1. 1 Chapter 8 Operator Overloading

2. 2 User-defined Meaning for “built-in” Operators zNatural, suggestive usage yComplex number addition, subtraction, etc. zSemantic integrity yAssignment for objects with pointers zUniformity with built-in types y<< operator with user defined types

3. 3 Format for Overloaded Operator s int& Array::operator+ (int i); array1 = array2 + 10  array2.operator+(10) Standard return type and Class Operator keyword Operator being overloaded Parameter

4. 4 Example Overloaded Operators for Array Class class Array { private: int array[20]; public: Array(int init = 0); // overload the index operator, [] int& operator[](int i); ~Array(); }; int& Array::operator[](int i) { assert(0 <= i && i < 20); return array[i]; } Interface Implementation Checks array bounds

5. 5 Using Overloaded Operator Array array; array[0] = 1; array[1] = 1; // compute first twenty // Fibonnaci numbers for (int i = 2; i < 20; i++) array[i] = array[i-1] + array[i-2];

6. 6 Mixing User-Defined and Built- In Operations Array safe; int regular[20]; regular[10] = safe[10]; safe[11] = regular[11]; safe[0] = safe[0] + regular[0] “Safe” array in terms of index bounds Typical array, no index bounds checking

7. 7 Implementing Overloaded Operators for Arrays int& Array::operator[](int i) { assert(0 <= i && i < 20); return array[i]; } Index operator

8. 8 Additional Overloaded Array Operators class Array { private: int array[20]; public: int& operator[](int i); // addition operator Array operator+(Array& other); // subtraction operator Array operator-(Array& other); ~Array(); }; Adding + and - operators

9. 9 Implementation of + and - Overloaded Operators Array Array::operator+(Array& other) { Array result; for(int i=0; i<20; i++) result[i] = array[i] + other[i]; return result; } Array Array::operator-(Array& other) { Array result; for(int i=0; i<20; i++) result[i] = array[i] - other[i]; return result; }

10. 10 Multiply Overloading an Operator class Array { private: int array[20]; public: // addition operator Array operator+(Array& other); // add increment to all Array operator+(int increment); }; Array Array::operator+(int increment){ Array result; for(int i = 0; i< 20; i++) result[i] = array[i] + increment; return result; } Both are + operators, but parameters differ

11. 11 Usage of Multiply Overloaded Operators Array one(1); // all elements are 1 // add 1 to each element Array two = one + 1; // pairwise addition of Arrays one and two Array three = one + two; Can we do this? Array four = 1 + one;

12. 12 When to Overload Operators with Nonmember Functions zOperators on primitive data types yE.g., int, float, arrays zClass source not always available yE.g., ostream zType casting

13. 13 Operator Lookup Rules zSuppose: zCompiler looks for ‘op’ in this order yMember function in X of form yNonmember function of form yReturn type is not part of lookup Y y; X x; x op y; operator op(Y) operator op(X,Y)

14. 14 Nonmember Function Operator Overload Array operator+(int increment, Array array) { Array result; for(int i=0; i<20; i++) result[i] = array[i] + increment; return result; } Now we can do this! // add 1 to each element Array four = 1 + one;

15. 15 Input-Output Using a Method class Array{ private: int array[20]; public: //... void Print(); ~Array(); };

16. 16 Input-Output Using a Method (cont’d) void Array::Print() { cout << "[ "; for(int i = 0; i < 19; i++) cout << array[i] << ", "; cout << array[19] << " ]"; } Array test; test.Print(); [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]

17. 17 Defining Stream Operators ostream& operator<< (ostream& os, Array& a) { os << "[ "; for(int i = 0; i < 19; i++) os << a[i] << ", "; os << a[19] << " ]"; } ofstream outFile("array.data"); Array a; // write values of a to outfile outFile << a; // write to standard output cout << a; Cool! We can write to any ostream!

18. 18 Overloading Stream Operators ofstream& operator<<(ofstream& ofs, Array& a){ ofs.write((char*)(a.array), 20*sizeof(int)); return ofs; }; ofstream binaryFile("array.bin"); Array a; // write a to file in binary form binaryFile << a; binaryFile.close(); How do we know the binary implementation will be invoked? But this is private data!

19. 19 Declaring a "friend" Stream I/O Operator class Array { //... friend ofstream& operator<< (ofstream& ofs, Array& a); }; But not: class Array { //... friend class ofstream; } Why not?

20. 20 How do I make an overloaded << operator for my Parts Class? class Part { public: Part(PartType type, int time, int identifier, float quality); int GetTime() {return time;} int GetIdentifier() {return identifier;} PartType GetType() {return type;} float GetQuality() {return quality;} protected: PartType type; int time; int identifier; float quality; }; Show definition and declaration changes and additions.

21. 21 Stream Operator for a Part/Piece Object /* This is not in the Parts class. */ ostream& operator<< (ostream &os, Part&p) { os << “ (“ << TranslateType(p.type) << “) ID# “; os << p.identifier << “ Quality: “; os << setw(4) << setprecision(2) << p.quality; os << “ Entered: “ << TranslateTime(p.time); os << “ #Recycles: << p.recycles << endl; } friend ofstream& operator<< (ofstream& ofs, Part& p); This is in the Parts class.

22. 22 Type Conversion Object of Class Y Need Object of Class X Have Type Conversion

23. 23 Implicit and Explicit Type Conversion int i; float f; f = i; // implicit conversion f = (float)i; // explicit conversion f = float(i); // explicit conversion

24. 24 Type Conversion Have: Counter count(0); // manipulate count if ( count.Value() > 0)... if ( count.Value() == 100)... cout << count.Value(); Want: Counter count(0); // manipulate count if ( count > 0)... if ( count == 100)... cout << count;

25. 25 Defining a Type Conversion Operator class Counter: public DisplayableNumber {... public:... operator int();... }; Simple int type conversion operator Used when implicit or explicit conversion done on a Counter

26. 26 Constructors as Type Conversion Mechanisms class Counter : public DisplayableNumber {... public: Counter(int i);... }; A constructor can be viewed as a way to convert an int to a Counter.

27. 27 Type Conversion and Operator Overloading zGiven a class X zConversion can be used to simplify yX + int yint + X yX + X zProvide a basic conversion from int to X

28. 28 Type Conversion and Operator Overloading: Colors zColor is yRed, Green, Blue combinations zEach primary color 0 - 63 yColor something(Red(?), Green(?), Blue(?))

29. 29 Type Conversion and Operator Overloading: Colors Color aqua(Red(0), Green(50), Blue(50)); Color darkRed = Red(63); Color purple = darkRed + aqua + Blue(13); Color peach = purple - Blue(13) - Green(3); Color test1 = darkRed + peach; Color test2 = peach + darkRed; cout << aqua << endl; cout << purple << endl; cout << peach << endl; cout << darkRed << endl;