Sorry if this question been asked. I'm trying to activate certain function in a class using a macro defined in the class from main.cpp. I'm using jrowberg MPU6050 library if that matters.


#include "gyro.h"
gyro gyro(Serial);

void setup(){
void loop(){


  #include "gyro.h"
      // display Euler angles in degrees
      mpu.dmpGetQuaternion(&q, fifoBuffer);
      mpu.dmpGetEuler(euler, &q);
      serial_.print(euler[0] * 180/M_PI);
      serial_.print(euler[1] * 180/M_PI);
      serial_.println(euler[2] * 180/M_PI);

If I define OUTPUT_READABLE_EULER in gyro.h, it works. But I want to do it from main.cpp.

  • Try putting the define before you include the file.
    – Delta_G
    Sep 16, 2020 at 2:03

1 Answer 1


You cannot not, but also you should not try to define a macro in a cpp file and use it in another cpp file.

Instead, you have to define it in a h (header) file, which is included in both cpp files.

Why would you define it in main.cpp or main.h anyway if you don't use it in those files?

You have mainly three options:

  • Define it in gyro.h so you include it in gyro.cpp where you use it. You can later use it in other files as well.
  • Define it in gyro.cpp so you can use it only in that file. This prevents the macro being used in other files (which can be safer).
  • Define it in a separate file, e.g. euler.h and include this header file everywhere you need it.

See also Juraj's comments below.

  • 1
    this applies if the define should by in a source file. this kind of defines is usually set on compiler command line as -D. the Arduino IDE doesn't have -D define configuration for a project.
    – Juraj
    Apr 18, 2020 at 12:40
  • @Juraj in that case it could be an option to create a specific header file with those macros (one in this case) and include them in every file. Apr 18, 2020 at 12:45
  • 1
    yes. it is the way if the macro should be used only in the sketch's files (project). inconvenient is it for libraries. then the user must go to library folder and a modifiy a file there, but then it applies for all projects. if the setting is boards specific or one has a customized board definition for a project, then -D can be set in boards.local.txt as build.extra_flags. for example in Eclipse -D can be set in project's settings
    – Juraj
    Apr 18, 2020 at 12:51
  • @Juraj Thanks for that additional info. However, that doesn't work in the IDE I guess ?(as the Arduino IDE is not eclipse based) Apr 18, 2020 at 14:25

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