MyLibrary

MyLibrary.cpp

#include <MyLibrary.h>

MyLibrary::MyLibrary()
{}

void MyLibrary::begin(void)
{
  Serial.print("MyLibrary text1: ");
  Serial.println(text1);
  Serial.print("MyLibrary text2: ");
  Serial.println(text2);
#ifdef text3
  Serial.print("MyLibrary text3: ");
  Serial.println(text3);
#endif
  Serial.println("Library out");
}

MyLibrary.h

#ifndef MyLibrary_h
#define MyLibrary_h

 #define text1 "Text 1 Library"
 #define text2 "Text 2 Library"
 
 #include <Arduino.h>

class MyLibrary
{
public:

// Class constructor
MyLibrary();
void begin(void);

private:

};

#endif

MyLibraryTest.ino

#ifdef text2
#undef text2
#define text2 "Text 2 Main before include"
#endif

#include <Arduino.h>
#include <MyLibrary.h>

#ifdef text1
#undef text1
#define text1 "Text 1 Main after include"
#endif

#define text3 "Text 3 Main only"

MyLibrary mylib = MyLibrary();

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  Serial.println();
  
  Serial.print("Main text1: ");
  Serial.println(text1);
#ifdef text2
  Serial.print("Main text2: ");
  Serial.println(text2);
#endif
  Serial.print("Main text3: ");
  Serial.println(text3);

  mylib.begin();
}

void loop() {
  // put your main code here, to run repeatedly:

}

output.txt

Main text1: Text 1 Main after include
Main text2: Text 2 Library
Main text3: Text 3 Main only
MyLibrary text1: Text 1 Library
MyLibrary text2: Text 2 Library
Library out