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

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.

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?

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

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)

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

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.

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); }

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

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

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);

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

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

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.

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;

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

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

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

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

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

Next lecture Object Life-times Aggregation vs composition