how to overload the plus operator ?

[masterinex]

I keep getting the error too many parameters for this operator function.

  
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

using namespace std;


class Book {

private:

	string author_ = "unknown";
	int isbn_ = 0;
	int price_ = 0;
	string title_ = "unknown";


public:

	string getAuthor() {

	  return author_; 
	
	}


	string getTitle() {

		return title_;

	}


	int getIsbn() {

		return isbn_;

	}

	int getPrice() {

		return price_;

	}


	void setAuthor(string author) {
		author_ = author; 		
	}

	/*
	// Overload + operator to add two Books
	Book operator+(const Book& b) {
		Book b3;
		b3.author_ = this->author_ + b.author_;
		b3.isbn_ = this-> isbn_ + b.isbn_;
		b3.price_ = this-> price_ + b.price_;
		b3.title_ = this->title_ + b.title_;
		return b3;
	}*/



	Book operator + (const Book& b1 , const Book& b2) {
		Book b3;
		b3.author_ = b1.author_ + b2.author_;
		b3.isbn_ =  b1.isbn_ + b2.isbn_;
		b3.price_ = b1.price_ + b2.price_;
		b3.title_ = b3.title_ + b3.title_;
		return b3;
	}





};


bool sortbyauthor(Book lhs, Book rhs)
{
	return lhs.getAuthor() < rhs.getAuthor(); 
}


ostream& operator<<(ostream& os, Book& dt)
{
	os << dt.getAuthor() << '/' << dt.getIsbn() << '/' << dt.getPrice() << '/' << dt.getTitle() << endl; 
	return os;
}





int main()
{
	Book b1;
	b1.setAuthor("bb");

	Book b2;
	b2.setAuthor("aa");

	vector <Book> v;

	v.push_back(b1); 
	v.push_back(b2);

	cout << "before sorting by author" << endl; 

	for (auto x : v)
		cout << x ;

	sort(v.begin(), v.end(), sortbyauthor);

	cout << "after sorting by author" << endl;

	for (auto x : v)
		cout << x ; 


	cout << "after sorting by author displaying using ranged for loop " << endl;
	for(int unsigned i = 0; i < v.size(); ++i)
	{
		cout << v[i] ;
	}
	
	Book b3;
	b3 = b2 + b1; 
	cout << " b1 + b2 is " << b3 << endl; 


}

[Duthomhas]

Member functions only need one additional argument.
Non-member functions need both arguments.

I find Wikipedia to be useful:
https://en.wikipedia.org/wiki/Operators_in_C_and_C%2B%2B

[masterinex]

if i use this function :

Book operator+(const Book& b) {
Book b3;
b3.author_ = this->author_ + b.author_;
b3.isbn_ = this-> isbn_ + b.isbn_;
b3.price_ = this-> price_ + b.price_;
b3.title_ = this->title_ + b.title_;
return b3;
}


and at the line in main():

b3 = b2 + b1;

how do i know which Book (b1 or b2) is referred by the "this" keyword in the overloaded function ?

how come this always refers to the Book which comes second in the expression :

b3 = b2 + b1;

which is b1 in this case ?

[JLBorges]

> how do i know which Book (b1 or b2) is referred by the "this" keyword in the overloaded function ?

b2 + b1 is equivalent to either b2.operator+(b1) or operator+( b2, b1 )


> how to overload the plus operator ?

This is the canonical way:

struct A
{
    // step one: first overload the compound assignment operator +=
    A& operator+= ( const A& that ) { this->a += that.a ; return *this ; }
    A& operator+= ( int n ) { this->a += n ; return *this ; }

    int a = 0 ;
};

// step two: then overload the + operator in terms of the += operator
A operator+ ( A first, const A& second ) { return first += second ; }
A operator+ ( A first, int n ) { return first += n ; }
A operator+ ( int n, A first ) { return first += n ; }

[coder777]

You can make the function a friend and would work like you expect:

friend Book operator + (const Book& b1 , const Book& b2) { // Note: friend in front

[masterinex]

Hi JLBorges,

i think u might have gotten it mixed up.


> how do i know which Book (b1 or b2) is referred by the "this" keyword in the overloaded function ?

b2 + b1 is equivalent to either b2.operator+(b1)

it should be

b2 + b1 is equivalent to either b1.operator+(b2) , because the this keyword refers to b1,
as I have tried it.

[JLBorges]

If operator+ is a member function, b2 + b1 is equivalent to b2.operator+(b1)
The member function is invoked on object b2 and the this pointer points to b2
(the object on the left hand side of the +)

[masterinex]

I am totally sure that the this keyword

for the line

b3 = b2 + b1;

points to b1.

you can try it out with my code.

[Chervil]

you can try it out with my code.

OK, here goes:

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

using namespace std;

class Book {

private:

    string author_ = "unknown";
    int isbn_      = 0;
    int price_     = 0;
    string title_  = "unknown";

public:

    string getAuthor() { return author_; }
    string getTitle()  { return title_;  }
    int getIsbn()      { return isbn_;   }
    int getPrice()     { return price_;  }

    void setAuthor(string author) { author_ = author; } 

    friend ostream& operator<<(ostream& os, Book& dt);

    Book operator+(const Book& b)
    {
        cout << "this is " << *this << '\n';
        Book b3;
        b3.author_ = this->author_ + b.author_;
        b3.isbn_   = this->isbn_   + b.isbn_;
        b3.price_  = this->price_  + b.price_;
        b3.title_  = this->title_  + b.title_;
        return b3;
    }
};


bool sortbyauthor(Book lhs, Book rhs)
{
    return lhs.getAuthor() < rhs.getAuthor(); 
}


ostream& operator<<(ostream& os, Book& dt)
{
    os << dt.getAuthor() << '/' << dt.getIsbn() << '/' 
       << dt.getPrice() << '/' << dt.getTitle() << endl; 
    return os;
}


int main()
{
    Book b1;
    b1.setAuthor("b1");

    Book b2;
    b2.setAuthor("b2");

    cout << "b1 is " << b1 << '\n';
    cout << "b2 is " << b2 << '\n';
    
    Book b3;
    b3 = b2 + b1; 
    cout << " b2 + b1 is " << b3 << endl; 
}

b1 is b1/0/0/unknown

b2 is b2/0/0/unknown

this is b2/0/0/unknown

 b2 + b1 is b2b1/0/0/unknownunknown