C＋＋库研究笔记——赋值操作符operator=的正确重载方式（三个准则）

C++ Operator Overloading Guidelines

最终设计：

  MyClass& MyClass::operator=(const MyClass &rhs) {    // Check for self-assignment!    if (this == &rhs)      // Same object?      return *this;        // Yes, so skip assignment, and just return *this.    ... // Deallocate, allocate new space, copy values...    return *this;  }

设计要求：
  a, b, c, d, e;


  a = b = c = d = e = 42;
This is interpreted by the compiler as:
  a = (b = (c = (d = (e = 42))));
  MyClass a, b, c;
  ...
  (a = b) = c;  // What??
  // Take a const-reference to the right-hand side of the assignment.
  // Return a non-const reference to the left-hand side.
//对应上条
  MyClass& MyClass::operator=(const MyClass &rhs) {
    ...  // Do the assignment operation!


    return *this;  // Return a reference to myself.
  }

  MyClass& MyClass::operator=(const MyClass &rhs) {    // 1.  Deallocate any memory that MyClass is using internally    // 2.  Allocate some memory to hold the contents of rhs    // 3.  Copy the values from rhs into this instance    // 4.  Return *this  }

但上述设计仍然不完整，a=a会出现什么情况？ 当然还要考虑性能问题

  MyClass mc;
  ...
  mc = mc;     // BLAMMO.
 CHECK FOR SELF-ASSIGNMENT.


所以正确且安全的设计如下：So, the correct and safe version of the MyClass assignment operator would be this:

  MyClass& MyClass::operator=(const MyClass &rhs) {    // Check for self-assignment!    if (this == &rhs)      // Same object?      return *this;        // Yes, so skip assignment, and just return *this.    ... // Deallocate, allocate new space, copy values...    return *this;  }

或者：

  MyClass& MyClass::operator=(const MyClass &rhs) {    // Only do assignment if RHS is a different object from this.    if (this != &rhs) {      ... // Deallocate, allocate new space, copy values...    }    return *this;  }

总之，重载操作符的三个准则
1.右端值要为const 类型（不想改变他的值）Take a const-reference for the argument (the right-hand side of the assignment).
2.要返回一个引用，以便于实现（a=b=c…… (Do this by returning *this.)

3.检查是否为自我赋值Check for self-assignment, by comparing the pointers (this to &rhs).

－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－

类似的，我们可以设计这样的运等符(数据传输）
a<<(b<<c) 与上文一样的
当然如果我们只允许a<<b非连锁的操作（指a<<b<<c不允许），我们可以抛弃掉规则2

  void MyClass::operator=(const MyClass &rhs) {    // Only do assignment if RHS is a different object from this.    if (this != &rhs) {      ... // Deallocate, allocate new space, copy values...    }    return *this;  }