PIMPL & Compile-Time Firewalls

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// widget.hpp
class Widget {
public:
    Widget();
    ~Widget();                       // must be declared here
    Widget(Widget&&) noexcept;
    Widget& operator=(Widget&&) noexcept;
    void draw();
private:
    struct Impl;
    std::unique_ptr<Impl> p_;
};

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// widget.cpp
struct Widget::Impl {
    std::vector<Shape> shapes;
    Renderer           renderer;
};

Widget::Widget() : p_(std::make_unique<Impl>()) {}
Widget::~Widget() = default;         // Impl is complete HERE
void Widget::draw() { p_->renderer.draw(p_->shapes); }