我想实现一个抽象工厂模式，但也想成为一个单例。classWindowFactory{protected:virtualScrollbar*createScrollbar()=0;};classMacWindowFactory:publicWindowFactory{virtualScrollbar*createScrollbar(){//returnainstance};};classLinuxWindowFactory:publicWindowFactory{virtualScrollBar*createScrollbar(){//returnainstance};};谁能帮我提供一