Operator overloading. Instructor: Bakhyt Bakiyev

Transcription

1 Operator overloading Instructor: Bakhyt Bakiyev

2 content Operator overloading

3 Operator Overloading CONCEPT: C++ allows you to redefine how standard operators work when used with class objects.

4 Approaches to Operator Overloading There are two approaches you can take to overload an operator: 1. Make the overloaded operator a member function of the class. This allows the operator function access to private members of the class. It also allows the function to use the implicit this pointer parameter to access the calling object. 2. Make the overloaded member function a separate, stand-alone function. When overloaded in this manner, the operator function must be declared a friend of the class to have access to the private members of the class. Some operators, such as the stream input and output operators >> and <<, must be overloaded as stand-alone functions. Other operators may be overloaded either as member functions or stand-alone functions.

5 Overloading the = Operator class NumberArray private: double *aptr; int arraysize; public: void operator=(const NumberArray &right); // Overloaded operator. NumberArray(const NumberArray &); NumberArray(int size, double value); ~NumberArray() if (arraysize > 0) delete [ ] aptr; void print(); void setvalue(double value); ;

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7 To change the meaning of = operator class Weird private: int value; public: Weird(int v) value = v; void operator=(const Weird &right) cout << right.value << endl; ;

8 Although the operator= function overloads the assignment operator, the function doesn t perform an assignment. All the overloaded operator does is display the contents of right.value. Consider the following program segment: Weird a(5), b(10); a = b; Although the statement a = b looks like an assignment statement, it actually causes the contents of b s value member to be displayed on the screen: 10

9 Example 1 class complex float x,y; public: complex() x=0;y=0; ; complex(float real, float imag) x=real; y=imag; friend complex operator+(complex a, complex b) return complex(a.x+b.x, a.y+b.y); friend complex operator-(complex a, complex b) return complex(a.x-b.x, a.y-b.y); void display(); ; void complex::display(void) cout<<x<<" + i"<<y<<endl;

10 Example 2 class complex float x; float y; public: complex() x=y=0; ; complex(float real, float imag) x=real; y=imag; complex operator+(complex c) complex temp; temp.x=x+c.x; temp.y=y+c.y; return(temp); complex operator-(complex c) complex temp; temp.x=x-c.x; temp.y=y-c.y; return(temp); void display(); ; void complex::display(void) cout<<x<<" + i"<<y<<endl;

11 main() int n; complex c1,c2,c3; c1=complex(2.5,3.5); c2=complex(1.6,2.7); cout<<"1-add/2-subtract :"; cin>>n; if(n==1) c3=c1+c2; if(n==2)c3=c1-c2; cout<<"c1="; c1.display(); cout<<"c2="; c2.display(); cout<<"c3="; c3.display(); return 0;

12 Overloading the Arithmetic and Relational Operators friend Length operator+(length a, Length b); friend Length operator-(length a, Length b); friend bool operator<(length a, Length b); friend bool operator==(length a, Length b);

13 Length operator+(length a, Length b) Length result(a.len_inches + b.len_inches); return result; or more succinctly: Length operator+(length a, Length b) return Length(a.len_inches + b.len_inches);

14 Example class Length private: int len_inches; public: Length(int feet, int inches) setlength(feet, inches); Length(int inches) len_inches = inches; int getfeet() return len_inches / 12; int getinches() return len_inches % 12; void setlength(int feet, int inches) len_inches = 12 *feet + inches; friend Length operator+(length a, Length b); friend Length operator-(length a, Length b); friend bool operator< (Length a, Length b); friend bool operator== (Length a, Length b); ;

15 Length operator+(length a, Length b) return Length(a.len_inches + b.len_inches); Length operator-(length a, Length b) return Length(a.len_inches - b.len_inches); bool operator==(length a, Length b) return a.len_inches == b.len_inches; bool operator<(length a, Length b) return a.len_inches < b.len_inches;

16 int main() Length first(0), second(0), third(0); int f, i; cout << "Enter a distance in feet and inches: "; cin >> f >> i; first.setlength(f, i); cout << "Enter another distance in feet and inches: "; cin >> f >> i; second.setlength(f, i); // Test the + and - operators third = first + second; cout << "first + second = "; cout << third.getfeet() << " feet, "; cout << third.getinches() << " inches.\n"; third = first - second; cout << "first - second = "; cout << third.getfeet() << " feet, "; cout << third.getinches() << " inches.\n"; // Test the relational operators cout << "first == second = "; if (first == second) cout << "true"; else cout << "false"; cout << "\n"; cout << "first < second = "; if (first < second) cout << "true"; else cout << "false"; cout << "\n"; return 0; Program Output Enter a distance in feet and inches: 6 5 Enter another distance in feet and inches: 3 10 first + second = 10 feet, 3 inches. first - second = 2 feet, 7 inches. first == second = false first < second = false

17 Choosing Between Stand-Alone and Member-Function Operators To sum up, the addition operator (as well as other arithmetic and relational operators) can be overloaded equally well as member functions or as stand-alone functions. It is generally better to overload binary operators that take parameters of the same type as stand-alone functions. This is because, unlike stand-alone operator overloading, member-function overloading introduces an artificial distinction between the two parameters by making the left parameter implicit. This allows convert constructors to apply to the right parameter but not to the left, creating situations where changing the order of parameters causes a compiler error in an otherwise correct program: Length a(4, 2), c(0); c = a + 2; // Compiles, equivalent to c = a.operator+(2) c = 2 + a; // Does not compile: equivalent to c = 2.operator+(a);

18 Overloading the Prefix ++ Operator class Length private: int len_inches; public: // Declaration of prefix ++ Length operator++(); // Rest of class not shown ; Length Length::operator++() len_inches ++; return *this; Given this overload, the user-friendly notation ++b is equivalent to the call b.operator++(). Either notation may be used in your program.

19 Overloading the Postfix ++ Operator Length Length::operator++(int) Length temp = *this; len_inches ++; return temp; The first difference you will notice is that the function has a dummy parameter of type int that is never used in the body of the function. This is a convention that tells the compiler that the increment operator is being overloaded in postfix mode. The second difference is the use of a temporary local variable temp to capture the value of the object before it is incremented. This value is saved and is later returned by the function.

20 Overloading the Stream Insertion and Extraction Operators Length b(4, 8), c(2, 5); cout << b; cout << b + c; appear to the compiler as Length b(4, 8), c(2, 5); operator<<(cout, b); operator(cout, b + c);

21 insertion operator should be written as ostream &operator<<(ostream& out, Length a) out << a.getfeet() << " feet, " << a.getinches() << " inches"; return out; the header for the stream input operator looks like this: istream &operator>>(istream &in, Length &a);

22 Example class Length private: int len_inches; public: Length(int feet, int inches) setlength(feet, inches); Length(int inches) len_inches = inches; int getfeet() return len_inches / 12; int getinches() return len_inches % 12; void setlength(int feet, int inches) len_inches = 12 *feet + inches; friend Length operator+(length a, Length b); friend Length operator-(length a, Length b); friend bool operator<(length a, Length b); friend bool operator==(length a, Length b); Length operator++(); Length operator++(int); friend ostream &operator<<(ostream &out, Length a); friend istream &operator>>(istream &in, Length &a); ;

23 istream &operator>>(istream &in, Length &a) int feet, inches; cout << "Enter feet: "; in >> feet; cout << "Enter inches: "; in >> inches; // Modify the object a with the data and return a.setlength(feet, inches); return in; ostream &operator<<(ostream& out, Length a) out << a.getfeet() << " feet, " << a.getinches() << " inches"; return out;

24 Length Length::operator++() len_inches ++; return *this; Length Length::operator++(int) Length temp = *this; len_inches ++; return temp;

25 Length operator+(length a, Length b) return Length(a.len_inches + b.len_inches); Length operator-(length a, Length b) return Length(a.len_inches - b.len_inches); bool operator==(length a, Length b) return a.len_inches == b.len_inches; bool operator<(length a, Length b) return a.len_inches < b.len_inches;

26 int main() Length first(0), second(1, 9), c(0); cout << "Demonstrating prefix ++ operator and output operator.\n"; for (int count = 0; count < 4; count++) first = ++second; cout << "First: " << first << ". Second: " << second << ".\n"; cout << "\ndemonstrating postfix ++ operator and output operator.\n"; for (int count = 0; count < 4; count++) first = second++; cout << "First: " << first << ". Second: " << second << ".\n"; cout << "\ndemonstrating input and output operators.\n"; cin >> c; cout << "You entered " << c << "." << endl; return 0;

27 Program Output Demonstrating prefix ++ operator and output operator. First: 1 feet, 10 inches. Second: 1 feet, 10 inches. First: 1 feet, 11 inches. Second: 1 feet, 11 inches. First: 2 feet, 0 inches. Second: 2 feet, 0 inches. First: 2 feet, 1 inches. Second: 2 feet, 1 inches. Demonstrating postfix ++ operator and output operator. First: 2 feet, 1 inches. Second: 2 feet, 2 inches. First: 2 feet, 2 inches. Second: 2 feet, 3 inches. First: 2 feet, 3 inches. Second: 2 feet, 4 inches. First: 2 feet, 4 inches. Second: 2 feet, 5 inches. Demonstrating input and output operators. Enter feet: 3 Enter inches: 4 You entered 3 feet, 4 inches.

28 Overloading the [ ] Operator class IntArray private: int *aptr; int arraysize; void suberror(); // Handles subscripts out of range public: IntArray(int); // Constructor IntArray(const intarray &); // Copy constructor ~IntArray(); // Destructor int size() return arraysize; int &operator[](int); // Overloaded [] operator ;

29 //constructor IntArray::IntArray(int s) arraysize = s; aptr = new int [s]; for (int count = 0; count < size; count++) *(aptr + count) = 0; //copy constructor IntArray::IntArray(const IntArray &obj) arraysize = obj.arraysize; aptr = new int [arraysize]; for(int count = 0; count < arraysize; count++) *(aptr + count) = *(obj.aptr + count); //destructor IntArray::~IntArray() if (arraysize > 0) delete [] aptr;

30 The [] operator is overloaded similarly to other operators: int &IntArray::operator[](int sub) if (sub < 0 sub >= arraysize) suberror(); return aptr[sub];

31 Example 1 class IntArray private: int *aptr; int arraysize; void suberror(); // Handles subscripts out of range public: IntArray(int); // Constructor IntArray(const IntArray &); // Copy constructor ~IntArray(); // Destructor int size() return arraysize; int &operator[](int); // Overloaded [] operator ;

32 IntArray::IntArray(int s) arraysize = s; aptr = new int [s]; for (int count = 0; count < arraysize; count++) *(aptr + count) = 0; IntArray::IntArray(const IntArray &obj) arraysize = obj.arraysize; aptr = new int [arraysize]; for(int count = 0; count < arraysize; count++) *(aptr + count) = *(obj.aptr + count); IntArray::~IntArray() if (arraysize > 0) delete [] aptr;

33 void IntArray::subError() cout << "ERROR: Subscript out of range.\n"; exit(0); int &IntArray::operator[](int sub) if (sub < 0 sub >= arraysize) suberror(); return aptr[sub];

34 int main() IntArray table(10); // Store values in the array. for (int x = 0; x < table.size(); x++) table[x] = (x * 2); // Display the values in the array. for (int x = 0; x < table.size(); x++) cout << table[x] << " "; // Use the built-in + operator on array elements. for (int x = 0; x < table.size(); x++) table[x] = table[x] + 5; // Display the values in the array. for (int x = 0; x < table.size(); x++) cout << table[x] << " "; // Use the built-in ++ operator on array elements. for (int x = 0; x < table.size(); x++) table[x]++; // Display the values in the array. for (int x = 0; x < table.size(); x++) cout << table[x] << " "; return 0; Program Output

35 Example 2 int main() IntArray table(10); // Store values in the array. for (int x = 0; x < table.size(); x++) table[x] = x; // Display the values in the array. for (int x = 0; x < table.size(); x++) cout << table[x] << " "; cout << endl; Program Output Attempting to store outside the array bounds: ERROR: Subscript out of range. cout << "Attempting to store outside the array bounds:\n"; table[table.size()] = 0; return 0;

36 Type Conversion Operators CONCEPT: Special operator functions may be written to convert a class object to any other type. Suppose a program uses the following variables: int i; double d; The following statement automatically converts the value in i to a double and stores it in d: d = i; Likewise, the following statement converts the value in d to an integer (truncating the fractional part) and stores it in i: i = d; The same functionality can also be given to class objects.

37 Example class Length private: int len_inches; public: Length(int feet, int inches) setlength(feet, inches); Length(int inches) len_inches = inches; int getfeet() return len_inches / 12; int getinches() return len_inches % 12; void setlength(int feet, int inches) len_inches = 12 *feet + inches; // Type conversion operators operator double(); operator int() return len_inches; // Overloaded stream output operator friend ostream &operator<<(ostream &out, Length a); ;

38 Length::operator double() return len_inches /12 + (len_inches %12) / 12.0; ostream &operator<<(ostream& out, Length a) out << a.getfeet() << " feet, " << a.getinches() << " inches"; return out;

39 int main() Length distance(0); double feet; int inches; distance.setlength(4, 6); cout << "The Length object is " << distance << "." << endl; // Convert and print feet = distance; inches = distance; cout << "The Length object measures " << feet << " feet." << endl; cout << "The Length object measures " << inches << " inches." << endl; return 0; Program Output The Length object is 4 feet, 6 inches. The Length object measures 4.5 feet. The Length object measures 54 inches.

40 Literature Tony Gaddis, Judy Walters, Godfrey Muganda, Starting out with C++, 2011.