How to resolve this circular dependency?

Question

I have a interface for element that returns the mesh for the current element. I have mesh which is collection of element. The mesh also has reference to mesh because it want to manipulate mesh. When ever I try to compile I get error saying invalid use of incomplete type ‘class Mesh’.

AbsElem.h

class Mesh;

class AbsElem{
  virtual Mesh& GetMesh()const = 0;
};

Elem1.h

#include "AbsElem.h"
#include <vector>

class Elem1 : public AbsElem{

  Elem1(Mesh& m):my_mesh(m){}
  virtual Mesh& GetMesh() const{return my_mesh;}

  void DoStuff(){
   my_mesh.GetVal();
  }

  Mesh& my_mesh;
};

Mesh.h

#include "Elem1.h"

class Mesh{
  public: 
  int GetVal(){return 1;}
  private:
  std::vector<Elem1> collect_elem_;
};

main.cpp

#include "Mesh.h"

int main(){
  Mesh m;
}

compile by : g++ main.cpp

compilation error :

In file included from Mesh.h:1, from main.cpp:1: Elem1.h: In member function ‘void Elem1::DoStuff()’: Elem1.h:10:4: error: invalid use of incomplete type ‘class Mesh’ my_mesh.GetVal(); ^~~~~~~ In file included from Elem1.h:1, from Mesh.h:1, from main.cpp:1: AbsElem.h:2:7: note: forward declaration of ‘class Mesh’ class Mesh; ^~~~