1. Structures Revisited
what is an aggregate construct? What aggregate constructs have we studied?
what is a structure? what is the keyword to define a structure?
what are structures used for?
is a structure definition an executable statement? Can it be put in a header file?
why includeing a header file with structures definition multiple times is a problem? how is this problem solved?
what are #define #ifndef #endif?
what is a structure variable?
what is the term for elements of a structure?
do elements of the same structure (different structures) have to have unique names?
how can structures be initialized? what happens when one structure variable is assigned the value of another?
can structure variables be directly compared? passed as parameters? by value? by reference? can a function return a structure?
can one structure be included in another structure? What is a substructure?
can a structure include an array? how can an array of structures be declared?

2. Classes

3. What’s Wrong with Structures?
structure is an aggregate construct providing encapsulation
encapsulation - combining a number of items in a single entity
consider implementing date:
as structure:
struct Date{
int month;
int day;
int year; };
and a set of functions manipulating dates:
void setDate(Date &d, int m, int d, int y);
void addYear(Date &d, int n);
bool compare(Date &d1, Date &d2);
problems:
there is no explicit connection between data type (structure) and these functions
it does not specify that the functions listed should be the only ones that access and modify date variables
if there is a bug in Date manipulation - it can be anywhere in the program
if modification of Date is needed - all program needs to be updated

4. Class Definition class is an aggregate construct
class may contain member variables (attributes) and member functions (methods)
member variables and member function prototypes are declared within class definition
member functions can manipulate member variables without accepting them as parameters
class Date { // class name
public: // ignore this for now
void set(int, int, int);
int getDay();
int month;
int day;
int year;
}; // don’t forget the semicolon
a variable of type class is called object (how is a variable of type structure called?)
Date mybday;
the object is said to belong to the class or be an instance of the class
each object has member variables and can call member functions of its class
values of all member variables is the state of the object
addressing the members is done using dot-operator:
mybday.set(10, 26, 68);
cout << mybday.day;

5. Public and Private Members
public/private attributes control the way the class members are accessed (why do we want to do that?)
public member can be accessed within member functions (no scope resolution needed) and outside (with dot operator)
private member - can only be accessed within member functions
class Date { // class name
public:
void set(int, int, int);
int getday();
private:
int month;
int day;
int year;
}; // don’t forget semicolon
make member variables private, make functions either public or private. This restricts manipulation of variables to member function which makes debugging and changes in class easier
examples:
mybday.set(10, 5, 99); // good
mybday.year=99; // ERROR - private member

6. Member Function Definitions
functions can be defined either inside or outside class definition
outside definition - class name (called type qualifier) and scope resolution operator (::) precedes function name:
void Date::set(int m, int d, int y){
month=m; // no dot with member variables
day=d; // no declaration of member variables
year=y; }
to define inside - replace prototype with definition
class Date {
public:
void set(int, int, int);
int getDay(){return day;}
private:
int month;
int day;
int year;
}; // don’t forget semicolon
function defined inside is called in-line function
for style: use for very small functions. One line is good

7. Mutators and Accessors
accessor function - member function that does not modify the state of an object (only returns the information about the object’s state)
mutator function - member function that modifies the state of an object
accessors should be marked with const type modifier so that compiler can find accidental object state modification
also remember to use const with parameters that are not modified
class Date {
public:
// mutator
void set(int, int, int); // mutator
int getMonth() const; // accessor
int getDay() const {return(day);} // accessor in-line
private:
int month;
int day;
int year; };
separate mutators and accessors - a function should not do both
since variables are private they all need accessor
are we missing an accessor in Date?

8. Constructors
constructor - mutator that is invoked automatically when object is declared
used to assign an initial state to the object (initialize object)
constructor it has the same name as class
constructor does not return a value
do not put void as a return value of constructor
class Date{
public:
Date(int, int, int); // constructor
private:
int month, day, year; };
outside constructor definition (can also be inlined)
Date::Date(int m, int d, int y){
month=m;
day=d;
year=y; }
constructor is invoked at declaration: Date mybday(10,26,99); or directly: yourbday=Date(10,26,00);

9. Multiple/Void Constructors
class can have multiple constructors
which constructor to call is determined by number and type of arguments
function overloading – same name functions distinguished by number/type of arguments
it is bad style to define constructors that differ by type of arguments only - confusing
if no constructors declared - object can be defined without initialization
void (or default) constructor - a constructor that does not take arguments
caveat: if at least one constructor declared - object has to be always initialized (needs a void constructor)
class Date {
public:
Date(int, int, int); // constructor
Date(int, int); // another constructor
Date(); // void or default constructor
private:
int month;
int day;
int year; };
calling default constructor (if constructors defined): Date mybday;
if array of objects, default constructor is invoked Date friendsbday[20];
what is this? Date mybday();
It looks like a default constructor call but to the compiler it looks like a function prototype

10. Constructors for Member Objects
(void) constructors for member objects are invoked before constructor for containing object
example
class one{ public: one() { cout << ”one's constructor" << endl; }; };
class two{ public: two() { cout << ”two's constructor" << endl; };
private: one o;
int main() {
two ob2; }
prints
one’s constructor two’s constructor
note, if containing object does not have a constructor, member object (void) constructors are still invoked
It looks like a default constructor call but to the compiler it looks like a function prototype

11. Friend Functions
to allow non-member function to access private class members it should be declared as friend function:
class Date {
public:
// compares two dates
friend bool equal(const Date&, const Date&);
private:
int month;
int day;
int year; };
a friend-function is not a class member - it’s definition should not be preceded with type qualifier (Date::)
no separate prototype (outside class definition) is necessary for friend
equal is invoked as non-member function - without dot operator:
if(equal(date1, date2)) { ...
friend-functions circumvent information hiding and should be used as rare as possible and with great care
only use friend-function if the task requires access to private members of more than one object and the objects are used symmetrically

12. Static Constants
the scope of static constant is the class: it is available to all methods
it is stored only once but every object of this class has access to it
can be private or public
class Date {
public:
...
private:
static const int CENT21 = 2001; };
variables can also be declared static. Careful with those since they are in effect global variables (not as problematic, why?)

13. Program Layout program with objects is laid out as follows:
header - class definition (inline function definitions), global constants and other non-executable constructs related to class
program file - member function definitions
multiple related classes may be put in one header/program file pair

14. Classes Review What is class? Why are classes needed? What is object?
What is the difference between object and class? What do we mean when we say object belongs to a class?
What’s member variable? Member function? Method? Attribute?
What’s an in-line function definition?
What are these operators used for? “.” “::”
What is the difference between public and private members of class?
What is the state of an object?
What is mutator? Accessor? How is accessor function distinguished?
What is constructor? Is constructor mutator or accessor?
What is the name of constructor?
How many parameters can constructor take? Can there be more than one constructor? What is constructor overloading? What is void constructor?
What is the return value of constructor?
What is friend function? Why is it needed?
What are static attributes?