Why do I get compile errors if I comment out an include?

Libraries
1

I added a dependency to lib_deps in platformio.ini:

adafruit/Adafruit MCP23017 Arduino Library@^2.1.0

Then I ran “pio run” from the command line to compile it.

I got a bunch of errors about not being able to find headers for Adafruiit_BusIO (which is a dependency). The error messages suggest adding it as a dependency, so I did, but it didn’t help; then it complains about not being able to find Wire.h.

If I then add the include to main:

#include <Adafruit_MCP23X08.h>

Then everything compiles fine, even though I removed the dependency on BusIO. Comment it out, and everything is broken again.

This behavior is puzzling. Why should commenting out an unused #include break things? I’m getting compile errors in code that I’m not actually using. Why is it compiled at all?

Also, why can PlatformIO find the dependency of a dependency when the #include is there, but it can’t find it if it’s not there?

Also, where in the documentation does it explain how the build system works?

2

The LDF will resolve dependencies to the things it finds an #include for – that’s documented here and here.

All build options are documented. Under the hood, PlatformIO uses SCons (currently 4.3), which is documented. Some more technical details are here.