Chapter Summary

小结

In addition to defining the operations on objects of its type, a class also defines what it means to copy, assign, or destroy objects of the type. Special member functionsthe copy constructor, the assignment operator, and the destructor define these operations. Collectively these operations are referred to as the "copy control" functions.

类除了定义该类型对象上的操作，还需要定义复制、赋值或撤销该类型对象的含义。特殊成员函数（复制构造函数、赋值操作符和析构函数）可用于定义这些操作。这些操作统称为“复制控制”函数。

If a class does not define one or more of these operations, the compiler will define them automatically. The synthesized operations perform memberwise initialization, assignment, or destruction: Taking each member in turn, the synthesized operation does whatever is appropriate to the member's type to copy, assign, or destroy that member. If the member is a class type, the synthesized operation calls the corresponding operation for that class (e.g., the copy constructor calls the member's copy constructor, the destructor calls its destructor, etc.). If the member is a built-in type or a pointer, the member is copied or assigned directly; the destructor does nothing to destroy members of built-in or pointer type. If the member is an array, the elements in the array are copied, assigned, or destroyed in a manner appropriate to the element type.

如果类没有定义这些操作中的一个或多个，编译器将自动定义它们。合成操作执行逐个成员初始化、赋值或撤销：合成操作依次取得每个成员，根据成员类型进行成员的复制、赋值或撤销。如果成员为类类型的，合成操作调用该类的相应操作（即，复制构造函数调用成员的复制构造函数，析构函数调用成员的析构函数，等等）。如果成员为内置类型或指针，则直接复制或赋值，析构函数对撤销内置类型或指针类型的成员没有影响。如果成员为数组，则根据元素类型以适当方式复制、赋值或撤销数组中的元素。

Unlike the copy constructor and assignment operator, the synthesized destructor is created and run, regardless of whether the class defines its own destructor. The synthesized destructor is run after the class-defined destructor, if there is one, completes.

与复制构造函数和赋值操作符不同，无论类是否定义了自己的析构函数，都会创建和运行合成析构函数。如果类定义了析构函数，则在类定义的析构函数结束之后运行合成析构函数。

The hardest part of defining the copy-control functions is often simply recognizing that they are necessary. 定义复制控制函数最为困难的部分通常在于认识到它们的必要性。

Classes that allocate memory or other resources almost always require that the class define the copy-control members to manage the allocated resource. If a class needs a destructor, then it almost surely needs to define the copy constructor and assignment operator as well.

分配内存或其他资源的类几乎总是需要定义复制控制成员来管理所分配的资源。如果一个类需要析构函数，则它几乎也总是需要定义复制构造函数和赋值操作符。