plus operator overloading

#include<iostream>
using namespace std;
class box
{
    private:
    double length,breadth,height;

    public:
    double getvolume()
    {
        return length*breadth*height;
    }
    void setlength(double l)
    {
        length=l;
    }
    void setbreadth(double b)
    {
        breadth=b;
    }
    void setheight(double h)
    {
        height=h;
    }
    box operator+(const box&b)
    {
        box box;
        box.length=this->length+b.length;
        box.breadth=this->breadth+b.breadth;
        box.height=this->height+b.height;
        return box;
    }
    box operator-(const box&m)
    {
        box box;
        box.length=this->length-m.length;
        box.breadth=this->breadth-m.breadth;
        box.height=this->height-m.height;
        return box;
    }
};
int main()
{
    double vol1,vol2,vol3,vol4,vol5;
    box box1;
    box box2;
    box box3;
    box box4;
    box box5;
    box1.setlength(2.5);
    box1.setbreadth(5.5);
    box1.setheight(7.5);

    box2.setlength(12.5);
    box2.setbreadth(15.5);
    box2.setheight(17.5);

    box4.setlength(2.5);
    box4.setbreadth(1.5);
    box4.setheight(1.5);

    vol1=box1.getvolume();
    vol2=box2.getvolume();
    vol4=box4.getvolume();
    cout<<endl<<"box1 volume is "<<vol1<<endl;
    cout<<"box2 volume is "<<vol2<<endl;
    cout<<"box4 volume is "<<vol4<<endl;

    box3=box1+box2+box4;
    vol3=box3.getvolume();
    box5=box2-box1;
    vol5=box5.getvolume();
    cout<<endl<<"overloads unuary + operator:"<<endl;
    cout<<"box3 volume is "<<vol3<<endl; //box3=box1+box2+box4
    cout<<endl<<"overloads unuary - operator:"<<endl;
    cout<<"box5 volume is "<<vol5<<endl; //box5=box2-box1
    
    cout<<endl<<"overloads [] operator:"<<endl;
    int intArray[1024];
    for (int i = 0, j = 0; i < 1024; i++) 
    {
        intArray[i] = j++;
    }
    // 512
    cout << intArray[512] << endl;
    // 257
    cout << 257 [intArray] << endl;
}