Unary operator sign change in c + +

#include<iostream.h>
#include<conio.h>
class unary
{
private:
int a,b,c;
public:
void input(int x,int y,int z)
{
a=x;
b=y;
c=z;
}
void display()
{
cout<<"\n a="<<a;
cout<<"\n b="<<b;
cout<<"\n c="<<c;
}
void operator-()
{
a=-a;
b=-b;
c=-c;
}
};
int main()
{
clrscr();
unary u1;
//u1.input();
u1.input(56,-45,-32);
u1.display();
-u1;
u1.display();
getch();
return 0;
}