Questions on Classes

[specter113]

Im studying classes and have questions regarding this sample code that was given in the tutorial section.

1)What is this line of code? int area() {return width*height;}
2)When declaring a function that is to be used in the class why is :: being used?
3)How do you declare something to fall underneath the created class? Rectangle rect;? Can you not simply call the class like you would a function? Any tips on how to utilize classes.

Thanks!

  // classes example
#include <iostream>
using namespace std;

class Rectangle {
    int width, height;
  public:
    void set_values (int,int);
    int area() {return width*height;}
};

void Rectangle::set_values (int x, int y) {
  width = x;
  height = y;
}

int main () {
  Rectangle rect;
  rect.set_values (3,4);
  cout << "area: " << rect.area();
  return 0;
}

[HalfNOoB]

1) the method int area() {return widht*height} is nothing else than

int Rectangle::area()
{
    return width * height;
}

2) Dunno... Just do it ;) Those :: can be used for static objects as well! (Forget it if it confuses you)

3) Creating a class is like creating a new datatype (e.g. int / bool / float / double......). You are creating a datatype called Rectangle. How do you declare new variables?

 
Datatype Variablename;
int       Width;
Rectangle Rect; //<-- A new object of type Rectangle 

When your Declaration isnt a pointer, you use the "." Operator to Access functions from this datatype.
If your Declaration is a pointer, you use the "->" Operator to Access the function...

Classes are making c++ a OOP (Object Oriented Programming)-language and will be veeeeery useful in your future ;)

[specter113]

Sorry for the late response I had some drama at work. Once you build a class how do you use it and what is the overall purpose for creating it?

[TheIdeasMan]

Hi,

Just to mention the Information, Tutorials, and Reference and Articles section at the top left of this page. There is a wealth of information there.

Q2:

:: is the scope resolution operator: so the compiler knows which class the function belongs to.

[xenovia12]

1)What is this line of code? int area() {return width*height;}

you can actually define a member function that is use to set a value inside the class

2)When declaring a function that is to be used in the class why is :: being used? 

::operator is use to link the declaration and definition of a class.
it is like a prototype and the :: operator is use to define of a function

Rectangle rect;?  Can you not simply call the class like you would a function? 

well , you can simply call a function that is declared(prototype) at the top of the main function because it is global ( global scope ).
but how do you call function that comes from another files? simply, you should include the header where that function is declared because it has a difference scope ( file scope )

but definitely this is not the reason why you create an object.

[mutexe]

Can you not simply call the class like you would a function?

OP:
http://www.tutorialspoint.com/cplusplus/cpp_classes_objects.htm
I think learning the differences between classes and objects would be good.
A class is a one-time 'creation' that allows to you create (instantiate) as many objects as you want.

for example:

 class Wibble
{
.....
};


int main()
{
      Wibble w1;
      Wibble w2;
      Wibble w3;
      Wibble w4;

// you now have FOUR objects. Your Wibble class (of 
// which you have one and always one) is the
// thing that creates them
}