Friend function implementation

/* Friend function implementation
*/
#include <iostream>
#include <cstdlib>

using namespace std;

class Metre;
class Centimetre{
    double length;
    public:
        inline Centimetre();
        inline Centimetre(double);
        inline ~Centimetre();
        inline void input();
        inline friend void compare(Centimetre&, Metre&);
};

class Metre{
    double length;
    public:
        inline Metre();
        inline Metre(double);
        inline ~Metre();
        inline void input();
        friend void compare(Centimetre&, Metre&);
};

void compare(Centimetre &c_obj, Metre & m_obj){
    if(c_obj.length > (m_obj.length * 100)){
        cout << c_obj.length << " cm is greater\n";
    }
    else {
        cout << m_obj.length << " m is greater\n";
    }
}

Metre::Metre(){
    length = 0.0;
}

Centimetre::Centimetre(){
    length = 0.0;
}

Centimetre::Centimetre(double length){
    Centimetre::length = length;
}

Centimetre::~Centimetre(){
    cout << "Centimetre object is destroyed!" << endl;
}

Metre::Metre(double length){
    Metre::length = length;
}

Metre::~Metre(){
    cout << "Metre object is destroyed!" << endl;
}

void Centimetre::input(){
    cout << "Enter the length in cm: ";
    cin >> length;
}

void Metre::input(){
    cout << "Enter the length in m: ";
    cin >> length;
}

int main() {
    system("cls");
    Centimetre c_obj;
    Metre m_obj;
    c_obj.input();
    m_obj.input();
    compare(c_obj, m_obj);
    system("pause");
    return 0;
}