编译通过,但是连接出错，请问是什么原因？

#include <iostream.h>
#include <stdlib.h>

const int len=50;
class stack
{
protected:
    int head;
    int stack[len];
public:
    stack（){head=0;}
    void push（int val)
    {
        head++;
        stack[head]=val;
    }
    int pop（)
    {
        int temp;
        temp=stack[head];
        head--;
        return temp;
    }
};

class opstack:public stack
{
public:
    int push（int val)
    {
        if（head>len)
        {
            cout<<"stack underflow!"<<endl;
            exit（1);
        }
        return stack::pop（);
    }
};

class usestack:public opstack
{
public:
    void productstack（)
    {
        int val;
        cout<<"input data:";
        cin>>val;
        while（（val<0) || （val>100))
        {
            cout<<"input error!"<<endl;
            cout<<"please input again:";
            cin>>val;
        }
        opstack::push（val);
    }
};

void main（)
{
    usestack a;
    a.productstack（);
    a.productstack（);
    cout<<"->"<<a.pop（)<<endl;
    cout<<"->"<<a.pop（)<<endl;
}

#include <iostream> [color=#ff0000]//修改的地方[/color]
#include <string> [color=#ff0000]//修改的地方[/color]
using namespace std; [color=#ff0000]//修改的地方[/color]

const int len=50;

class station
{
protected:
    char fromstation[len];
    char tostation[len];
public:
    station（char fs[],char ts[])
    {
        strcpy（fromstation,fs);
        strcpy（tostation,ts);
    }

    void inputvalue（)
    {
        cout<<"enter from station:";
        cin>>fromstation;
        cout<<"enter to station:";
        cin>>tostation;
    }
    void display（)
    {
        cout<<"going from "<<fromstation<<" station to "<<tostation<<" station";
    }
};

class mile
{
protected:
    int mile;
public:
    mile（int m)
    {
        mile=m;
    }
    void inputmile（)
    {
        cout<<"enter mile:";
        cin>>mile;
    }
    void display（)
    {
        cout<<"is "<<mile<<" miles";
    }
};

class price:public station,public mile
{
    int price;
public:
    price（char ff[],char tt[],int mm,int pp):station（ff,tt),mile（mm)
    {
        price=pp;
    }
    void getprice（)
    {
        station::inputvalue（);
        mile::inputmile（);
        cout<<"enter price:";
        cin>>price;
    }
    void display（)
    {
        station::display（);
        mile::display（);
        cout<<",the price is "<<price<<endl;
    }
};

int main（) [color=#ff0000]//修改的地方[/color]
{
    price a（"beijing","shanghai",1200,50);
    a.display（);
}