211 lines
5.7 KiB
Markdown
211 lines
5.7 KiB
Markdown
rvalue 是指：
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- 等號右邊的值
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- 臨時的值，例如運算的結果
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- 無法被取址（address-of）的物件
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## rvalue reference
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一般的參考只能參考[[lvalue]]，如下的程式是ok的：
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```cpp
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int a = 10;
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int& b = a;
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```
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但是像這樣就不行了：
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```cpp
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int a = 10;
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int b = 5;
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int& c = a + b;
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```
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因為`a+b`是一個 rvalue（臨時的值，沒辦法取址），所以無法參考。
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但是可以用`&&`來參考 rvalue。例如：
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```cpp
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int a = 10;
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int b = 5;
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int&& c = a + b; // c = 15
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```
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而不用這樣：
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```cpp
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int a = 10;
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int b = 5;
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int r = a + b;
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int& c = r;
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```
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了解 rvalue reference 之後，就可以實作類別的 move constructor 跟 move assignment operator。這可以減少複製的成本。
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## Move constructor
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假設我們有一個 class 叫 BigBuffer，定義如下：
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```cpp
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class BigBuffer {
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public:
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BigBuffer(int size=100*1024*1024) :
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bufferSize(size)
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{
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std::cout << "BigBuffer constructor\n";
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this->buffer = std::make_unique<uint8_t[]>(bufferSize);
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}
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~BigBuffer() {
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std::cout << "BigBuffer destructor\n";
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}
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BigBuffer(const BigBuffer& src) {
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std::cout << "BigBuffer copy constructor\n";
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bufferSize = src.bufferSize;
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buffer = std::make_unique<uint8_t[]>(bufferSize);
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std::memcpy(buffer.get(), src.buffer.get(), bufferSize);
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}
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BigBuffer& operator= (BigBuffer& src) {
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std::cout << "BigBuffer copy operator\n";
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bufferSize = src.bufferSize;
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buffer = std::make_unique<uint8_t[]>(bufferSize);
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std::memcpy(buffer.get(), src.buffer.get(), bufferSize);
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return *this;
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}
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private:
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int bufferSize = 0;
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std::unique_ptr<uint8_t[]> buffer = nullptr;
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};
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```
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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：
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```cpp
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BigBuffer buf1;
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// Do something with buf1
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// Assign to buf2
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BigBuffer buf2 = buf1;
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```
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執行訊息：
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```
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BigBuffer constructor // create buf1
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BigBuffer copy constructor, copy 104857600Bytes // copy buf1 to buf2
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...
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```
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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 constructor 就可以派上用場了。
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幫 BigBuffer 加一個 Move constructor：
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```cpp
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class BigBuffer {
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public:
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...
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BigBuffer(BigBuffer&& src) noexcept {
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std::cout << "BigBuffer move constructor\n";
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bufferSize = src.bufferSize;
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buffer = std::move(src.buffer);
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src.buffer.reset();
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src.bufferSize = 0;
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}
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...
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};
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```
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這個 move constructor 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。
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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 constructor 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：
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```cpp
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BigBuffer buf1;
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// Do something with buf1
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// Assign to buf2
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BigBuffer buf2 = std::move(buf1);
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```
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我們用 `std::move()` 來「移動」物件，這時輸出變成
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```
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BigBuffer constructor // create buf1
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BigBuffer move constructor // move buf1 to buf2, buf1 has nullptr now
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...
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```
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另外一個情形也可以受益於此，假如我們有個 function 會產生 `BigBuffer`，如下：
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```cpp
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BigBuffer BigBufferCreator() {
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std::cout << "BigBufferCreator: Create a BigBuffer!\n";
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BigBuffer tempb;
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// do something
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std::cout << "BigBufferCreator: return\n";
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return tempb;
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}
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BigBuffer b = BigBufferCreator(); // copy tempb to b
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```
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在沒有 Move constructor 的情況下，上面的程式會先產生一個 `tempb`，然後複製給 `b`，訊息：
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```
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BigBufferCreator: Create a BigBuffer!
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BigBuffer constructor
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BigBufferCreator: return
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BigBuffer copy constructor, copy 104857600Bytes // Copy 100MB!
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...
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```
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在有 Move constructor 的情況下，訊息就變成：
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```
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BigBufferCreator: Create a BigBuffer!
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BigBuffer constructor
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BigBufferCreator: return
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BigBuffer move constructor // Use MOVE!
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BigBuffer destructor
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BigBuffer destructor
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```
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因為 `BigBufferCreator()` 產生的就是一個 `BigBuffer` rvalue，所以 compiler 會使用 move constructor（`BigBuffer(BigBuffer&& src)`） 而不是 copy constructor。
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## Move assignment operator(`=`)
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Move assignment operator 的行為跟 move constructor 是一樣的，幫 `BigBuffer` 加入 move assignment operator：
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```cpp
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class BigBuffer {
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public:
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...
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BigBuffer& operator=(BigBuffer&& src) noexcept {
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std::cout << "BigBuffer move operator\n";
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bufferSize = src.bufferSize;
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buffer = std::move(src.buffer);
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src.buffer.reset();
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src.bufferSize = 0;
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return *this;
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}
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...
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};
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```
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測試程式：
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```cpp
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BigBuffer b1, b2;
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b2 = b1;
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```
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訊息：
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```
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BigBuffer constructor
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BigBuffer constructor
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BigBuffer copy operator, copy 104857600Bytes
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```
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還是使用 copy assignment operator 來複製，理由是一樣的，需要一個明確的 `std::move()` 來表示「轉移」的行動，把程式改成：
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```cpp
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BigBuffer b1, b2;
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b2 = std::move(b1);
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```
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這樣就可以了。訊息：
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```
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BigBuffer constructor
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BigBuffer constructor
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BigBuffer move operator // Use MOVE!
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```
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## 參考
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- [Value categories - cppreference.com](https://en.cppreference.com/w/cpp/language/value_category)
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- [rvalue 參考](https://openhome.cc/Gossip/CppGossip/RvalueReference.html)
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- [Move constructors - cppreference.com](https://en.cppreference.com/w/cpp/language/move_constructor)
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- [Move assignment operator - cppreference.com](https://en.cppreference.com/w/cpp/language/move_assignment)
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