declarations in libraries

Using Arduino Programming Questions
10

As for declaring variables in .h files being standard practice, let's look at EEPROM.h/.cpp:

#ifndef EEPROM_h
#define EEPROM_h

#include <inttypes.h>

class EEPROMClass
{
  public:
    uint8_t read(int);
    void write(int, uint8_t);
};

extern EEPROMClass EEPROM;    // <<< reference to a variable declared elsewhere

#endif

#include <avr/eeprom.h>
#include "Arduino.h"
#include "EEPROM.h"

uint8_t EEPROMClass::read(int address)
{
	return eeprom_read_byte((unsigned char *) address);
}

void EEPROMClass::write(int address, uint8_t value)
{
	eeprom_write_byte((unsigned char *) address, value);
}

EEPROMClass EEPROM;   // <<< variable declaration is in .cpp file