Simplified and improved the convenience and encapsulation of the system.

library/phx/system.hpp

@@ -23,8 +23,6 @@
 #ifndef LIBRARY_PHX_SYSTEM_HPP_
 #define LIBRARY_PHX_SYSTEM_HPP_
 
-#include <string>
-
 #include "phx/export.hpp"
 #include "phx/frame_timer.hpp"
 #include "phx/loggable.hpp"
@@ -34,21 +32,13 @@ class Engine;
 
 class PHOENIX_EXPORT System : public Loggable {
  public:
-  System() = delete;
   virtual ~System() = default;
 
   virtual void Prepare() {}
   virtual void Update(const FrameTimer::TimeInfo&) {}
 
-  Engine* GetEngine() const;
-
  protected:
-  explicit System(Engine* engine);
-  System(const System&) = delete;
-  System(System&&) = default;
-
-  System& operator=(const System&) = delete;
-  System& operator=(System&&) = default;
+  friend Engine;
 
   Engine* engine_ = nullptr;
 };