Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-1 Concordia University Department of Computer Science and Software Engineering COMP345 – Advanced Program Design with C++ Lecture 8: Operator overloading

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-2 Learning Objectives  Basic Operator Overloading  Unary operators  As member functions  Friends and Automatic Type Conversion  Friend functions, friend classes  Constructors for automatic type conversion

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-3 Operator Overloading Introduction  Operators +, -, %, ==, etc.  Really just functions!  Simply "called" with different syntax: x + 7  "+" is binary operator with x & 7 as operands  We "like" this notation as humans  Think of it as: +(x, 7)  "+" is the function name  x, 7 are the arguments  Function "+" returns "sum" of it’s arguments

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-4 Operator Overloading Perspective  Built-in operators  e.g., +, -, =, %, ==, /, *  Already work for C++ built-in types  In standard "binary" notation  We can overload them!  To work with OUR types!  To add "Chair types", or "Money types"  As appropriate for our needs  In "notation" we’re comfortable with  Always overload with similar "actions"!

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-5 Overloading Basics  Overloading operators  VERY similar to overloading functions  Operator itself is "name" of function  Example Declaration: const Money operator +(const Money& amount1, const Money& amount2);  Overloads “+” for operands of type Money  Uses constant reference parameters for efficiency  Returned value is type Money  Allows addition of "Money" objects

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-6 Overloaded "+"  Given previous example:  Note: overloaded "+" not a member function  Definition is "more evolved" than number "add"  Requires issues of Money type addition  Must handle negative/positive values  Operator overload definitions generally very simple  Just perform "addition" particular to "your" type

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-7 Overloading as external operator  The operator is overloaded as a “free function is defined, i.e. outside of the class:  Note: no Money:: in the operator definition header, as the operator is not a member of the Money class class Money { public: … // no declaration of + operator in the class … } const Money operator +(const Money m1, const Money m2) {…}

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-8 Money "+" Definition: Display 8.1 Operator Overloading  Definition of "+" operator for Money class:

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-9 Overloaded "=="  Equality operator, ==  Enables comparison of Money objects  Declaration: bool operator ==(const Money& amount1, const Money& amount2);  Returns bool type for true/false equality  Again, it’s a non-member function (like "+" overload)

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-10 Overloaded "==" for Money: Display 8.1 Operator Overloading  Definition of "==" operator for Money class:

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-11 Returning by const Value  Consider "+" operator overload again: const Money operator +(const Money& amount1, const Money& amount2);  Returns a "constant object“?  Why?  Consider impact of returning "non-const" object to see… 

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-12 Returning by non-const Value  Consider "no const" in declaration: Money operator +(const Money& amount1, const Money& amount2);  Consider the expression: m1 + m2  Where m1 and m2 are Money objects  Calls the operator declared above  Object returned is Money object  We can "do things" with objects!  Like call member functions…

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-13 What to do with Non-const Object  Can call member functions:  We could invoke member functions on the object returned by expression m1+m2:  (m1+m2).output(); //Legal, right?  Not a problem: doesn’t change anything in the (m1+m2) object  (m1+m2).input(); //Legal!  PROBLEM! //Legal, but MODIFIES the (m1+m2) object!  Allows modification of "anonymous" object!  Can’t allow that here!  Even more:  (m1 + m2) = m3; //Legal!!!  So we define the return object as const to avoid these problems  Ideally:  const Money operator +(const Money&, const Money&) const;

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-14 Overloading Unary Operators  C++ has unary operators:  Defined as taking one operand  e.g., - (negation)  x = -y; // Sets x equal to negative of y  Other unary operators:  ++, --  Unary operators can also be overloaded

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-15 Overload "-" for Money  Overloaded "-" function declaration  Placed outside class definition: const Money operator –(const Money& amount);  Notice: only one argument  Since only 1 operand (unary)  "-" operator is overloaded twice!  For two operands/arguments (binary)  For one operand/argument (unary)  Definitions must exist for both

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-16 Overloaded "-" Definition  Overloaded "-" function definition: const Money operator –(const Money& amount) { return Money(-amount.getDollars(), -amount.getCents()); }  Applies "-" unary operator to built-in type  Operation is "known" for built-in types  Returns anonymous object again

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-17 Overloaded "-" Usage  Consider: Money amount1(10), amount2(6), amount3; amount3 = amount1 – amount2;  Calls binary "-" overload amount3.output();//Displays $4.00 amount3 = -amount1;  Calls unary "-" overload amount3.output()//Displays -$10.00

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-18 Overloading as Member operator  Previous examples: standalone functions  Defined outside a class  Can overload as "member operator"  Considered "member function" like others  When operator is member function:  Only ONE parameter, not two!  Calling object serves as 1 st parameter

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-19 Overloading as Member operator  Declared and defined as a member function, with operator declaration syntax:  Note: operator is declared as const : it is not changing the values of the data members of its calling object (not necessary but informative and precautious). class Money { public: … const Money operator +(const Money m1) const; … } const Money Money::operator +(const Money m1) const {…}

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-20 Member Operator in Action  Money cost(1, 50), tax(0, 15), total; total = cost + tax;  If "+" overloaded as member operator:  Variable/object cost is calling object  Object tax is single argument  Think of as: total = cost.+(tax);  Declaration of "+" in class declaration:  const Money operator +(const Money& amount);  Notice only ONE argument

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-21 const Functions  When to make function const ?  Constant functions not allowed to alter class member data  Constant objects can ONLY call constant member functions  Good style dictates:  Any member function that will NOT modify data should be made const  Use keyword const after function declaration and heading  const Money operator +(const Money&, const Money&) const;

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-22 Overloading Operators: Which Method?  Object-Oriented-Programming  Principles suggest member operators  Many agree, to maintain "spirit" of OOP  Member operators more efficient  No need to call accessor & mutator functions  At least one significant disadvantage  (Later…)

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-23 Friend Functions  Nonmember functions  Recall: operator overloads as nonmembers  They access data through accessor and mutator functions  Very inefficient (overhead of calls)  Friends can directly access private class data  No need to use accessors/mutators, no overhead, more efficient  So: best to make nonmember operator overloads as friend operators

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-24 Friend Functions  Friend function of a class  Not a member function  Has direct access to private members  Just as member functions do  In function declaration (inside the class):  Use keyword friend in front of function declaration  Specified IN class definition  But they’re NOT member functions!  Even if members are declared as private or protected, they are considered public from the perspective of the friend  In function definition:  Don’t use keyword friend  Don’t use ClassName:: in front of function name: it is not a member of ClassName

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-25 Friend Function Uses  Operator Overloads  Most common use of friends  Improves efficiency  Avoids need to call accessor/mutator member functions  Operator must have access anyway  Might as well give full access as friend  Friends can be any function  If a member function is declared as friend, it must be accessible

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-26 Friend function example // friend functions #include using namespace std; class CRectangle { int width, height; public: void set_values (int, int); int area () {return (width * height);} friend CRectangle duplicate (CRectangle); }; void CRectangle::set_values (int a, int b) { width = a; height = b; } CRectangle duplicate (CRectangle rectparam) { CRectangle rectres; rectres.width = rectparam.width*2; rectres.height = rectparam.height*2; return (rectres); } int main () { CRectangle rect, rectb; rect.set_values (2,3); rectb = duplicate (rect); cout << rectb.area(); return 0; }

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-27 Friend class  A class B can be declared as friend to another class A  Allows B to freely access all members of class A  Friend is widely considered a “rogue feature” of C++  It conflicts with information hiding  Complexifies inheritance

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-28 Friend class example class CSquare; class CRectangle { int width, height; public: int area (){ return (width * height); } void convert (CSquare a); }; class CSquare { private: int side; public: void set_side (int a) { side=a; } friend class CRectangle; }; void CRectangle::convert (CSquare a) { width = a.side; height = a.side; } int main () { CSquare sqr; CRectangle rect; sqr.set_side(4); rect.convert(sqr); cout << rect.area(); return 0; }

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-29 Friend operators: automatic type conversion  In case of member-defined operators:  Checks for a Money member to call on baseAmount :  Money Money::operator + (int)  If not, we might have a constructor Money(int) that will be called to translate 25 into $ If not: compilation error. Money baseAmount(100,60), fullAmount fullAmount = baseAmount + 25; // problem? fullAmount.output; // $125.60

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-30 Friend operators: automatic type conversion  However:  As operator “+” of int is certainly not overloaded for a Money object parameter, 25 cannot be a calling object, and type conversion using a constructor does not work for calling objects.  This is a great disadvantage of overloading binary operators as class members  Solution: overload the operator as a friend Money baseAmount(100,60), fullAmount; fullAmount = 25 + baseAmount; // problem!! fullAmount.output; // $125.60

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-31 Operator overloading as a friend  Overload the operator externally, then include it as a friend:  Note: no Money:: and no friend in the definition.  This way, both operands can be automatically converted using constructors (as the left operand is not a calling object), and the operator can refer to private data members in its operation (thus saving computation time by not using accessors/mutators). class Money { public: … friend const Money operator +(const Money m1, const Money m2); … } const Money operator +(const Money m1, const Money m2){…}

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-32 Operator overloading: 3 ways  As non-member non-friend  Defined as external to the class  Not related to the class, except by its operands type  Implementation code cannot refer to private members of its operands, leading to inefficient implementation through the use of accessors/mutators.  Has two declared operands and no calling object  May have type conversion for both of its operators  As member  Defined as a member of the class  May use data members, as the operator is a member of the class  Is clear that the operator belongs to the class (as opposed to non-member)  Best from the point of view of OO principles, but…  Used as the left parameter being the calling object of the operator i.e. o1 + o2 =~= o1.+(o2)  Suffers from lack of type conversion of the left operand  As friend  Defined as external to the class, but introduced inside the class as a friend  At least, it is mentioned in the class declaration… though it is not a member.  May use private data members, thus more efficient than non-member non-friend  Has two declared operands and no calling object  May have type conversion for both of its operators

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-33 Summary 1  C++ built-in operators can be overloaded  To work with objects of your class  Operators are really just functions that can be called in infix mode i.e. a+b instead of +(a,b)  Friend functions have direct private member access  Operators can be overloaded as member functions  1 st operand is calling object

Copyright 2006 Pearson Addison-Wesley, 2008, 2013 Joey Paquet 2-34 Summary 2  Friend external operators add efficiency only  Not required if sufficient accessors/mutators available, though inefficient  Use const return type, unless in exceptional cases.