Hi,guys, I've encountered a problem regarding the order of declaration and definition and I hope that you guys can help me out. Basically, I was trying to write a class named Sales_data with a non-member class related function named read and the code is shown below.
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#include <iostream>
#include <iomanip>
using namespace std;
struct Sales_data
{
Sales_data(): bookNo("\0"), units_sold(0),revenue(0) { }
Sales_data(const string &s): bookNo(s) { }
Sales_data(const string &s, unsigned n, double p): bookNo(s), units_sold(n), revenue(p*n) { }
Sales_data(istream &is) {read(is, *this);}
string isbn() const {return bookNo;}
Sales_data& combine(const Sales_data&);
double avg_price() const;
string bookNo;
unsigned units_sold = 0;
double revenue = 0.0;
};
istream &read(istream &is, Sales_data &item)
{
double price = 0;
is >> item.bookNo >> item.units_sold >> price;
item.revenue = price * item.units_sold;
return is;
}
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Notice here that the read function takes a Sales_data class object as one of its argument, and in the definition of class Sales_data, this read function is used in the definition of a constructor.
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Sales_data(istream &is) {read(is, *this);}
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When I tried to run this program, it generated an error saying that in
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Sales_data(istream &is) {read(is, *this);}
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'read' had not been declared, so I tried to put a declaration before the definition of class Sales_data, but this time it said that 'Sales_data' had not been declared.
Of course I could define the constructor
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Sales_data(istream &is) {read(is, *this);}
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outside of the class definition as
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#include <iostream>
#include <iomanip>
using namespace std;
struct Sales_data
{
Sales_data(): bookNo("\0"), units_sold(0),revenue(0) { }
Sales_data(const string &s): bookNo(s) { }
Sales_data(const string &s, unsigned n, double p): bookNo(s), units_sold(n), revenue(p*n) { }
Sales_data(istream &);
string isbn() const {return bookNo;}
Sales_data& combine(const Sales_data&);
double avg_price() const;
string bookNo;
unsigned units_sold = 0;
double revenue = 0.0;
};
istream &read(istream &is, Sales_data &item)
{
double price = 0;
is >> item.bookNo >> item.units_sold >> price;
item.revenue = price * item.units_sold;
return is;
}
Sales_data::Sales_data(istream &is) {read(is, *this);}
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but I was trying to see whether there is a way to definite the constructor in-class, so I put the declarations of both Sales_data class and read function at the beginning and it worked out.
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#include <iostream>
#include <iomanip>
using namespace std;
struct Sales_data;
istream &read(istream &, Sales_data &);
struct Sales_data
{
Sales_data(): bookNo("\0"), units_sold(0),revenue(0) { }
Sales_data(const string &s): bookNo(s) { }
Sales_data(const string &s, unsigned n, double p): bookNo(s), units_sold(n), revenue(p*n) { }
Sales_data(istream &is) {read(is, *this);}
string isbn() const {return bookNo;}
Sales_data& combine(const Sales_data&);
double avg_price() const;
string bookNo;
unsigned units_sold = 0;
double revenue = 0.0;
};
istream &read(istream &is, Sales_data &item)
{
double price = 0;
is >> item.bookNo >> item.units_sold >> price;
item.revenue = price * item.units_sold;
return is;
}
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But this thing is I have never seen a forward class declaration before and I'm not sure if it's syntatically correct, so is this the way to do it?
