Mircea Murar

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9 years, 11 months ago.

Classes question

Dear all,

I am planning to create an application that will write on a text LCD through a I2C IO Expander (PCF8574).

I have a custom made library for generating a menu functionality on a display, still I would also like to write data from the main procedure or other function from different cpp files.

The question is how to achieve this in the right way.

What I have tested already, was to create a "common.h" with the following content (and load it in every .h file):

common.h file

#ifndef __COMMON_H_INCLUDED__
#define __COMMON_H_INCLUDED__

#include "mbed.h"

I2C i2c_bus_2(PTE0, PTE1);
TextLCD_I2C lcd(&i2c_bus_2, 0x40, TextLCD::LCD20x4); // I2C bus, PCF8574 Slaveaddress, LCD Type

#endif

Reading about C++ classes I found out that this is not a healthy way for some reasons. Could you please guide me or explain how to create my application the right way.

Thanks, Mircea.

1 Answer

Martin Kojtal
9 years, 11 months ago.

Hi Mircea Murar,

don't place objects (variable, class, etc..) definitions in a header file, you might end up with multiply definitions (waste space) .

You can create an object in one code file (translation unit) globally and export it through declaration in a header file, so it becomes visible to other translations units.

The examples can be found here for example http://mylinuxbook.com/the-concept-of-extern-variables-in-c/.

The another way to do it, it's to create it locally (on stack/heap), and pass it around the program as parameter using a pointer type.

See this question http://programmers.stackexchange.com/questions/222626/is-it-better-to-use-an-external-variable-or-to-pass-around-a-pointer

Regards,
0xc0170

Accepted Answer

So in this instance you would want:

common.h

#ifndef __COMMON_H_INCLUDED__
#define __COMMON_H_INCLUDED__
 
#include "mbed.h"
#include "TextLCD_I2C.h" // the header file that defines your LCD driver
 
extern I2C i2c_bus_2;
extern TextLCD_I2C lcd;
 
#endif

main.cpp

#include "common.h"
I2C i2c_bus_2(PTE0, PTE1);
TextLCD_I2C lcd(&i2c_bus_2, 0x40, TextLCD::LCD20x4); // I2C bus, PCF8574 Slaveaddress, LCD

This way anything that includes common.h can assess both i2c_bus_2 and lcd directly but they are only declared in a single file.

posted by Andy A 30 May 2014

Hi Martin Kojtal

thanks for your tips, using extern seems to work fine.

Thanks again, Mircea

posted by Mircea Murar 03 Jun 2014