1. 自賦值的情況: if (this == &rhs)
2. 返回值爲對象的reference: 爲了連等的情況。 (a=b)=c
3. exceptional safe 的assignment operator:
A. http://www.cplusplus.com/articles/y8hv0pDG/
B. Exceptional C++中, Item 9
C. why can't destructor throw? More effective C++, Item 11
D. What is exception safe?
4. 左值，右值問題