Classes in C++

[Mr.Anderson]

A programming tutorial: C++ classes
Written by Jo "Mr. Anderson" Cat.

Greetings and salutations, citizens.
I'm Mr. Anderson, and I've come to teach you about classes in C++. This is actually my first C++ tutorial, but don't let that deter you. In reading this tutorial, I'm assuming you have a remotely basic knowledge of C++. Code snippets are tabbed at least one space. Let's dive right in.

A class, in its most basic form, is a struct with functions.
Easy enough.

To create a class named NiceClass, start with...

	#ifndef NiceClass_h
	#define NiceClass_h

Notice that these statements don't have semicolons at the end.
They don't need them because they're special. In this sense,
all #ifndef and #define do is check to see if something else already included them.

Next, we add to our code by tagging on the 'introductory' part of the class. This tells the compiler that this is the name of the class, and this is where it begins.

	#ifndef NiceClass_h
	#define NiceClass_h

	class NiceClass
	{

We then go on to add the _private_ members of our class.
Isn't that neat. Private members are variables that are only accessible to the internal functions of the class. This is to be sure you don't accidentally change one of them.

	#ifndef NiceClass_h
	#define NiceClass_h

	class NiceClass
	{
		private:
			int NiceNumber;
			char NiceLetter;

Notice that there's no initialization in this step, only declaration.
You _CANNOT_ initialize a variable in the private or public class, leave that for the constructor. We'll get to that right about... wait for it... now.

	#ifndef NiceClass_h
	#define NiceClass_h

	class NiceClass
	{
		private: // Can't be accessed by anyone but the class.
			int NiceNumber; // A number.
			char NiceLetter; // A letter.
		public:
			NiceClass(); // The  constructor!
			void setNiceNumber( int ); // This sets the value of the number.
			void setNiceLetter( char ); // This sets the value of the letter.
			int getNiceNumber( void ); // This returns the value of the number.
			char getNiceLetter( void ); // THis returns the value of the letter.
	}; // END!

Pretty simple. Though there's something missing.

	#ifndef NiceClass_h
	#define NiceClass_h

	class NiceClass
	{
		private:
			int NiceNumber; // A number.
			char NiceLetter; // A letter.
		public:
			NiceClass(); // This is the all important constructor. This doesn't have any return type.
			void setNiceNumber( int ); // This sets the value of the number.
			void setNiceLetter( char ); // This sets the value of the letter.
			int getNiceNumber( void ); // This returns the value of the number.
			char getNiceLetter( void ); // THis returns the value of the letter.
	};

	#endif // The real end!

Of course, for every beginning, there must come an end. Just add an #endif and you've created the class.
So, we need to actually make the functions of the class. Do this by saying...

whatever classname::classfunction( whatever )

Though it should be noted that all functions should be included in a .cpp file. NOT the header file. (Thanks to Joe_Burin for pointing this out.)

In our case...

// NiceClass.h
	#ifndef NiceClass_h
	#define NiceClass_h

	class NiceClass
	{
		private: // Can't be accessed by anyone but the class.
			int NiceNumber; // A number.
			char NiceLetter; // A letter.
		public:
			NiceClass(); // This is the all important constructor. NO RETURN TYPE!
			void setNiceNumber( int ); // This sets the value of the number.
			void setNiceLetter( char ); // This sets the value of the letter.
			int getNiceNumber( void ); // This returns the value of the number.
			char getNiceLetter( void ); // THis returns the value of the letter.
	}; // END!

	#endif

// NiceClass.cpp

	#include "NiceClass.h"

	NiceClass::NiceClass()
	{
		NiceNumber = 0;
		NiceLetter = 'a';
	}

	void NiceClass::setNiceNumber( int userNumber )
	{
		NiceNumber = userNumber;
	}

	void NiceClass::setNiceLetter( char userLetter )
	{
		NiceLetter = userLetter;
	}

	int NiceClass::getNiceNumber( void )
	{
		return NiceNumber;
	}

	char NiceClass::getNiceLetter( void )
	{
		return NiceLetter;
	}

Whew. That's a lot to learn, but don't worry. Once you get the hang of it, writing classes becomes a real time saver. To access your class, save the above text as a header file (.h, usually), then

	#include "NiceClass.h"

and declare a variable like you would in your main function...

	NiceClass NiceVariable;

Then, to set a value...

	NiceVariable.setNiceLetter('s');

And to return a value...

	SomeOtherVariable = NiceVariable.getNiceLetter();
	cout << NiceVariable.getNiceLetter();

That's all, folks. I hope this was helpful to you.
If you have any questions or comments,
e-mail me at FloatingPoint001@hotmail.com
with the subject "YourTutorial".

Best of luck to you.
--Jo "Mr. Anderson" Cat

Post Scriptum:
I've attached the .cpp and .h files.
I've tested the script, but I'm far from perfect.
If you see something that's wrong, e-mail me.

Many thanks to Joe_Burin for pointing out a reather significant error.

[joe_bruin]

defining the member functions of a class inside a header function is BAD! other than the fact that it's bad form to put executable code in header files anyway, if this header was included in multiple .cpp files, you would get a "function redefinition" error by the linker.

the correct way to do this is to have a header (.h) file that contains the class declaration, and a .cpp file (usually of the same name) that defines the methods. using the author's examples:

/************* NiceClass.h ************/
#ifndef NiceClass_h
#define NiceClass_h

class NiceClass
{
private: // Can't be accessed by anyone but the class.
  int NiceNumber; // A number.
  char NiceLetter; // A letter.
public:
  NiceClass(); // This is the all important constructor. NO RETURN TYPE!
  void setNiceNumber( int ); // This sets the value of the number.
  void setNiceLetter( char ); // This sets the value of the letter.
  int getNiceNumber( void ); // This returns the value of the number.
  char getNiceLetter( void ); // THis returns the value of the letter.
}; // END!

#endif

/************* NiceClass.cpp ************/
#include "NiceClass.h"

NiceClass::NiceClass()
{
  NiceNumber = 0;
  NiceLetter = 'a';
}

void NiceClass::setNiceNumber( int userNumber )
{
  NiceNumber = userNumber;
}

void NiceClass::setNiceLetter( char userLetter )
{
  NiceLetter = userLetter;
}

int NiceClass::getNiceNumber( void )
{
  return NiceNumber;
}

char NiceClass::getNiceLetter( void )
{
  return NiceLetter;
}

[Mr.Anderson]

Originally posted by joe_bruin
defining the member functions of a class inside a header function is BAD! other than the fact that it's bad form to put executable code in header files anyway, if this header was included in multiple .cpp files, you would get a "function redefinition" error by the linker.

the correct way to do this is to have a header (.h) file that contains the class declaration, and a .cpp file (usually of the same name) that defines the methods. using the author's examples:

/************* NiceClass.h ************/
#ifndef NiceClass_h
#define NiceClass_h

class NiceClass
{
private: // Can't be accessed by anyone but the class.
  int NiceNumber; // A number.
  char NiceLetter; // A letter.
public:
  NiceClass(); // This is the all important constructor. NO RETURN TYPE!
  void setNiceNumber( int ); // This sets the value of the number.
  void setNiceLetter( char ); // This sets the value of the letter.
  int getNiceNumber( void ); // This returns the value of the number.
  char getNiceLetter( void ); // THis returns the value of the letter.
}; // END!

#endif

/************* NiceClass.cpp ************/
#include "NiceClass.h"

NiceClass::NiceClass()
{
  NiceNumber = 0;
  NiceLetter = 'a';
}

void NiceClass::setNiceNumber( int userNumber )
{
  NiceNumber = userNumber;
}

void NiceClass::setNiceLetter( char userLetter )
{
  NiceLetter = userLetter;
}

int NiceClass::getNiceNumber( void )
{
  return NiceNumber;
}

char NiceClass::getNiceLetter( void )
{
  return NiceLetter;
}

I suppose you're right, it's simply a matter of my (probably idiotic) tendancies to make evrything compact. Thanks for pointing that out. I owe you one.

-- Jo "Mr. Anderson" Cat

Thanks again.

[sde]

We're still getting used to this tutorial submission process.

The Original Post is the one that shows up in the tutorial viewer.

If there are agree with the corrections that joe bruin suggested, .. could you please 'edit' the first post?

This is the only way the tutorial viewer will show the correct tutorial.

Thanks
-=SDE

[Rotkiv]

am i aloud to post here?

just wanted to say that was an excelent tutorial! finally figured out what that public/privat thing was all about.