From fa039f46411e6fb7d7ab3e69b31480cd91dae41e Mon Sep 17 00:00:00 2001
From: Awin Huang <awinhuang@gmail.com>
Date: Thu, 9 Jun 2022 14:10:26 +0800
Subject: [PATCH] vault backup: 2022-06-09 14:10:26
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Affected files:
02. PARA/03. Resources（資源）/C++17/rvalue.md
---
 .../03. Resources（資源）/C++17/rvalue.md     | 39 ++++++++++++++++++-
 1 file changed, 38 insertions(+), 1 deletion(-)

diff --git a/02. PARA/03. Resources（資源）/C++17/rvalue.md b/02. PARA/03. Resources（資源）/C++17/rvalue.md
index 7088a41..341acb3 100644
--- a/02. PARA/03. Resources（資源）/C++17/rvalue.md	
+++ b/02. PARA/03. Resources（資源）/C++17/rvalue.md	
@@ -80,10 +80,47 @@ BigBuffer buf1;
 BigBuffer buf2 = buf1;
 ```
 
+執行訊息：
+```
+BigBuffer constructor
+BigBuffer copy constructor, copy 104857600Bytes
+BigBuffer destructor
+BigBuffer destructor
+```
+
 這會先產生buf1，然後把buf1 copy給buf2。如果我們想要省下copy的成本，這時候 Move assignment operator（就是 `=`）就可以派上用場了。
-幫
+幫 BigBuffer 加一個 Move assignment operator：
+```cpp
+class BigBuffer {
+public:
+    ...
+    
+    BigBuffer& operator=(BigBuffer&& src) noexcept {
+        std::cout << "BigBuffer move operator\n";
+        bufferSize = src.bufferSize;
+        buffer = std::move(src.buffer);
+        
+        src.buffer.reset();
+        src.bufferSize = 0;
+        return *this;
+    }
+    ...
+```
 
+這個 Move assignment operator 的參數就是一個 rvalue reference，我們把來源的 bufferSize 跟 buffer指標「移到」我們這邊，而不是完整的複製一份。在轉移之後呢，當然也要把來源清空，讓轉移更加明確。
 
+有了 Move assignment operator 之後，在執行一次原本的程式，你會發現訊息......沒有變，還是一樣呼叫 copy assignment operator 來複製了100MB的buffer，這時我們需要明確的告訴 compiler 我們要「移動」物件，而不是複製它，把原本的程式改為：
+```cpp
+BigBuffer buf1;
+// Do something with buf1
+// Assign to buf2
+BigBuffer buf2 = std::move(buf1);
+```
+
+我們用 `std::move()`  來「移動」物件，這時輸出變成
+```
+
+```
 
 ## 參考
 - [Value categories - cppreference.com](https://en.cppreference.com/w/cpp/language/value_category)