Beginner who wants to migrate to platformIO but a lot of things that I do not understand (my sketch works on Arduino IDE)

PlatformIO IDE
4

Exactly, the code was meant to be an .ino file and not a .cpp file; The Arduino IDE does pre-processing on the file, namely, scanning the file for every function implementation and declaring it at the top of the file.

This is invalid C++ code (but valid .ino because of the preprocessing):

void setup() {
   doStuff();
}

void doStuff() { 
  Serial.println("Doing stuff..");
}

because at the time of usage, doStuff() is not previously declared, thus unknown to the compiler. This is fixed by adding the function declaration void doStuff(); over the setup() function.

You can:

  • rename your main.cpp file to main.ino to make PIO also do the same preprocessing
  • or fix the source by making it actually valid C++; see the documentation / FAQ
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