大家帮我看一下这个程序哪里有问题

设计个计算器类Calculator，它只有一个用于计数的数据成员count。该计算器的有效计数范围是0~65535，实现计数器的前自增，后自增，前自减，后自减，两个计数器想加减等运算。
下面是我写的程序，但是总是“有一个无法解析的外部xx”，哪里有问题阿？？？
#include<iostream>
using namespace std;
class Calculator{
private:
    int count;
public:
    Calculator(int i=0){count=i;}
    Calculator & operator++();
    Calculator & operator++(int);
    Calculator & operator--();
    Calculator & operator--(int);
    Calculator operator+(Calculator a);
    Calculator operator<<(Calculator a);
    Calculator operator>>(Calculator a);
};
Calculator & Calculator::operator++(){
    ++count;
    return *this;
}
Calculator & Calculator::operator++(int){
    count++;
    return *this;
}
Calculator & Calculator::operator--(){
    --count;
    return *this;
}
Calculator & Calculator::operator--(int){
    count--;
    return *this;
}
Calculator Calculator::operator+(Calculator a){return Calculator(count+a.count);}
Calculator Calculator::operator<<(Calculator a)
{cout<<count<<endl;
return count;}
Calculator Calculator::operator>>(Calculator a)
{cin>>count;
return count;}
 
void main()
{
    int a;
    Calculator b;
    Calculator sum;
    Calculator c;
    cout<<"请输入数字： "<<endl;
    cin>>a;
    if(a>65535||a<0)
    {cout<<"该数字超出该计数器的有效计数范围！"<<endl;}
    else{
    cout<<"请再次输入该数字： "<<endl;
     b.operator>>(b);
    ++b;
    cout<<"前自增的结果是："<<endl;
     b.operator<<(b);
    b++;
    cout<<"后自增的结果是："<<endl;
    b.operator<<(b);
    --b;
    cout<<"前自减的结果是："<<endl;
    b.operator<<(b);
    b--;
    cout<<"后自减的结果是："<<endl;
    b.operator<<(b);
    cout<<"请输入要加的数字："<<endl;
    c.operator>>(c);
    sum=c + b;
    sum.operator<<(sum);}
}