how to create a variable in .h file

Projects Programming
1

Hey everone,
I'm having trouble with what I believe should be fairly straightforward. I'm trying to create a variable in a header file and use it a corresponding .cpp file. the code is a follows:

project.ino:

#include <EEPROM.h>
#include "Config.h"

void setup() 
{
  init();
}

void loop() 
{
  
}

Config.h:

//variables
unsigned char stored;     //data stored in EEPROM

void initPen();

Config.cpp:

#include <Arduino.h>
#include <EEPROM.h>

//Initialize everything
void init() 
{
  //Get stored data from EEPROM.
  stored = EEPROM.read(0); //read value at address 0
}

When compiling I get the following error:
Config.cpp:8: error: 'stored' was not declared in this scope

  • stored = EEPROM.read(0); //read value at address 0*

Shouldn't the variable 'stored' be available for the .cpp file with the same name as the .h file? or am I misguided here?

2

Shouldn't the variable 'stored' be available for the .cpp file with the same name as the .h file? or am I misguided here?

Yes, you are misguided.
You need to #include the file - there is nothing automatic about files with similar names being included.

3

You forgot to include that header file in you .cpp as well :wink:

But that will give you a new problem which is guarding. Add

#pragma once

at the top of the .h.

4

You also need to decide if that variable needs to be known in multiple files or only in config.cpp.

If only in config.cpp, you can simply declare the variable in the config.cpp instead of in the h file. Or use a different .h file (e.g. config_private.h) and include that file in config.cpp.

5

You shouldn't be "creating" the variable in the .h file at all. If you need to access that variable from multiple source files (the ONLY reason to have it in a .h file at all), then you must DECLARE it in the .h file. You then #include the .h file into every source file that needs to access the variable. Finally, you must DEFINE that variable as a global in exactly ONE of the source files. And, use Include Guards. Thus:

HeaderFile.h:

#ifndef _HEADER_FILE_H_
#define _HEADER_FILE_H_

extern uint8_t globalVariable;   // Declare the variable

#endif

MainFile.ino:

#include "HeaderFile.h"  // #include header in all files that need the variable

uint8_t globalVariable;  // Define the variable once and only once

void setup() {
  globalVariable = 7;  // Use the global variable
.
.
.
}

AnotherFile.cpp:

#include "HeaderFile.h"  // #include header in all files that need the variable

void aFunction() {  // some function
  globalVariable = 2;   // Use the global variable
.
.
.
}
6

AWOL:
You need to #include the file - there is nothing automatic about files with similar names being included.

  • Duh - thank you! I've been staring at this way too long.

septillion:
Add

#pragma once

at the top of the .h.

Thanks! That's a neat way of handling the guarding instead of the ifndef stuff.

sterretje:
If only in config.cpp, you can simply declare the variable in the config.cpp instead of in the h file. Or use a different .h file (e.g. config_private.h) and include that file in config.cpp.

Thanks! Yes I do need the data to be globally available.

gfvalvo:
You shouldn't be "creating" the variable in the .h file at all. If you need to access that variable from multiple source files (the ONLY reason to have it in a .h file at all), then you must DECLARE it in the .h file. You then #include the .h file into every source file that needs to access the variable. Finally, you must DEFINE that variable as a global in exactly ONE of the source files. And, use Include Guards.

Thank you. That's what I get for having english as a second language. Thank you for the example!

Conclusion:
No - cpp files don't inherit anything from their .h file counterparts. Remember to include those.
Use '#pragma once' in top of header files to be sure they're not included multiple times -> compile errors