复制作业应该做什么

Question

我正在寻找以下解释的答案（来自“C++ 之旅”一书）

MyClass& operator=(const MyClass&) // copy asignment: clean up target and copy

我从来没有清理过目标 （或者至少我不明白这是什么意思）复制时：

• 复制的想法不就是拥有两个相同的东西吗？
• 如果我清理目标，那不是move吗？

• 清理目标到底是什么意思？

  • 参考也是const 所以我不会 可以修改它

---

在书中它指出：

MyClass& operator=(MyClass&&) // move assignment: clean up target and move

在这里清理目标是有意义的，因为这就是我理解move -ing 的工作原理

Answer 1

假设 MyClass 有一个拥有指针

class MyClass {
  Owned *that;
public:
...
  MyClass& operator=(const MyClass&other) // copy asignment: clean up target and copy
  {
     Owned = other->owned;
  }

指向的内存会发生什么？它被泄露了。所以改为这样做

  MyClass& operator=(const MyClass&other) // copy asignment: clean up target and copy
  {
     if (this == &other)  // prevent self assignment as this would in this case be a waste of time.
       return *this;
     delete that; // clean up
     that = new Owned(*other->that); // copy
     return *this; // return the object, so operations can be chained.
  }

感谢@PaulMcKenzie && @Eljay

  MyClass& operator=(const MyClass&other) // copy asignment: clean up target and copy
  {
     Owned *delayDelete = that;
     that = new Owned(*other->that); // copy, if this throws nothing happened
     delete delayDelete; // clean up
     return *this; // return the object, so operations can be chained.
  }