From f8e28d70faf13c93413308c75478852fc34750d2 Mon Sep 17 00:00:00 2001
From: Awin Huang <awinhuang@gmail.com>
Date: Thu, 9 Jun 2022 14:00:26 +0800
Subject: [PATCH] vault backup: 2022-06-09 14:00:26
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Affected files:
02. PARA/03. Resources（資源）/C++17/rvalue.md
---
 .../03. Resources（資源）/C++17/rvalue.md     | 49 ++++++++++++++++++-
 1 file changed, 47 insertions(+), 2 deletions(-)

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 0ffb5d0..7088a41 100644
--- a/02. PARA/03. Resources（資源）/C++17/rvalue.md	
+++ b/02. PARA/03. Resources（資源）/C++17/rvalue.md	
@@ -35,8 +35,53 @@ int& c = r;
 
 了解rvalue reference之後，就可以實作類別的 move constructor 跟move assignment operator。
 
-## Move constructor
-Mov
+## Move assignment operator
+假設我們有一個class叫BigBuffer，定義如下：
+```cpp
+class BigBuffer {
+public:
+    BigBuffer(int size=100*1024*1024) :
+        bufferSize(size)
+    {
+        std::cout << "BigBuffer constructor\n";
+        this->buffer = std::make_unique<uint8_t[]>(bufferSize);
+    }
+
+    ~BigBuffer() {
+        std::cout << "BigBuffer destructor\n";
+    }
+
+    BigBuffer(const BigBuffer& src) {
+        std::cout << "BigBuffer copy constructor\n";
+        bufferSize = src.bufferSize;
+        buffer = std::make_unique<uint8_t[]>(bufferSize);
+        std::memcpy(buffer.get(), src.buffer.get(), bufferSize);
+    }
+    
+    BigBuffer& operator= (BigBuffer& src) {
+        std::cout << "BigBuffer copy operator\n";
+        bufferSize = src.bufferSize;
+        buffer = std::make_unique<uint8_t[]>(bufferSize);
+        std::memcpy(buffer.get(), src.buffer.get(), bufferSize);
+        return *this;
+    }
+
+private:
+    int bufferSize = 0;
+    std::unique_ptr<uint8_t[]> buffer = nullptr;
+};
+```
+
+這個class的特色就是每一次使用都會佔用100MB的記憶體空間，想像下面的程式的動作： 
+```cpp
+BigBuffer buf1;
+// Do something with buf1
+// Assign to buf2
+BigBuffer buf2 = buf1;
+```
+
+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