How can I move functions to an external file?

Hi, I have recently started to develop locally. As my app grows, I find myself distracted by a bunch of functions at the beginning of my one application.cpp file. I am looking for a way to clean up my code. I know I could write a few libraries for them, but the functions are very diverse and rather unconnected - I would prefer to simply move them to a separate file to have them put of the way, and to somehow bake them into the compilate.

I am pretty new to c++ and could not find anything that made me feel I was on the right track. Maybe someone can help? I’d be glad!

Yes, you can do this:

  • create a new header file (e.g. myapp.h) in the inc/ folder, and put your function prototypes in there. Add the include to “application.cpp”
    #include "myapp.h"

myapp.h might look like this:

void myfunc();
int doSomethingAmazing(int a, String s);
  • Create a new .cpp file in the src/ folder, say myapp.cpp. You put the function bodies in there, like this:
#include "myapp.h"

void myfunc() {
    // do stuff

int doSomethingAmazing(int a, String b) {
   return 42;
  • Finally, to get the build system to recongnize your new cpp file, add this to src/

CPPSRC += $(TARGET_SRC_PATH)/myapp.cpp

And that’s it!

Oh, that was easy. Great! One question though, if you don’t mind.
When I do define a configuration const in application.cpp, how can I make it be seen in my new external function file as well?

let’s say you have this const in your .cpp file:

const int magicNumber = 42;

Then you declare it in the header file like this:

extern const int magicNumber;

What this tells the compiler is that somewhere, there is a constant, and then it is resolved at link time.

There is also an easier option - move the const to the header file - simply copy and paste:

const int magicNumber = 42;

Then include the header whenever you need to access the constant. Most compilers optimize away the storage for the constant, unless you take the address of the constant.

Ok, so I have my standard application.cpp file:

const int RGB_FEEDBACK_DURATION = 250;
#include "helpers.h"


#include "application.h"
extern const int RGB_FEEDBACK_DURATION;
void rgb_black ();


#include "helpers.h" // I added this because of (A), see below.
void rgb_black () {
     RGB.color(0, 0, 0);

Even though I honestly don’t know what I am doing here, the reasons for adding (A): The compiler would complain about not declared in this scope for RGB, which is a built-in library, and RGB_FEEDBACK_DURATION.

What’s the status now? The compile error goes away. (error: 'RGB_FEEDBACK_DURATION' was not declared in this scope)

I now have a new (linker?) error, undefined reference to 'RGB_FEEDBACK_DURATION'

Mysterious! I’d be happy for any further help.

Try putting “helpers.h” before RGB_FEEDBACK_DURATION in application.cpp - that should fix the linker problem.

The reason you had to include helpers.h in helpers.cpp is to also include application.h (which is in helpers.h) - this includes all the built-in libraries, including, RGB.color().

Thank you for your patience and help. It all works now and I do have the impression I have learned something. Great!

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Great to hear you got it working!

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