14.4. Assignment Operators

14.4. 赋值操作符

We covered the assignment of one object of class type to another object of its type in Section 13.2 (p. 482). The class assignment operator takes a parameter that is the class type. Usually the parameter is a const reference to the class type. However, the parameter could be the class type or a nonconst reference to the class type. This operator will be synthesized by the compiler if we do not define it ourselves. The class assignment operator must be a member of the class so the compiler can know whether it needs to synthesize one.

第 13.2 节讨论了类类型对象对同类型其他对象的赋值。类赋值操作符接受类类型形参，通常，该形参是对类类型的 const 引用，但也可以是类类型或对类类型的非 const 引用。如果没有定义这个操作符，则编译器将合成它。类赋值操作符必须是类的成员，以便编译器可以知道是否需要合成一个。

Additional assignment operators that differ by the type of the right-hand operand can be defined for a class type. For example, the library string class defines three assignment operators: In addition to the class assignment operator, which takes a const string& as its right-hand operand, the string class defines versions of assignment that take a C-style character string or a char as the right-hand operand. These might be used as follows:

可以为一个类定义许多附加的赋值操作符，这些赋值操作符会因右操作符类型不同而不同。例如，标准库的类 string 定义了 3 个赋值操作符：除了接受 const string& 作为右操作数的类赋值操作符之外，类还定义了接受 C 风格字符串或 char 作为右操作数的赋值操作符，这些操作符可以这样使用：

     string car ("Volks");
     car = "Studebaker"; // string = const char*
     string model;
     model = 'T'; // string = char

To support these operations, the string class contains members that look like

为了支持这些操作符，string 类包含如下成员：

     // illustration of assignment operators for class string
     class string {
     public:
         string& operator=(const string &);      // s1 = s2;
         string& operator=(const char *);        // s1 = "str";
         string& operator=(char);                // s1 = 'c';
         // ....
      };

Assignment operators can be overloaded. Unlike the compound-assignment operators, every assignment operator, regardless of parameter type, must be defined as a member function. 赋值操作符可以重载。无论形参为何种类型，赋值操作符必须定义为成员函数，这一点与复合赋值操作符有所不同。

Assignment Should Return a Reference to *this

赋值必须返回对 *this 的引用

The string assignment operators return a reference to string, which is consistent with assignment for the built-in types. Moreover, because assignment returns a reference there is no need to create and destroy a temporary copy of the result. The return value is usually a reference to the left-hand operand. For example, here is the definition of the Sales_item compound-assignment operator:

string 赋值操作符返回 string 引用，这与内置类型的赋值一致。而且，因为赋值返回一个引用，就不需要创建和撤销结果的临时副本。返回值通常是左操作数的引用，例如，这是 Sales_item 复合赋值操作符的定义：

     // assumes that both objects refer to the same isbn
     Sales_item& Sales_item::operator+=(const Sales_item& rhs)
     {
        units_sold += rhs.units_sold;
        revenue += rhs.revenue;
        return *this;
     }

Ordinarily, assignment operators and compound-assignment operators ought to return a reference to the left-hand operand. 一般而言，赋值操作符与复合赋值操作符应返回操作符的引用。