编程中碰到个问题~

能帮我看一下这个吗？谢谢。

#include "stdafx.h"
#include <iostream>
using namespace std;
class distance
{
    public:
        float inch;
        float feet;
    distance()
        {
            inch = 0;
            feet = 0;
        }
        distance(float k, float t)
        {
            inch = k;
            feet = t;
        }
        float displaydistance()
        {
            cout << "i= " << inch << endl;
            cout << "f= " << feet << endl;
        }
        distance operator-()
        {
            inch = -inch;
            feet = -feet;
            return distance(inch, feet);
        }    };
int main()
{
    distance d1(11, 10), d2(-5, 11);
    -d1;
    -d2;
    d1.displaydistance();
    d2.displaydistance();
    return 0;
}
每次系统都提示主函数的 'distance': ambiguous symbol。