C++ won't let me make friends

Question

I have two classes, Mesh and MeshList. I want MeshList to have a function that can change the private members of Mesh. But it won't compile and I don't know why. Here's my code.

Mesh.h

#ifndef _MESH_H #define _MESH_H  #include "MeshList.h" #include <iostream>  class Mesh { private:     unsigned int vboHandle_;     friend void MeshList::UpdateVBOHandle(); public:     inline void Out() {std::cout << vboHandle_;} }; #endif 

Mesh.cpp

#include "Mesh.h" 

MeshList.h

#ifndef _MESH_LIST_H #define _MESH_LIST_H  #include "Mesh.h"   class MeshList {  public:     Mesh *mesh; //Line 11 Error     void UpdateVBOHandle(); }; #endif 

MeshList.cpp

#include "MeshList.h"  void MeshList::UpdateVBOHandle() {     *mesh->vboHandle_ = 4; } 

I get these errors:

MeshList.h (Line 11)

• error C2143: syntax error : missing ';' before '*'
• error C4430: missing type specifier - int assumed. Note: C++ does not support default-int

• error C4430: missing type specifier - int assumed. Note: C++ does not support default-int

• mesh.h(11) : error C2653: 'MeshList' : is not a class or namespace name

• meshlist.cpp(5) : error C2248: 'Mesh::vboHandle_' : cannot access private member declared in class 'Mesh'
• mesh.h(10) : see declaration of 'Mesh::vboHandle_'
• mesh.h(8) : see declaration of 'Mesh'
• meshlist.cpp(5) : error C2100: illegal indirection

Answer 1

When you compile Mesh.cpp, it includes Mesh.h, which includes MeshList.h, which starts to include Mesh.h but stops early because _MESH_H is now defined. Then (back in MeshList.h) there's a reference to Mesh -- but that hasn't been declared yet. Hence, e.g., your C2143 error.