vault backup: 2022-06-08 11:05:41

Affected files:
02. PARA/03. Resources（資源）/C++17/rvalue.md

02. PARA/03. Resources（資源）/C++17/rvalue.md

@@ -15,4 +15,14 @@ rvalue reference可以減少無間的複製成本。例如：
 std::string longStr = "qwertyuiopasdfghjklzxcbnm";
 std::string anotherLong = longStr;
 ```
-這時longStr會被複製一份anotherLong，把longStr換成很大的buffer，
+這時longStr會被複製一份anotherLong，把longStr換成很大的buffer，其複製成本就更大了。用rvalue reference可以消除這個複製成本。如下：
+```cpp
+std::string longStr = "qwertyuiopasdfghjklzxcbnm";
+std::string&& anotherLong = longStr;
+
+std::cout << anotherLong; // "qwertyuiopasdfghjklzxcbnm"
+//std::cout << longStr;   // Undefined bahavior, don't do this
+```
+上面的例子，longStr的內容被「轉移」給anotherLong，所以不可以再longStr，這時候longStr的內容取決於move operator的作法。
+
+不過