Thread: Classes

In addition to the above:

There are three levels of access in classes (by classes, I mean classes or structs):
public, private, and protected

Any symbol (typedefs, functions, variables, member classes, etc.) that are declared public can be accessed by anything. Those symbols which are declared as private can only be accessed by the class itslef, and classes/functions declared to be 'friends' of the class (not even derived classes have access to these). Any symbol that is protected can be accessed by friends and derived classes.

Constructors (that have the same name as the class, and any arguments you choose) are initializers. They are responsible for creating an instance of the class, and constructing its 'this' pointer. Desctrutors (same name, prefixed by '~') take no arguments, and destroy the object.

Neither the constructors nor the destructor need to be public, and as it is possible to have a static build function in a class, as well as have the class commit suicide (delete the 'this' pointer). These are rarely used, however.

One other thing. The difference between a class and a struct in C++ is simply that if you don't provide an access specifier (private, protected, public), then a class defaults to private, and a struct defaults to public.

This is a rather rapid (and incomplete) overview, but a start, I suppose.

A class is a user-defined datatype. It´s through classes you can create ADT (abstract datatypes). ADT are used to make computerlanguage more humanlanguage (in simple terms). I will illustrate this with an example

Lets say you want to code a program that handles infomation about employes in a company. Without classes

Code:

//Simulate an employe-object
//Code snippet 1
char *name = "Smith";
int age = 28;
double salery = 40000.0;

with classes

Code:

class employee
{
//Code snippet 2
//Methods should be public
public:
employee(char *pName = "Smith", int pAge = 28, double pSalery = 40000.0);
//Datamembers should be private
//private:
char *mname;
int mage;
double msalery;
};

Now of you study the code snippet 1 you can see that the information is scattered (it nothing that keeps all infomation together). This can (will) lead that the code is gonna be hard to read and maintain, especially when a program grows in size. But when using classes (code snippet 2) you actually encapsulate all necessary infomation to a single unit. But code snippet 2 only declares (dont allocates memory) what an employee is. If you want an employee-object (an instance) you have to define one.

Code:

employee Eric;

and to acces the elements of an employee-object

Code:

std::cout << "Employee name is: " << Eric.mname << std::endl;
std::cout << " and is " << Eric.mage << " years old" << std::endl;
std::cout << "and has a salery " << Eric.msalery << std::endl;

Classes is a big subject and don´t feel stupid if you don´t understand everything I said. It is hard.

I personlly like books more than tutorials because they have more depth.