c++ - Odd behavior with classes in separate files -

this question has answer here:

• when can use forward declaration? 12 answers

i have classes this:

world.h:

#ifndef world_h_ #define world_h_ #include "gameobject.h" #include <vector>  class world { public: std::vector<gameobject*> objects;  world(); virtual ~world(); void add(gameobject*); void initialize(); void update(); void render(); };  #endif /* world_h_ */ 

gameobject.h:

#ifndef gameobject_h_ #define gameobject_h_ #include "util/point.h" #include "world.h"  class gameobject { public: world *world; point *position; gameobject(); virtual ~gameobject();  virtual void update(); virtual void render(); };  #endif /* gameobject_h_ */ 

why give error:
"world.h, line 9 - 'gameobject' has not been declared " and
"world.h, line 13 - 'gameobject' not declared in scope"?

i using linux gcc 4.8.1 compiler.

you have circular include dependency. in case, can use forward declarations instead of includes:

#ifndef world_h_ #define world_h_ #include <vector>  class gameobject; // forward declaration  class world {   // before }; #endif 

and

#ifndef gameobject_h_ #define gameobject_h_  class world; class point;  class gameobject {   // before }; #endif 

then, include headers in implementation files need them.

for more details, see when use forward declarations.