class Foo
{
public:
    void   operator() ()
    {
        cout <<"Foo operator"<<endl;
    }
    int operator() (int val)
    {
        return val*10;
    }
};

Foo f;
f();    // Foo operator
cout<< f(5) <<endl;  // 50

Foo是定义了调用操作符()的类,它的对象就相当于函数名,因此operator()取名叫函数对象

---

struct Data {
    double dist;
    double confidence;

    bool operator <= ( double d ) const
    {
        return(dist <  d);
    }
    bool operator <= ( Data& d ) const
    {
        return(dist <=  d.dist);
    }
};


Data a, b;
a.dist = 1;
a.confidence = 1;

b.dist = 1.3;
b.confidence = 1;
if(a <= 1.0)
       cout << " 00000 " << endl;
if(a <= b)
       cout << " 11111 " << endl;