今天继续看书中，在练习的时候发现了以下的问题：

 

#include<iostream>
using namespace std;

enum sign{plus,minus};

class Currency{
 friend ostream & operator<<(ostream &,const Currency &);
public:
 Currency(sign s=plus,unsigned long d=0,unsigned int c=0);
 ~Currency(){}

 bool Set(sign s,unsigned long d,unsigned int c);
 bool Set(float a);

 sign Sign() const
 {
  if(amount<0) return minus;
  else return plus;
 }
 unsigned long Dollars() const
 {
  if(amount<0) return (-amount)/100;
  else return amount/100;
 }
 unsigned int Cents() const
 {
  if(amount<0)
   return -amount-Dollars()*100;
  else
   return amount-Dollars()*100;
 }

 Currency operator+(const Currency &x) const;
 Currency& operator+=(const Currency &x)
 {
  amount+=x.amount;
  return *this;
 }

private:
 long amount;
};

Currency::Currency(sign s,unsigned long d,unsigned int c)
{
 if(c>99)
 {
  //cerr<<"Cents should be<100"<<endl;
  exit(1);
 }

 amount=d*100+c;
 if(s==minus) amount=-amount;
}

bool Currency::Set(sign s,unsigned long d,unsigned int c)
{
 if(c>99) return false;

 amount=d*100+c;
 if(s==minus) amount=-amount;
 return true;
}

bool Currency::Set(float a)
{
 sign sgn;
 if(a<0) {sgn=minus;a=-a;}
 else sgn=plus;

 amount=(a+0.001)*100;

 if(sgn==minus) amount=-amount;

 return true;
}

Currency Currency::operator+(const Currency &x) const
{
 Currency y;
 y.amount=amount+x.amount;
 return y;
}

ostream & operator<<(ostream & out,const Currency &x)
{
 long a=x.amount;
 if(a<0) {out<<'-';a=-a;}
 
 long d=a/100;
 out<<'$'<<d<<'.';
 
 int c=a-d*100;
 if(c<10) out<<'0';
 out<<c;
 return out;
}

int main()
{
 Currency g,h(plus,3,50),i,j;

 g.Set(minus,2,25);
 i.Set(-6.45f);

 j=h+g;
 cout<<j<<endl;
 //print(j);cout<<endl;

 i+=h;
 cout<<i<<endl;
 //print(i);cout<<endl;

 j=i+g+h;
 cout<<j<<endl;
 //print(j);cout<<endl;

 j=(i+=g)+h;
 cout<<j<<endl;
 //print(j);cout<<endl;
 cout<<i<<endl;
 //print(i);cout<<endl;

 return 0;
}

 

--------------------Configuration: 005 - Win32 Debug--------------------
Compiling...
005.cpp
C:\Documents and Settings\Administrator\桌面\005\005.cpp(88) : error C2248: 'amount' : cannot access private member declared in class 'Currency'
        C:\Documents and Settings\Administrator\桌面\005\005.cpp(41) : see declaration of 'amount'
C:\Documents and Settings\Administrator\桌面\005\005.cpp(108) : error C2593: 'operator <<' is ambiguous
C:\Documents and Settings\Administrator\桌面\005\005.cpp(112) : error C2593: 'operator <<' is ambiguous
C:\Documents and Settings\Administrator\桌面\005\005.cpp(116) : error C2593: 'operator <<' is ambiguous
C:\Documents and Settings\Administrator\桌面\005\005.cpp(120) : error C2593: 'operator <<' is ambiguous
C:\Documents and Settings\Administrator\桌面\005\005.cpp(122) : error C2593: 'operator <<' is ambiguous
Error executing cl.exe.

005.exe - 6 error(s), 0 warning(s)

 

　　这个问题很棘手，看了半天也没有明白到底是哪里出了问题。在网上搜索了一下，发现了类似的情况。这个问题是命名空间和友元的冲突，据说是VC++6.0编译器的BUG所导致的。

　　按照下面的方式修改的话，就能够通过编译了：

 

#include<iostream.h>
//using namespace std;

enum sign{plus,minus};

class Currency{
 friend ostream & operator<<(ostream &,const Currency &);
public:
 Currency(sign s=plus,unsigned long d=0,unsigned int c=0);
 ~Currency(){}

 bool Set(sign s,unsigned long d,unsigned int c);
 bool Set(float a);

 sign Sign() const
 {
  if(amount<0) return minus;
  else return plus;
 }
 unsigned long Dollars() const
 {
  if(amount<0) return (-amount)/100;
  else return amount/100;
 }
 unsigned int Cents() const
 {
  if(amount<0)
   return -amount-Dollars()*100;
  else
   return amount-Dollars()*100;
 }

 Currency operator+(const Currency &x) const;
 Currency& operator+=(const Currency &x)
 {
  amount+=x.amount;
  return *this;
 }

private:
 long amount;
};

Currency::Currency(sign s,unsigned long d,unsigned int c)
{
 if(c>99)
 {
  //cerr<<"Cents should be<100"<<endl;
  //exit(1);
 }

 amount=d*100+c;
 if(s==minus) amount=-amount;
}

bool Currency::Set(sign s,unsigned long d,unsigned int c)
{
 if(c>99) return false;

 amount=d*100+c;
 if(s==minus) amount=-amount;
 return true;
}

bool Currency::Set(float a)
{
 sign sgn;
 if(a<0) {sgn=minus;a=-a;}
 else sgn=plus;

 amount=(a+0.001)*100;

 if(sgn==minus) amount=-amount;

 return true;
}

Currency Currency::operator+(const Currency &x) const
{
 Currency y;
 y.amount=amount+x.amount;
 return y;
}

ostream & operator<<(ostream & out,const Currency &x)
{
 long a=x.amount;
 if(a<0) {out<<'-';a=-a;}
 
 long d=a/100;
 out<<'$'<<d<<'.';
 
 int c=a-d*100;
 if(c<10) out<<'0';
 out<<c;
 return out;
}

int main()
{
 Currency g,h(plus,3,50),i,j;

 g.Set(minus,2,25);
 i.Set(-6.45f);

 j=h+g;
 cout<<j<<endl;
 //print(j);cout<<endl;

 i+=h;
 cout<<i<<endl;
 //print(i);cout<<endl;

 j=i+g+h;
 cout<<j<<endl;
 //print(j);cout<<endl;

 j=(i+=g)+h;
 cout<<j<<endl;
 //print(j);cout<<endl;
 cout<<i<<endl;
 //print(i);cout<<endl;

 return 0;
}