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
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 slidesclass 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 slidesclass 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
Examplesfloat * 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 deleteclass CPatient {private:
char * p;int age;// etc.
constructorCPatient::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 memoryp = new char[ strlen(n) + 1];strcpy(p,n);// etc
destructorCPatient::~CPatient(){
delete p[ ];}
Constructor allocates memory dynamicallyallocating only the memory required for the
string
Destructor now required to de-allocate memory
name is replaced with p; a pointer to char
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
class CPatient {private:
char * p;int age;// etc.
constructorCPatient::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 memoryp = new char[ strlen(n) + 1];strcpy(p,n);// etc
destructorCPatient::~CPatient(){
delete p[ ];}
Constructor allocates memory dynamicallyallocating only the memory required for the string
Destructor now required to de-allocate memory
name is replaced with p; a pointer to char
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