesized $\mathtt{copy}$ constructor will be =delete
  • if the class has a member whose own copy constructor is =delete or inaccessible, or
  • if the class has a member with a =delete or inaccessible destructor.
• The synthesized $\mathtt{copy}$-assignment operator will be =delete
  • if the class has a member has a =delete or inaccessible copy-assignment operator, or
  • if the class has a const or reference member.
• The synthesized default constructor will be =delete
  • if the class has a member with a =delete or inaccessible destructor, or
  • if the class has a reference member that does not have an in-class initializer (in-class initializer 其实就是指在 class 内部直接 define member 的语句，比如 class Foo { int i = 47; }), or
  • if the class has a const member whose type does not explicitly define a default constructor and that member does not have an in-class initializer.

In essence, these rules mean that if a class has a data member that cannot be default constructed, copied, assigned, or destroyed, then the corresponding function/operator will be =delete.

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In order to define these 5 operations, we first have to decide what copying an object of our type will mean. In general, we have two choices: we can define the copy operations to make the class behave

1. like a value, or
2. like a pointer.

Classes that behave like values have their own state. When we copy a valuelike object, the copy and the original are independent of each other. Changes made to the copy have no effect on the original, and vice versa.

Classes that act like pointers (e.g. Smart Pointers) share state. When we copy objects of such classes, the copy and the original use the same underlying data. Changes made to the copy also change the original, and vice versa.

• 这个时候你可能需要参考 C++ overloading operator=: shallow copy vs. deep copy / reference counting / copy on write