What difference does const make?

College(string clg_name, int num_dept, Department &dp,  Student &s)
    {
        m_clgName=clg_name;  
        m_numDept=num_dept;
        dep=dp;
        st=s;
    }

error: No matching function for call to 'College::College(const char[5],int ,Department, student)
 no known conversion for argument 3 'Department' from 'Department&'

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 Solution either: College(string clg_name, int num_dept, Department dp,  Student s)
  or    College(string clg_name, int num_dept, const Department &dp, const Student &s)

#ifndef STUDENT_H
#define STUDENT_H

#include <iostream>
#include <cstring>

using namespace std;

class Student
{
    string m_stName;
    int m_stID;
    //
public:
    Student(){}
    Student(string name, int id){StdInfo(name,id);}
    string GetStdName() {return m_stName;}
    int GetStdID(){return m_stID;}
    void StdInfo(string name, int id)
    {
        m_stName=name;
        m_stID=id;
    }
    friend ostream& operator<<(ostream &os, const Student &st);
    friend istream& operator>>(istream &is, Student &st);
};
ostream& operator<<(ostream &os, const Student &st)
{
    os<<"\nStudent's name: "<<st.m_stName<<", ID: "<<st.m_stID<<endl;
    return os;
}
istream& operator>>(istream &is, Student &st)
{
    cout << "\nEnter student's name: ";
    is >> st.m_stName;
    cout << "\nEnter student's ID: ";
    is >> st.m_stID;
    return is;
}
#endif // STUDENT_H 

#ifndef DEPARTMENT_H
#define DEPARTMENT_H

#include <iostream>
#include <cstring>

using namespace std;

class Department
{
    string m_DeptName;
//
public:
    Department(){}
    Department(string name){ setDepName(name);}
    void setDepName(string name){ m_DeptName=name;}
    string getDeptName(){ return m_DeptName;}
    friend ostream& operator<<(ostream &os, const Department &dp);
    friend istream& operator>>(istream &is, Department &dp);
};

ostream& operator<<(ostream &os, const Department &dp)
{
    os<<"Department's name: "<<dp.m_DeptName<<endl;
    return os;
}
istream& operator>>(istream &is, Department &dp)
{
    cout << "\nEnter Department name: ";
    is >> dp.m_DeptName;
    return is;
}
#endif // DEPARTMENT_H 

#include <iostream>
#include <string>
#include <cstring>
#include "Student.h"
#include "Department.h"

using namespace std;


class College
{
    string m_ClgName;
    int m_numDept;
    Department dep;
    Student st;
    //Department *dep=new Department[3];
    //Student *st=new Student[4];
    College(){}
public:
    College(string clg_name, int num_dept, Department &dp, Student &s)
    {
        m_ClgName=clg_name;
        m_numDept=num_dept;
        dep=dp;
        st=s;
    }
    friend ostream& operator<<(ostream &os, const College &cl);
    friend istream& operator>>(istream &is , College &cl);
    void SetClgDetails()
    {cin >> dep;cin >> st;}
};

ostream& operator<<(ostream &os, const College &cl)
{
    os <<cl.m_ClgName<<" has "<<cl.m_numDept<<" departments\n";
    cout <<cl.dep;
    cout <<cl.st<<endl;
}
istream& operator>>(istream &is , College &cl)
{
    cout << "\nEnter college name :";
    is >> cl.m_ClgName;
    cout << "\nEnter no. of departments: ";
    is >> cl.m_numDept;
}
int main()
{

    College clg("ABCD", 1, Department("Computer_Science"),Student("Rajender",249));
    cout << clg;
    cout << "===============================================";
    cin >>clg;
    clg.SetClgDetails();
    cout <<clg;
    return 0;
} 

Department *dp = new Department[size_d];
Student *st= new Student[size_s];