vc6.0 中关于友元的一个问题

c++中友元函数无法访问私有成员的问题。

近来学习c++的类时，用vc6.0编译一个测试类的程序时，总是出现一个错误，但在其他的编译器上却可以编译通过。现在把这个错误记录下来。

#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);Money(long dollars,int sents);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 << "Enter an amount of money:";your_amount.input(cin);cout<<"your amount is ";cout<<endl;your_amount.output(cout);cout<<"my amount is ";my_amount.output(cout);cout<<endl;if(equal(your_amount,my_amount))cout<<"We have the same amounts./n";elsecout<<"One of us is richer/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);}Money::Money(long dollars,int cents){if(dollars*cents<0){cout<<"wrong";exit(1);}all_cents=dollars*100+cents;}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;}elsenegative=false;ins>>dollars>>decimal_point>>digit1>>digit2;if(one_char!='$'||decimal_point!='.'||!isdigit(digit1)||!isdigit(digit2)){cout<<"wrong";exit(1);}cents=digit_to_int(digit1)*10+digit_to_int(digit2);all_cents=dollars*100+cents;if(negative)all_cents=-all_cents;}void Money::output(ostream& outs){long positive_cents,dollars,cents;positive_cents=labs(all_cents);dollars=positive_cents/100;cents=positive_cents%100;if(all_cents<0)outs<<"-$"<<dollars<<'.';elseouts<<"$"<<dollars<<'.';if(cents<10)outs<<'0';outs<<cents;}int digit_to_int(char c){return(static_cast<int>(c)-static_cast<int>('0'));}

在vc6.0上总是报错：'all_cents' : cannot access private member declared in class 'Money'；

只有按下面这样改才可以运行。希望大家可以方便查找错误

将using namespace std；改为

using std::cin;
using std::endl; using std::cout;
using std::ostream;
using std::istream;
using std::ostream;

原因:

这据说是VC的一个经典BUG。和namespace也有关.

只要含有using namespace std; 就会提示友员函数没有访问私有成员的权限。

解决方法：去掉using namespace std;换成更小的名字空间。

以下内容转自：（http://hi.baidu.com/zgjxwl/blog/item/f0382a6e4ea08ddc80cb4a3d.html）

例如:
含有#include <string>就要加上using std::string
含有#include <fstream>就要加上using std::fstream
含有#include <iostream>就要加上using std::cin; using std::cout; using std::ostream; using std::istream; using std::endl; 等等,需要什么即可包含什么.