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Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Week 6 Lab 2 is marked Hand in Lab 3 Questions from Last Week Operator Overloading.

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Presentation on theme: "Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Week 6 Lab 2 is marked Hand in Lab 3 Questions from Last Week Operator Overloading."— Presentation transcript:

1 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Week 6 Lab 2 is marked Hand in Lab 3 Questions from Last Week Operator Overloading Lab 4

2 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Schedule

3 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Stack vs. Heap Allocate on the stack: if (x > 0) { Rectangle r(3,4); cout << r.area(); } Object exists only while execution is between the brace brackets

4 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Stack vs. Heap Allocate on the heap: if (x > 0) { Rectangle* pr = new Rectangle(3,4); cout area(); } Object continues to exist until it is deleted If you plan to save a pointer and use it elsewhere, it must point to a location on the heap

5 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Gets and Sets class BankAccount { private: int balance; //in pennies public: int getbalance() {return balance;} void setbalance(int bal) {balance = bal;} // other stuff };

6 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Encapsulation to the Rescue class BankAccount { private: float dollarbalance; //in dollars public: int getbalance() {return (int) (dollarbalance*100);} void setbalance(int bal) {dollarbalance = bal/100.0;} // other stuff };

7 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Overloading Revisited When a two functions have the same name, they are overloaded class foo { public: int something(int x); int something(); };

8 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Operator Overloading All languages have operator overloading int x = 1 + 1; float f = ; Usually, the compiler is the only one who “gets to play”

9 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Operator Overloading How do you compare two objects? Date d1, d2; // they get values somehow if (d1.equals(d2)) { // something }

10 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Operator Overloading Wouldn’t this be nicer? Date d1, d2; // they get values somehow if (d1 == d2) { // something }

11 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Operator Overloading How about this? Date d1, d2; // they get values somehow if (d1 != d2) { d1 = d2 + 3; }

12 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel How do you overload an operator? You write a function The name is operator followed by the symbol operator+ operator==

13 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Where does the function go? For binary operators: class A, B; A a; B b; // give them values somehow int x = a + b;

14 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Operator Overload as Member Function // in A.h class A { // whatever else it has int operator+(B b); }; // in A.cpp int A::operator+(B b) { // something }

15 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Operator Overload as Global Function // not in any class int operator+(A a, B b); Typically the declaration is in the header file for A and the implementation is in the implementation file for A

16 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Operator Overload as Global Function // in A.h class A { // whatever else it has }; int operator+(A a, B b); // in A.cpp int operator+(A a, B b) { // something }

17 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Your class isn’t always on the left class A; A a; int x = A + 2; int y = 2 + A; What functions does the compiler look for?

18 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Your class on the left int x = A + 2; A::operator+(int i) operator+(A a, int i) The choice is yours

19 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Your class on the right int x = 2 + A; int::operator+(A a) –Not possible! operator+(int i, A a) –Your only choice

20 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Coding a Global Operator Sometimes it’s easy: class A { private: int x; public: int operator+(int arg); //other stuff }; int operator+(int arg, A a);

21 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Coding a Global Operator Here’s a neat trick int A::operator+(int arg); { return x + arg; } int operator+(int arg, A a); { return a + arg; }

22 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Coding a Global Operator Sometimes the operator is not reversible like that The global function will need access to private member variables of the class it works with It needs to be an honourary member of the class –friend

23 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Where does the function go? For unary operators: class U; U u1, u2; // give them values somehow u2 = !u1;

24 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Operator Overload as Member Function // in U.h class U { // whatever else it has U operator!(); }; // in U.cpp U U::operator+() { // something }

25 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Operator Overload as Global Function // in U.h class U { // whatever else it has }; U operator!(U u); // in U.cpp U operator!(U u) { // something }

26 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel What can you overload? Binary + - * / % += -= *= /= %= (pre and post) ^ & | > = <= == != []

27 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel What can you overload? Unary + - ! Some other scary ones eg &

28 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Operator Consistency Operator+ should do something that feels like adding –Matrix add, complex number add –Container (list, queue) add an element –String concatenate –Increase date Don’t mess with people’s heads

29 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Operator Consistency If you have defined similar operators, they should work the same way. These three expressions should all have the same result: A a; a = a + 1; a += 1; a++;

30 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Tip Use one operator to implement the others: bool A::operator==(const A& arg) { // whatever } bool A::operator!=(const A& arg) { return !(*this == arg); }

31 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel More Rules You can’t change the order of operations You can’t invent new operators, including unary versions of binary-only operators such as /. You can’t overload operators that work on only fundamental types: int operator+(int i, int j)

32 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Overloading << cout << “my number is “ << 3 << endl; Employee e; cout << e << endl; Compiler is looking for ostream& operator<<(ostream& o, Employee e) Typically the code is just like a display() function Return the ostream& that was passed

33 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Simple Array Class Implement an Array class with –Range checking –Array assignment –Arrays that know their size –Outputting entire arrays with << –Array comparisons with == and != –Element access with []

34 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel array.h class Array { friend ostream& operator<<( ostream& o, const Array& a); public: Array( int size = 10 ); Array( const Array& a); //copy constructor ~Array(); int getSize() const { return size; } const Array& operator=( const Array& a); bool operator==( const Array& a) const; bool operator!=( const Array& right ) const { return !( *this == right ); } int& operator[]( int ); const int& operator[]( int ) const; private: int size; int* ptr; // pointer to actual content };

35 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Using the Array class int main() { Array integers1(7), integers2; cout << "integers1:" << integers1 << endl; cout << "integers2:" << integers2 << endl; integers1[5] = 1000; cout << integers1[5] << endl; if ( integers1 == integers2 ) cout << "They are equal\n\n"; else cout << "They are not equal\n"; return 0; }

36 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Implementing Array - constructor Array::Array( int arraySize ) { size = ( arraySize > 0 ? arraySize : 10 ); ptr = new int[ size ]; for ( int i = 0; i < size; i++ ) ptr[ i ] = 0; }

37 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Implementing Array - destructor Array::~Array() { delete [] ptr; } Whenever your destructor does something destructive, you must code a copy constructor and an assignment operator

38 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Destructive Destructors Not all classes have a destructor that actually cleans up –Free memory –Close file –Release lock or database connection or... When the destructor cleans up you need to be sure it will never go off accidentally

39 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Shallow and Deep Copies An object holds a resource (pointer to memory, name of file, pointer to connection object) If you copy the object bit-for-bit, now two objects hold the same pointer (or handle or name or whatever) When one goes out of scope, the destructor cleans up –The other has a pointer to nowhere!

40 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Implementing Array – copy constructor Array::Array( const Array& init ) : size( init.size ), ptr(new int[init.size]) { for ( int i = 0; i < size; i++ ) ptr[i] = init.ptr[i]; }

41 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Copy Constructor takes a reference Imagine this code: Array a2(a1); This uses the copy constructor to construct a2 Takes by value: need to make a copy of a1 to pass to the function –Use the copy constructor. –But that takes by value, so need to make a copy Use the copy constructor...

42 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Implementing Array – assignment operator const Array& Array::operator=( const Array right ) { // always check for self-assignment if ( &right != this ) { if ( size != right.size ) { delete [] ptr; size = right.size; ptr = new int[ size ]; } for ( int i = 0; i < size; i++ ) ptr[ i ] = right.ptr[ i ]; } return *this; // enables x = y = z; }

43 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Copy Constructor vs Assignment operator It’s not about whether the = operator is used It’s about whether something is being constructed Array a1(10); Array a2(a1); Array a3 = a2; a1 = a3;

44 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Implementing Array – equality test bool Array::operator==( const Array& right ) const { if ( size != right.size ) return false; // arrays of different sizes for ( int i = 0; i < size; i++ ) if ( ptr[ i ] != right.ptr[ i ] ) return false; // arrays are not equal return true; // arrays are equal }

45 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Implementing Array – element access int& Array::operator[]( int subscript ) { if( subscript >= 0 && subscript < size ); return ptr[ subscript ]; else // should throw exception or something exit(1); } Why does it return a reference? integers1[5] = 1000;

46 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Implementing Array const int& Array::operator[]( int subscript ) const { if( subscript >= 0 && subscript < size ); return ptr[ subscript ]; // const reference else // should throw exception or something exit(1); } For use with const arrays (since it’s const) Guarantees the object will stay const by not providing a reference to the inside, which could be changed.

47 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Implementing Array ostream& operator<<( ostream& output, const Array& a ) { int i; for ( i = 0; i < a.size; i++ ) { output << a.ptr[i] << endl; } return output; //enables cout << x << y; }

48 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel Remember The Requirements Implement an Array class with –Range checking –Array assignment –Arrays that know their size –Outputting entire arrays with << –Array comparisons with == and != –Element access with []

49 Monday, Feb 10, 2003Kate Gregory with material from Deitel and Deitel For Next class Enjoy Reading Week – see you Feb 24 Read chapter 9 Do Lab 4 Study for Midterm –Feb 24, class time, this room –Will cover everything till today –You may be asked to write code in handwriting –One hour, closed book –Worth 25%


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