C++ “extern”

“extern” allow us using the same variable among different transition units(cpp files).

RegisterDay.cpp: here the count is declared & defined the first time in the whole program.

namespace gym {
  int count;
  void FirstTraining() {
    count += 1;
  }
}

TrainingDay.hpp: I also want to add up to the same variable count, to do so, I need to tell the program this count has already been declared somewhere (in our case, in RegisterDay.cpp).

namespace gym {
  extern int count;
}

Now in TrainingDay.hpp, I have informed the existence of count, I’m able to use it in all the *.cpp files which include TrainingDay.hpp file. For example:

CardioDay.cpp:

namespace gym {
  void run() {
    count += 1;
  }
}