c++ - Why doesn't foward declaration of class work when class is included in another class -

this compiles

#include "sprite.h"  class gameobject {   public:       int x, y, w, h;       sprite sprite;   public:     gameobject();     gameobject(int _x, int _y, int _w, int _h);     virtual ~gameobject(); }; 

this doesn't

class sprite;  class gameobject {   public:       int x, y, w, h;       sprite sprite;   public:     gameobject();     gameobject(int _x, int _y, int _w, int _h);     virtual ~gameobject(); }; 

i know forward declare , use pointer sprite why doesn't forward declaration works here. doesn't "class sprite;" tells sprite exists? i'm trying #include classes in .cpp , avoid in .h @ cost. classes not including each other there no need use sprite*. guess understanding of forward declaring is wrong or because don't see reason why doesn't work.

thanks in advance.

pretend you're compiler. without having complete declaration of sprite @ disposal, how possible determine whether sprite 1 byte big, or 1 hundred thousand bytes big?

you don't need know class when need pointer class (or reference class, or few other minor things); when need use class, mere forward declaration not sufficient. it's not enough know "sprite exists"; it's necessary know how big is, too. , without complete declaration, that's not possible.