#include<iostream>
#include<cstdlib>
#include<cctype>
using namespace std;
class Money
{
public:
    friend Money add(Money amount1,Money amount2);
    friend bool equal(Money amount1,Money amount2);//我已经把这个设为了友员函数为什么还为出现这样的错误
    Money(long dollars ,int cents);
    Money(long dollars);
    Money();
    double get_value();
    void input(istream& ins);
    void output(ostream& outs);
private:
    long all_cents;

};
int digit_to_int(char c);
int main()
{
  Money your_amount,my_amount(10,9),our_amount;
  cout<<"请输入一个金额：";
  your_amount.input(cin);
  cout<<"你的金额是：";
  your_amount.output(cout);
  cout<<endl;
  cout<<"我的金额是：";
  my_amount.output(cout);
  cout<<endl;
  if(equal(your_amount,my_amount))
      cout<<"你的钱和我的一样多：\n";
  else
      cout<<"我们一个人富点：\n";
  our_amount=add(your_amount,my_amount);
  your_amount.output(cout);
  cout<<"+";
  my_amount.output(cout);
  cout<<" equals";
  our_amount.output(cout);
  cout<<endl;
  return 0;

}
Money add(Money amount1,Money amount2)
{
  Money temp;
  temp.all_cents=amount1.all_cents+amount2.all_cents;
  return temp;
}
bool equal(Money amount1, Money amount2)
{
  return (amount1.all_cents==amount2.all_cents);

Text100.exe - 2 error(s), 0 warning(s)

}
Money::Money(long dollars):all_cents(dollars*100)
{}
Money::Money():all_cents(0)
{}
double Money::get_value()
{return (all_cents*0.01);}
void Money::input(istream& ins)
{
  char one_char,decimal_point,digit1,digit2;
  long dollars;
  int cents;
  bool negative;
  ins>>one_char;
  if(one_char=='-')
  {
    negative=true;ins>>one_char;
  }
  else negative=false;
  ins>>dollars>>decimal_point>>digit1>>digit2;
  if(one_char!='s'||decimal_point!='.'||!isdigit(digit1)||!isdigit(digit2))
  {
      cout<<"输入错误：\n";
          exit(1);
  }
   cents=digit_to_int(digit1)*10+digit_to_int(digit2);
   all_cents=dollars*100+cents;
   if(negative)
       all_cents=-all_cents;
}
void Money::output(ostream& outs)
{
  long positive_cents,dollars,cents;
  positive_cents=labs(all_cents);
  dollars=positive_cents/100;
  cents=positive_cents%100;
  if(all_cents<0)
      outs<<"-$"<<dollars<<'.';
  else
      outs<<"$"<<dollars<<'.';
  if(cents<10)
      outs<<'0';
  outs<<cents;

}
int digit_to_int(char c)
{return (int(c)-int(0));}



错误：ext100.cpp
D:\My Documents\Text100.cpp(54) : error C2248: 'all_cents' : cannot access private member declared in class 'Money'
        D:\My Documents\Text100.cpp(17) : see declaration of 'all_cents'
D:\My Documents\Text100.cpp(54) : error C2248: 'all_cents' : cannot access private member declared in class 'Money'
        D:\My Documents\Text100.cpp(17) : see declaration of 'all_cents'
Error executing cl.exe.

Text100.exe - 2 error(s), 0 warning(s)

return也是个函数 它不是友员所以不能直接访问成员变量,我建议你用if语句试一下

amount1.all_cents==amount2.all_cents
all_cents这个数据成员是PRIVATE，对象不能访问。
解决：
设一PUBLIC函数，处理PRIVATE变量。如：
public:
  long getVal(){return this->all_cents;}
在友元函数中：
amount1.getVal（）就可以操作私有变量。

上面是我回答错误，这里最主要大概是与stl中的equal同名了，所以用using namespace std是由漏洞的。
但是我也没搞明白stl怎么声明equal，让同名的没有重载，反而把友元给弄掉了，怪阿。顶阿。