127 lines
3.5 KiB
Markdown
127 lines
3.5 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。
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## Move assignment operator
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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
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BigBuffer copy constructor, copy 104857600Bytes
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BigBuffer destructor
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BigBuffer destructor
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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 assignment operator（就是 `=`）就可以派上用場了。
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幫 BigBuffer 加一個 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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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。
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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 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：
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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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```
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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) |