Implement pimpl idiom by using smart pointer

--- tags: c++, c++11, pimpl --- # Implement pimpl idiom by using smart pointer  --- autotime.h ```cpp= #ifdef _WIN32 #include <windows.h> #else #include <sys/time.h> #endif #include <string> ``` ```cpp=+ class AutoTimer { public: explicit AutoTimer(const std::string& name); ~AutoTimer(); //解構時顯示出經過了多久時間 private: double GetElapsed() const; std::string m_Name; #ifdef _WIN32 DWORD m_StartTime; #else struct timeval m_StartTime; #endif }; ```  --- autotime.h, Pimpl idiom ```cpp= #include <string> ``` ```cpp=+ class AutoTimer { public: explicit AutoTimer(const std::string& name); ~AutoTimer(); private: class Impl; // forward declaration Impl *m_Impl; // opaque pointer }; ```  --- autotime.cpp ```cpp= #include "autotime.h" #ifdef _WIN32 #include <windows.h> #else #include <sys/time.h> #endif ``` ```cpp=+ class AutoTimer::Impl { public: AutoTimer::Impl(const std::string& name) : m_Name(name) { ... } AutoTime::~Impl() { ... } private: double GetElapsed() const { ... } std::string m_Name; #ifdef _WIN32 DWORD m_StartTime; #else struct timeval m_StartTime; #endif }; AutoTimer::AutoTimer(const std::string& name) : m_Impl(new AutoTimer::Impl(name)) { ... } // 印出時間, delete m_Impl AutoTimer::~AutoTimer() { ... } ```  --- ## PIMPL Idiom (Pointer to IMPLementation) * 避免在header file公開private data的一種方式 * C++上的一種解決辦法，嚴格來說不算Design Pattern * 特殊情況的Bridge Pattern  --- ## Compilation Firewall ![](https://i.imgur.com/4hUl2wg.png) --- ## Pros * 資訊隱藏 * 解耦 * 編譯變快 * ABI compatible * Lazy allocation  --- ## Cons * Runtime overhead * Maintenance overhead  --- ## 前面範例的問題 * raw pointer * 需要記得實作copy ctor，免得有人誤用  --- ## Demo code [Pimpl Demo](https://github.com/weisunglee/PimplDemo)  --- ## Using smart pointer Widget.h ```cpp= #include <memory> class Widget { public: Widget(); void Foo(); private: class Impl; std::unique_ptr<Impl> m_Impl; }; ```  --- Widget.cpp ```cpp= #include "Widget.h" #include <iostream> ``` ```cpp=+ class Widget::Impl { public: void Foo() { std::cout << "Foo" << std::endl; } }; Widget::Widget() : m_Impl(std::make_unique<Impl>()) {} void Widget::Foo() { m_Impl->Foo(); } ```  --- # But...  --- ## Compile Error ```bash error: invalid application of 'sizeof' to an incomplete type 'Widget::Impl' static_assert(sizeof(_Tp) > 0, ^~~~~~~~~~~ ```  --- std::unique_ptr, defined in header \<memory\> ```cpp= template< class T, class Deleter = std::default_delete<T> > class unique_ptr; ``` * std::unique_ptr may be constructed for an incomplete type T  --- * complier 自動產生的 dtor 為inline * 需要自己定義widget的 dtor  --- ## Rule of three/five * destructor * copy constructor * copy assignment operator * move constructor (C++11) * move assignment operator (C++11)  --- std::shared_ptr, defined in header \<memory\> ```cpp= template< class T > class shared_ptr; ``` * 用shared_ptr就不需要自己定義dtor * 但通常pimpl idiom用unique_ptr比較符合現況  --- ## Pimpl template，讓實作pimpl時更方便  --- 當Widget::Impl需要帶有參數的ctor時該怎麼辦  --- 設計API時要注意const member function在pimpl下的合理性 > std::experimental::propagate_const  --- ## 結論 * 設計API時，使用pimpl idiom對用API的人來說是方便的 * 大型專案中，努力讓編譯更快是值得的  --- ## Reference * Martin Reddy, API Design for C++ * Scott Meyers, Effective Modern C++ * https://en.cppreference.com * https://www.fluentcpp.com * Sutter’s Mill, https://herbsutter.com/gotw/_100/ * Sutter’s Mill, https://herbsutter.com/gotw/_101/  --- ## Q&A  --- # C++20 : Modules