C++ Primer 1.5 Questions

[Shreddar]

So my question is I have my class which is the Sales_Item.h all set up and all. But the thing I am stuck on is that In the code

#include <iostream>
#include "Sales_item.h"


int main(){
	Sales_item item1, item2; // I just declared a object of Saltes_item
	std::cin >> item1 >> item2; 
	std::cout << item1 + item2 << std::endl;
	return 0;
}

But I am lost as what is connected to this item1 object as I cant seem to find it declared anywhere within the sales_item.h class
Also this is the sales_item.h class file

#ifndef SALESITEM_H
// we're here only if SALESITEM_H has not yet been defined
#define SALESITEM_H

// Definition of Sales_item class and related functions goes here
#include <iostream>
#include <string>

class Sales_item {
// these declarations are explained section 7.2.1, p. 270
// and in chapter 14, pages 557, 558, 561
friend std::istream& operator>>(std::istream&, Sales_item&);
friend std::ostream& operator<<(std::ostream&, const Sales_item&);
friend bool operator<(const Sales_item&, const Sales_item&);
friend bool
operator==(const Sales_item&, const Sales_item&);
public:
    // constructors are explained in section 7.1.4, pages 262 - 265
    // default constructor needed to initialize members of built-in type
    Sales_item() = default;
    Sales_item(const std::string &book): bookNo(book) { }
    Sales_item(std::istream &is) { is >> *this; }
public:
    // operations on Sales_item objects
    // member binary operator: left-hand operand bound to implicit this pointer
    Sales_item& operator+=(const Sales_item&);

    // operations on Sales_item objects
    std::string isbn() const { return bookNo; }
    double avg_price() const;
// private members as before
private:
    std::string bookNo;      // implicitly initialized to the empty string
    unsigned units_sold = 0; // explicitly initialized
    double revenue = 0.0;
};

// used in chapter 10
inline
bool compareIsbn(const Sales_item &lhs, const Sales_item &rhs)
{ return lhs.isbn() == rhs.isbn(); }

// nonmember binary operator: must declare a parameter for each operand
Sales_item operator+(const Sales_item&, const Sales_item&);

inline bool
operator==(const Sales_item &lhs, const Sales_item &rhs)
{
    // must be made a friend of Sales_item
    return lhs.units_sold == rhs.units_sold &&
           lhs.revenue == rhs.revenue &&
           lhs.isbn() == rhs.isbn();
}

inline bool
operator!=(const Sales_item &lhs, const Sales_item &rhs)
{
    return !(lhs == rhs); // != defined in terms of operator==
}

// assumes that both objects refer to the same ISBN
Sales_item& Sales_item::operator+=(const Sales_item& rhs)
{
    units_sold += rhs.units_sold;
    revenue += rhs.revenue;
    return *this;
}

// assumes that both objects refer to the same ISBN
Sales_item
operator+(const Sales_item& lhs, const Sales_item& rhs)
{
    Sales_item ret(lhs);  // copy (|lhs|) into a local object that we'll return
    ret += rhs;           // add in the contents of (|rhs|)
    return ret;           // return (|ret|) by value
}

std::istream&
operator>>(std::istream& in, Sales_item& s)
{
    double price;
    in >> s.bookNo >> s.units_sold >> price;
    // check that the inputs succeeded
    if (in)
        s.revenue = s.units_sold * price;
    else
        s = Sales_item();  // input failed: reset object to default state
    return in;
}

std::ostream&
operator<<(std::ostream& out, const Sales_item& s)
{
    out << s.isbn() << " " << s.units_sold << " "
        << s.revenue << " " << s.avg_price();
    return out;
}

double Sales_item::avg_price() const
{
    if (units_sold)
        return revenue/units_sold;
    else
        return 0;
}
#endif 

As well later on there is another part which is with methods/member functions

#include <iostream>
#include "Sales_item.h"


int main(){
	Sales_item item1, item2;
	std::cin >> item1 >> item2;
	if (item1.isbn() == item2.isbn()){ // This is where I am confused as when did I declare isbn as a type or am I missing something
	std::cout << item1 + item2 << std::endl;
	return 0;
	}else{
		std::cerr << "Data must refer to same ISBN" << std::endl;
		return -1;
	}
}

But I believe I am just not understanding what are all the class types associated with the class and member/methods I don''t understand how I just call a object without declaring it first

[Thomas1965]

isbn() is a function that returns a string.

[MikeyBoy]

A simple text search showed me that Sales_item::isbn() is defined on line 30 of your sales_item.h file:

class Sales_item {
    // ...
    std::string isbn() const { return bookNo; }
    // ...
}

But I believe I am just not understanding what are all the class types associated with the class and member/methods I don''t understand how I just call a object without declaring it first

All the member functions of the class are declared in sales_item.h . And you did declare an object; in fact, you declared two of them:

Sales_item item1, item2; // I just declared a object of Saltes_item

See - you even wrote a comment to say that you declared them!

EDIT: Why is this in the Lounge, which is the one forum that is supposed to be for things other than C++ coding?