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9 Overloading Operators In this chapter: Overloading the Assignment Operator The this Pointer Overloading Arithmetic Operators Overloading the Arithmetic Assignment Operators Overloading the Relational Operators Overloading the Stream Operators Conversion Operators Overloading the Increment and Decrement Operators Overloading the Subscript Operator
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C++ includes a rich store of operators that are defined automatically for fundamental types (int, float, etc.). When you create a new type (class) you can overload most C++ operators to this user-defined type. Overloading the Assignment Operator Of all the operators, the assignment operator = is probably used the most. Its purpose is to copy one object to another. Like the default constructor, the copy constructor, and the destructor, the assignment operator is created automatically for every class that is defined, but it can be defined explicitly in the class definition. Example 9.1 An Assignment Operator for the Rational Class Rational default and copy constructors and assignment operator: class Rational { public: Rational(int =0, int =1); // default const Rational (const Rational&); // copy constructor void operator=(const Rational&); // assignment // other declarations go here private: int num, den; }; The name of this member function is operator=. Its argument list is the same as that of the copy constructor: it contains a single argument of the same class, passed by constant reference. Here is the implementation of the overloaded assignment operator: void Rational: :operator=(const Rational& r) { num=r.num; den=r.den; } It copies the member data from r to the object that owns the call. The this Pointer C++ allows assignments to be chained together, like this: x = y = z = 3.14; This is executed first by assigning 3.14 to z, then to y, and finally to x.
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As Ex. 9.1 shows, the assignment operator is really a function named operator=. In this chain, the function is called three times nested, like this: f(x, f(y, f(z, 3.14))) As assignment operator returns the value it assigns, it should return a reference to the same type as the object being assigned: Rational& operator=(Rational& r) This allows assignments to be chained together. Example 9.2 Preferred Prototype for an Overloaded Assignment // assignment Rational& operator = (const Rational&); The function should return the object that is being assigned, in order for the assignment chain to work. Since there is no other name available for this owner object, C++ defines a special pointer, named this, which points to the owner object. Using the this pointer we can give the correct implementation of the overloaded assignment operator: Example 9.3 Implementation of the Rational Class Assignment Rational& Rational::operator = (const Rational& r) { num = r.num; den = r.den; return *this; } Now assignments for the Rational class can be chained together: Rational x, y, z(22,7); x = y = z;
Finally, note that an assignment is different from an initialization, even though they both use the equals sign: Rational Rational Rational Rational w = x; x(22,7); // this is y(x); // this is z = x; // this is w; // this is an initialization an initialization an initialization an assignment
An initialization calls the copy constructor. An assignment calls the assignment operator. Overloading Arithmetic Operators Most programming languages provide the standard arithmetic operators +,-,*, and / for numeric types. Therefore, it is natural to define these
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for user-defined types like the Rational class. In older programming languages, this is done by defining functions like this: Rational product(Rational x, Rational y) { Rational z(x.num*y.num, x.den*y.den); return z; } This works, but the function has to be called in the conventional way: z = product (x,y); C++ allows such functions to be defined using the standard arithmetic operator symbols, so that they can be called more naturally using infix notation (e.g., z = x*y; ). Like most operators in C++, the multiplication operator has a function name: operator*. Using this in place of "product" in the code above results in Rational operator*(Rational x, Rational y) { Rational z(x.num*y.num, x.den*y.den); return z; } However, this is not a member function of Rational. Since the overloaded arithmetic operators cannot be member functions, they cannot access the private member data num and den. Fortunately, C++ allows an exception to this by allowing us to declare the function as a friend of the Rational class.
Note! A friend function is a nonmember function that is given access to all members of the class within which it is declared. friends have all the privileges of member functions without actually being a class member. This attribute is used mostly with overloaded operators.
Example 9.4 Making the Multiplication Operator a friend Here a friend function overloads the multiplication operator: class Rational { friend Rational operator* (const Rational&, const Rational&); public: Rational(int =0, int =1); Rational (const Rational&);
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