Boost.Intrusive Containers - Elements with different size

Question

In the Boost.Intrusive docs in the chapter "When to use?" https://www.boost.org/doc/libs/1_72_0/doc/html/intrusive/usage_when.html, it is stated that you can use intrusive containers containing objects of different or unknown size.

#include <boost/intrusive/list.hpp>

using namespace boost::intrusive;

//An abstract class that can be inserted in an intrusive list
class Window : public list_base_hook<>
{
   public:
   //This is a container those value is an abstract class: you can't do this with std::list.
   typedef list<Window> win_list;

   //A static intrusive list declaration
   static win_list all_windows;

   //Constructor. Includes this window in the list
   Window()             {  all_windows.push_back(*this);  }
   //Destructor. Removes this node from the list
   virtual ~Window()    {  all_windows.erase(win_list::s_iterator_to(*this));  }
   //Pure virtual function to be implemented by derived classes
   virtual void Paint() = 0;
};

//The static intrusive list declaration
Window::win_list Window::all_windows;

//Some Window derived classes
class FrameWindow :  public Window
{  void Paint(){/**/} };

class EditWindow :  public Window
{  void Paint(){/**/} };

class CanvasWindow :  public Window
{  void Paint(){/**/} };

//A function that prints all windows stored in the intrusive list
void paint_all_windows()
{
   for(Window::win_list::iterator i(Window::all_windows.begin())
                                , e(Window::all_windows.end())
      ; i != e; ++i)
      i->Paint();
}

//...

//A class derived from Window
class MainWindow  :  public Window
{
   FrameWindow   frame_;  //these are derived from Window too
   EditWindow    edit_;
   CanvasWindow  canvas_;

   public:
   void Paint(){/**/}
   //...
};

//Main function
int main()
{
   //When a Window class is created, is automatically registered in the global list
   MainWindow window;

   //Paint all the windows, sub-windows and so on
   paint_all_windows();

   //All the windows are automatically unregistered in their destructors.
   return 0;
}

My problem is that I don't understand when someone would need that (example of usage?). And I dont understand which part in the given example shows this behaviour and why you cannot do this with standard containers.

Answer 1

My problem is that I don't understand when someone would need that

They have measured the performance of their program, and found that the slowdown of using std::vector<Window *> over this is unacceptable. The given example shows a "how to", it isn't an example of the "when to".

And I dont understand which part in the given example shows this behaviour and why you cannot do this with standard containers.

You can't have a std::vector<Window> (nor std::list<Window> etc), because Window is an abstract type. The std:: containers own the Window objects they contain, having allocated space for them.

The intrusive list works because the boost::intrusive::list_base_hook base contains the prev / next pointers, and the objects contain the hook. That means the Window objects can live anywhere, e.g. in main as the example with MainWindow window and it's member Windows.