1. More C++ Features True object initialisation
Composition (a type of aggregation)
Copy constructor and assignment operator =
Dynamic Memory
Pointers to objects

2. True object initialisation
Without a constructor function an object’s member data is created with random values
A constructor function can assign values to the member data overwriting the random values
True initialisation allows the constructor to create the member data with an initial value – subtly different from assignment!
Initialisation is achieved through the member initialisation list
Allows initialisation of constants in member data.

3. Object Assignment class CBulb {
public:
CBulb(int p, int s);
private:
int power;
int state; };
CBulb::CBulb (int p, int s) {
state = s;
power = p; }
Note: This is not a complete class definition. Only the statements relevant to the topic are shown
When the object is created :-
CBulb billy(60, 0);
state and power are created with random values and immediately overwritten with new values in the assignment
What about default function argument?
What about a default constructor?

4. Object Initialisation
class CBulb {
… As in previous slide };
CBulb::CBulb (int p, int s) : state(s), power(p) { }
Start of member initialisation list
comma separated list of member data with initial values in parentheses

5. Aggregation Object relationships Aggregation of objects
Aggregation forms object hierarchies
The “has a” or “part of” relationship between objects
1 to 1
1 to n (n = 0,1,… up to n)

6. Composition - CLamp class
A CBulb class – example code
A CSwitch class – example code
A lamp is composed of a bulb and a switch or
A lamp “has a “ bulb
A lamp “has a” switch
A composite class – CLamp
Clamp has an instance of a CSwitch and a CBulb as member data
Composition is one type of aggregation

7. Copy constructor and assignment
When a copy of an object is required (e.g. passing an object by value) a copy is required.
Assignment operator =
Consider :- CAny x, y; and the statement x = y;
binary operators such as = are interpreted as x.operator=(y)
i.e. x is an instance of a class with a member function called operator with an argument y.
operator= also requires a copy to be performed.
The compiler supplied default copy constructor does a member-wise copy; often this is sufficient.
The compiler supplied default operator= does a member-wise copy; again often sufficient.

8. User supplied copy constructor
consider class CPatient used in previous slides
class CPatient {
private:
char name[30];
int age;
char gender;
public:
CPatient (const CPatient & s); //copy constructor
// etc.
copy constructor is defined:-
CPatient::CPatient (const CPatient & s) {
gender = s.gender;
age = s.age;
strcpy(name, s.name); }

9. User supplied operator=
Consider class CPatient used in previous slides
class CPatient {
private:
char name[30];
int age;
char gender;
public:
const CPatient& operator= (const CPatient& s); // assignment operator
// etc.
Assignment operator is defined:-
const CPatient& CPatient::operator= (const CPatient& s) {
if (this == &s) // avoid self assignment – “this” is a pointer to the invoking object
gender = s.gender;
age = s.age;
strcpy(name, s.name); }
return (*this); // return the invoking object

10. The default copy constructor and operator=
The previous copy constructor and assignment operator are exactly the same as those that would be supplied by the compiler.
Therefore we can often use the defaults provided by the compiler
The operator= function is an example of operator overloading. Most of the standard operators may be overloaded
The operator= function must be a member function of the class

11. Pointers
In many cases it is often preferable to manipulate objects via pointers rather than directly.
Direct method
An object can be created :- CAny x;
manipulated directly using x.member_function(any_arguments);
Using pointers
CAny *op; //op is a pointer to an object of the class CAny
op = &x; //op is now pointing at x
member functions are invoked using ->
op->member_function(any_arguments);

12. Dynamic memory Global variables and objects
are stored in static memory
exist during the life of a program
Local variables and objects
are stored in the stack memory
use stack memory that is allocated and de-allocated automatically
are allocated at point of declaration
are de-allocated when they go out of scope
Dynamic variables and objects
Use the Heap memory
programmer is responsible for allocation and de-allocation of heap memory
new keyword allocates memory
delete keyword de-allocates memory

13. new keyword new allocates memory space from heap
new returns with a pointer to the allocated memory
new returns with 0 if no memory is available
Good for creating variable length arrays at run-time
General format
for primitive data types
pointer = new data_type; //single variable
pointer = new data_type[ number required]; //array
for user defined types i.e. classes
pointer = new classname(constructor arguments); //single object
pointer = new classname[number required]; //array – must have a default constructor

14. Dynamic memory Examples float * fp = new float; // creates 1 float
int * p = new int[50]; // creates an array of 50 int’s accessible either through the pointer p or using conventional array notation p[index]
CBulb * bp = new CBulb(60,0); // creates the bulb object in the heap memory and bp is set pointing at the bulb.

15. delete keyword
delete de-allocates the memory originally allocated by new.
must use the pointer that was allocated with new
Examples
delete fp;
delete [ ]p; // notice empty [ ] for releasing arrays
delete bp;

16. Use of new and delete Consider CPatient class in previous lecture
The data member name was a fixed char array of 30 characters
What if name is more than 30 characters?
Most names are less than 30 characters – wastes space
Use dynamic memory

17. Use of new and delete class CPatient { private: char * p;
int age;
// etc.
constructor
CPatient::CPatient(char * n, int a, char g) {
// find length of n and add 1 for null character //allocate that many chars and set p pointing //at the allocated memory
p = new char[ strlen(n) + 1];
strcpy(p,n);
// etc
destructor
CPatient::~CPatient()
delete p[ ]; }
name is replaced with p; a pointer to char
Constructor allocates memory dynamically
allocating only the memory required for the string
Destructor now required to de-allocate memory

18. Issues with use of new in classes
What about assignment operator =
What about copy constructor?
Defaults supplied by compiler perform “member-wise” copy
Problems with pointers!
member-wise copy only pointer is copied – shallow copy
copy pointer and data pointed to by pointer - Deep copy

19. Use of new and delete name is replaced with p; a pointer to char
class CPatient {
private:
char * p;
int age;
// etc.
constructor
CPatient::CPatient(char * n, int a, char g) {
// find length of n and add 1 for null character //allocate that many chars and set p pointing //at the allocated memory
p = new char[ strlen(n) + 1];
strcpy(p,n);
// etc
destructor
CPatient::~CPatient()
delete p[ ]; }
Constructor allocates memory dynamically
allocating only the memory required for the string
Destructor now required to de-allocate memory

20. Deep copy
Need to copy member data and any data that is used by pointers
May also need to modify other member functions
See Cpatient modifications

21. Next lecture
Object Life-times
Aggregation vs composition