Nạp chồng toán tử nhị phân trong C++

#include <iostream>
using namespace std;
 
class Box
{
public:
    double tinhTheTich(void)
    {
        return chieudai * chieurong * chieucao;
    }
    void setChieuDai(double dai)
    {
        chieudai = dai;
    }      
    void setChieuRong(double rong)
    {
        chieurong = rong;
    } 
    void setChieuCao(double cao)
    {
        chieucao = cao;
    }
 
    // Nap chong toa tu + de cong hai doi tuong Box.
    Box operator+(const Box& b)
    {
        Box box;
        box.chieudai = this->chieudai + b.chieudai;
        box.chieurong = this->chieurong + b.chieurong;
        box.chieucao = this->chieucao + b.chieucao;
        return box;
    }
private:
    double chieudai;      // chieu dai cua mot box
    double chieurong;     // Chieu rong cua mot box
    double chieucao;      // Chieu cao cua mot box
};
// ham main cua chuong trinh
int main()
{
    Box Box1;                // Khai bao Box1 la cua kieu Box
    Box Box2;                // Khai bao Box2 la cua kieu Box
    Box Box3;                // Khai bao Box3 la cua kieu Box
    double thetich = 0.0;    // Luu giu the tich cua mot box tai day
 
    // thong tin chi tiet cua box 1
    Box1.setChieuDai(3.0);
    Box1.setChieuRong(4.0);
    Box1.setChieuCao(5.0);
 
    // thong tin chi tiet cua box 2
    Box2.setChieuDai(6.0);
    Box2.setChieuRong(7.0);
    Box2.setChieuCao(8.0);
 
    // the tich cua box 1
    thetich = Box1.tinhTheTich();
    cout << "The tich cua Box1 la: " << thetich << endl;
 
    // the tich cua box 2
    thetich = Box2.tinhTheTich();
    cout << "The tich cua Box2 la: " << thetich << endl;
    // cong hai doi tuong nhu sau:
    Box3 = Box1 + Box2;
    // the tich cua box 3
    thetich = Box3.tinhTheTich();
    cout << "The tich cua Box3 la: " << thetich << endl;
    return 0;
}