Circular Dependency c++ - c++

Question

I have the following header files within my code. I know that the problem is that a circular dependency is occurring but I can't seem to solve it.
Any help to fix it?
project.h gets me this ERROR: field ‘location’ has incomplete type
#ifndef PROJECT_H_
#define PROJECT_H_
#include <string.h>
#include "department.h"
class department;
class project{
string name;
department location;
public:
//constructors
//Setters
//Getters
};
#endif
employee.h gets me this ERROR field "‘myDepartment’ has incomplete type"
#ifndef EMPLOYEE_H_
#define EMPLOYEE_H_
#include "department.h"
#include <vector>
class department;
class project;
class employee
{
//attributes
department myDepartment;
vector < project > myProjects;
public:
//constructor
// Distructor
//Setters
//Getters
#endif
department.h
#ifndef DEPARTMENT_H_
#define DEPARTMENT_H_
#include <string.h>
#include "employee.h"
#include "project.h"
#include <vector>
class project;
class employee;
class department{
private:
string name;
string ID;
employee headOfDepatment;
vector <project> myprojects;
public:
//constructors
//Setters
//Getters
};
#endif

Answer 1

You have cyclical #includes.
Try removing #include "employee.h" and #include "project.h" from department.h.
Or vice versa.

Answer 2

You have an include tree like this which will cause you
problems:
project.h
department.h
employee.h
department.h
department.h
employee.h
project.h
normally it is better to make your headers as independent of
other class headers as possible, to do this keep your forward
declarations but remove the includes, then in the .cpp file
include the headers.
e.g.
class project;
class employee;
class department {
...
employee* headOfDepartment;
vector<project*> myprojects;
then in department.cpp
include employee.h and project.h and instantiate the members in your constructor, to make it even better use unique_ptr so you don't need to bother about deleting them:
class department {
...
std::unique_ptr<employee> headOfDepartment;
std::vector<std::unique_ptr<project>> myprojects;
another tip is to not have using namespace std in the header, instead include the namespace e.g. std::vector<...>