Avoiding multiple inclusions of header files.

We sometimes need to #include header files inside other header files.

byte.h

typedef unsigned char Byte;

const Byte MIN_BYTE = 0;

const Byte MAX_BYTE = 255;

IPAddress.h

#include "byte.h"

class IPAddress

{

public:

...

private:

Byte IP[4];

};

The above is equivalent to coding:

typedef unsigned char Byte;

const Byte MIN_BYTE = 0;

const Byte MAX_BYTE = 255;

class IPAddress

{

public:

...

private:

Byte IP[4];

};

EthernetAddress.h

#include "byte.h"

class EthernetAddress

{

public:

...

private:

Byte Ethernet[6];

};

The above is equivalent to:

typedef unsigned char Byte;

const Byte MIN_BYTE = 0;

const Byte MAX_BYTE = 255;

class EthernetAddress

{

public:

...

private:

Byte Ethernet[6];

};

... but we will get a syntax error (Multiple declarations of Byte) if we #include IPAddress.h AND Ethernet.h in a file:

myApp.cpp

#include "IPAddress.h"

#include "EthernetAddress.h"

...

equivalent to:

typedef unsigned char Byte;

const Byte MIN_BYTE = 0;

const Byte MAX_BYTE = 255;

class IPAddress

{

public:

...

private:

Byte IP[4];

};

<= redefinition

typedef unsigned char Byte;

const Byte MIN_BYTE = 0;

const Byte MAX_BYTE = 255;

class EthernetAddress

{

public:

...

private:

Byte Ethernet[6];

};

...

There is a standard coding technique to avoid multiple inclusions of the same header file.

<= preprocessor directives

#ifndef BYTE_H

#define BYTE_H

typedef unsigned char Byte;

const Byte MIN_BYTE = 0;

const Byte MAX_BYTE = 255;

#endif