五谷杂粮配方大全比例:VC＋＋高手进来看一下！！！

#include <iostream.h>
#include <iomanip.h>

class PhoneNumber
{
friend ostream &operator<< ( ostream&, const PhoneNumber & );
friend istream &operator>> ( istream&, PhoneNumber & );
private:
char areaCode[ 4 ];
char exchange[ 4 ];
char line[ 5 ];
};

ostream &operator<< ( ostream &output, const PhoneNumber &num )
{
output << "(" << num.areaCode << ")" << num.exchange << "-" << num.line;
return output;
}

istream &operator>> ( istream &input, PhoneNumber &num )//为什么这里不可以是const类型而上面却可以？
{
input.ignore();
input >> setw( 4 ) >> num.areaCode;
input.ignore( 2 );
input >> setw( 4 ) >> num.exchange;
input.ignore();
input >> setw( 5 ) >> num.line;
return input;
}

void main()
{
PhoneNumber phone;
cout << "Enter phone number in the form (123) 456-7890:\n";
cin >> phone;
cout << "The phone number entered was: " << phone << endl;
}