Classes and Pointers: Some Peculiarities

Transcription

1 Classes and Pointers: Some Peculiarities Dr. Mardé Helbig Dr. Mardé Helbig, COS 110 1

2 Issues with Destructor ~ void print Which method is the constructor? How is the constructor different from other methods? What is the goal of the constructor? Dr. Mardé Helbig, COS 110 2

3 Issues with Destructor (cont.) ~ void print(); Order::Order() clientnr = 0; numberofitems = 10; items = new int[10]; Dr. Mardé Helbig, COS 110 3

4 Issues with Destructor (cont.) #include <iostream> #include Order.h int main() Order myorder; Which method gets called at end of main? What is the goal of the destructor? for (int i=0; i < 10; i++) myorder.getitems.add(100*i); myorder.print(); return 0; Dr. Mardé Helbig, COS 110 4

5 Issues with Destructor (cont.) ~ void print(); Order::~Order() cout << Destroying object << endl; Which attributes do myorder have? Dr. Mardé Helbig, COS 110 5

6 Issues with Destructor (cont.) ~ void print(); clientnr numberofitems items What happens to the attributes when the destructor is called? items is a pointer - use new to create - use delete to destroy => change destructor Dr. Mardé Helbig, COS 110 6

7 Issues with Destructor (cont.) ~ void print(); Order::~Order() delete [] items; NB: items must have a valid value for destructor to work properly Dr. Mardé Helbig, COS 110 7

8 Issues with Assignment Operator #include <iostream> #include Order.h int main() Order myorder1; Order myorder2; myorder1 clientnr 109 numberofitems 10 items for (int i=0; i < 10; i++) myorder1.additem(100*i); myorder2 = myorder1; clientnr numberofitems return 0; myorder2 items Shallow copy of pointers Dr. Mardé Helbig, COS 110 8

9 Issues with Assignment Operator (cont.) What happens if myorder1 goes out of scope? myorder1 clientnr numberofitems items of myorder1 will be deallocated, but also items of myorder2. So if we try to reference items of myorder2 it will be invalid items Solution: Need to override assignment operator to do a deep copy when attribute is a pointer myorder2 clientnr numberofitems items Shallow copy of pointers Dr. Mardé Helbig, COS 110 9

10 Issues with Copy Constructor ~ void print(); Which method is the copy constructor? Order::Order(const Order& myorder) clientnr = myorder.clientnr; numberofitems = myorder.numberofitems; items = myorder.items; Shallow copy of items same issue as with assignment operator Dr. Mardé Helbig, COS

11 Issues with Copy Constructor (cont.) #include <iostream> #include Order.h int main() Order myorder1; for (int i=0; i < 10; i++) myorder1.additem(100*i); myorder1 clientnr 109 numberofitems 10 items Order myorder2(myorder1); clientnr 109 return 0; myorder2 numberofitems items 10 Shallow copy of pointers Dr. Mardé Helbig, COS

12 Issues with Copy Constructor (cont.) ~ void print(); Order::Order(const Order& myorder) clientnr = myorder.clientnr; numberofitems = myorder.numberofitems; items = new int[myorder.numberofitems]; for (int i=0; i < myorder.numberofitems; i++) items[i] = myorder.items[i]; Deep copy of items Dr. Mardé Helbig, COS

13 Issues with Pass by Value ~ void print(); void Orders::removeOrder(Order myorder) myorder.setnumberofitems(0); delete [] myorder.getitems(); Orders orderlist; orderlist.removeorder(myobject1); What is the problem here? Dr. Mardé Helbig, COS

14 Issues with Pass by Value void Orders::removeOrder(Order myorder) myorder.setnumberofitems(0); delete [] myorder.getitems(); Orders orderlist; orderlist.removeorder(myobject1); Pass by value is used but default copy constructor leads to shallow copy. Not effect that we want. Dr. Mardé Helbig, COS

15 Issues with Pass by Value (cont.) Solution: Ensure that copy constructor is correctly implemented. When an object is passed by value to a function, the copy constructor is called. Copy constructor is called when: An object is initialised using the value of another object An object is passed by value as a parameter The return value of a function is an object Dr. Mardé Helbig, COS