11-08.cpp

将文件流中的数据以类的形式读入到另一个文件中

//DISPLAY 11.8 Overloading << and >>  
//Program to demonstrate the class Money. 
#include <iostream>
#include <fstream>
#include <cstdlib>
#include <cctype>
using namespace std;

//Class for amounts of money in U.S. currency. 
class Money
{
public:
    friend Money operator +(const Money& amount1, const Money& amount2);

    friend Money operator -(const Money& amount1, const Money& amount2);

    friend Money operator -(const Money& amount);

    friend bool operator ==(const Money& amount1, const Money& amount2);


Money(long dollars, int cents);

    Money(long dollars);

    Money( );

    double get_value( ) const;

    friend istream& operator >>(istream& ins, Money& amount);
    //Overloads the >> operator so it can be used to input values of type Money.
    //Notation for inputting negative amounts is as in -$100.00.
    //Precondition: If ins is a file input stream, then ins has already been
    //connected to a file. 

    friend ostream& operator <<(ostream& outs, const Money& amount);
    //Overloads the << operator so it can be used to output values of type Money.
    //Precedes each output value of type Money with a dollar sign.
    //Precondition: If outs is a file output stream, 
    //then outs has already been connected to a file.
private:
    long all_cents;
};
int digit_to_int(char c);
//Used in the definition of the overloaded input operator >>.
//Precondition: c is one of the digits '0' through '9'.
//Returns the integer for the digit; for example, digit_to_int('3') returns 3.

int main( )
{
    Money amount;
    ifstream in_stream;
    ofstream out_stream;

    in_stream.open("infile.dat");
    if (in_stream.fail( ))
    {
        cout << "Input file opening failed.\n";
        exit(1);
    }

    out_stream.open("outfile.dat");
    if (out_stream.fail( ))
    {
        cout << "Output file opening failed.\n";
        exit(1);
    }


in_stream >> amount;
    out_stream << amount 
              << " copied from the file infile.dat.\n";
    cout << amount 
         << " copied from the file infile.dat.\n";

    in_stream.close( );
    out_stream.close( );

    return 0;
}
//Uses iostream, cctype, cstdlib:
istream& operator >>(istream& ins, Money& amount)
{
    char one_char, decimal_point,
         digit1, digit2; //digits for the amount of cents
    long dollars;
    int cents;
    bool negative;//set to true if input is negative.

    ins >> one_char;
    if (one_char == '-')
    {
        negative = true;
        ins >> one_char; //read '$'
    }
    else
        negative = false;
    //if input is legal, then one_char == '$'

    ins >> dollars >> decimal_point >> digit1 >> digit2;

    if ( one_char != '$' || decimal_point != '.'
         || !isdigit(digit1) || !isdigit(digit2) )
    {
        cout << "Error illegal form for money input\n";
        exit(1);
    }

    cents = digit_to_int(digit1)*10 + digit_to_int(digit2);

    amount.all_cents = dollars*100 + cents;
    if (negative)
        amount.all_cents = -amount.all_cents;


return ins;
}       

int digit_to_int(char c)
{
    return ( static_cast<int>(c) - static_cast<int>('0') );
}

//Uses cstdlib and iostream:
ostream& operator <<(ostream& outs, const Money& amount)
{
    long positive_cents, dollars, cents;
    positive_cents = labs(amount.all_cents);
    dollars = positive_cents/100;
    cents = positive_cents%100;

    if (amount.all_cents < 0)
        outs << "-$" << dollars << '.';
    else
        outs << "$" << dollars << '.';

    if (cents < 10)
        outs << '0';
    outs << cents;

    return outs;
}

//The definitions of the member functions and other overloaded operators go here.
//See Display 11.3, 11.4, 11.5, and 11.6 for the definitions.