请高手帮帮忙看看小弟的程序

#ifndef NUMBER_H
#define NUMBER_H
class Number;
class Number
{
      private:
              double num;
      public:
             Number();
             Number(double);
             ~Number();
             void setNumber(double);
             double getNumber() const;
             Number operator = (const Number &);
             Number operator + (const Number &);
             Number operator - (const Number &);
             Number operator * (const Number &);
             Number operator / (const Number &);
             friend ostream &operator << (ostream &, const Number &);
             friend istream &operator >> (istream &, Number &);
};

#endif

#include "Number.h"
#include <iostream>
#include <conio.h>
using namespace std;
class Number;
Number::Number()
{
     num=0.0;
}

Number::Number(double n)
{
     num=n;
}

Number::~Number()
{
     // To destroy the memory in this program.
}

void Number::setNumber(double n)
{
     num=n;
}

double Number::getNumber() const
{
    return num;
}

Number Number::operator + (const Number &right)
{
       Number third;
       third.num=num+right.num;
       return third;
}

Number Number::operator - (const Number &right)
{
       Number third;
       third.num=num-right.num;
       return third;
}

Number Number::operator = (const Number &right)
{
       num=right.num;
       return num;
}
       
Number Number::operator * (const Number &right)
{
       Number third;
       third.num=num*right.num;
       return third;
}

Number Number::operator / (const Number &right) 
 
{
       Number third;
       third.num=num/right.num;
       return third;
}

/*ostream &operator << (ostream &strm, const Number &obj)
{
        strm << obj.num;
        return strm;
}
*/
istream &operator >> (istream &strm, Number &obj)
{
        strm >> obj.num;
        return strm;
}
        

#include <iostream>
#include <conio.h>
#include "Number.h"
using namespace std;
class Number;
int main()
{
    Number box1, box2, box3, box4, box5, box6, box7;
    cout << "Enter box1 >> ";
    cin >> box1;
    cout << "Enter box2 >> ";
    cin >> box2;
    box3=box1+box2;
    box4=box1-box2;
    box5=box4;
    box6=box1*box2;
    box7=box1/box2;
    cout << endl;
    cout << "box3 = box1 + box2 = " << box3 << endl;
    cout << "box4 = box1 - box2 = " << box4 << endl;
    cout << "box5 = box4 = " << box5 << endl;
    cout << "box6 = box1 * box2 = " << box6 << endl;
    cout << "box7 = box1 / box2 = " << box7 << endl; 
    getch();
    return 0;
}

小弟的程序不知为何不能运行，请高手帮帮忙下～ 注：小弟是新手～
[解决办法]
using namespace std;
class Number;//这行去掉试试
[解决办法]
这样改就OK了

#ifndef NUMBER_H
#define NUMBER_H

#include <iostream>
#include <conio.h>
using namespace std;

class Number
{
private:
    double num;
public:
    Number();
    Number(double);
    ~Number();
    void setNumber(double);
    double getNumber() const;
    Number operator = (const Number &); 
 
    Number operator + (const Number &);
    Number operator - (const Number &);
    Number operator * (const Number &);
    Number operator / (const Number &);
    friend ostream &operator << (ostream &, const Number &);
    friend istream &operator >> (istream &, Number &);
};

#endif

#include "Number.h"

Number::Number()
{
    num=0.0;
}

Number::Number(double n)
{
    num=n;
}

Number::~Number()
{
    // To destroy the memory in this program.
}

void Number::setNumber(double n)
{
    num=n;
}

double Number::getNumber() const
{
    return num;
}

Number Number::operator + (const Number &right)
{
    Number third;
    third.num=num+right.num;
    return third;
}

Number Number::operator - (const Number &right)
{
    Number third;
    third.num=num-right.num;
    return third;
}

Number Number::operator = (const Number &right)
{
    num=right.num;
    return num;
}

Number Number::operator * (const Number &right)
{
    Number third;
    third.num=num*right.num;
    return third;
}

Number Number::operator / (const Number &right)
{
    Number third;
    third.num=num/right.num;
    return third;
}

ostream &operator << (ostream &strm, const Number &obj)
{
    strm << obj.num;
    return strm;
}

istream &operator >> (istream &strm, Number &obj)
{
    strm >> obj.num;
    return strm;
}

#include "Number.h"

int main()
{
    Number box1, box2, box3, box4, box5, box6, box7;
    cout << "Enter box1 >> ";
    cin >> box1;
    cout << "Enter box2 >> ";
    cin >> box2;
    box3=box1+box2;
    box4=box1-box2;
    box5=box4;
    box6=box1*box2;
    box7=box1/box2;
    cout << endl;
    cout << "box3 = box1 + box2 = " << box3 << endl;
    cout << "box4 = box1 - box2 = " << box4 << endl; 
 
    cout << "box5 = box4 = " << box5 << endl;
    cout << "box6 = box1 * box2 = " << box6 << endl;
    cout << "box7 = box1 / box2 = " << box7 << endl;
    getch();
    
    return 0;
}