https://en.cppreference.com/w/cpp/language/value_category
介紹各種值類型
https://www.cnblogs.com/qicosmos/p/4283455.html
這里介紹左值、右值、左值引用、右值引用
https://eli.thegreenplace.net/2011/12/15/understanding-lvalues-and-rvalues-in-c-and-c
這個講得好,在本文檔下一篇《理解左值和右值》里做了全文引用