How to declare a global variable that could be used in the entire program

Question

I have a variable that I would like to use in all my classes without needing to pass it to the class constructor every time I would like to use it. How would I accomplish th

Answer 1

Just declare it outside the class:

Header file:

extern int x;

class A {
  int z;
  public:
  A() : z(x++) {}
};

One source file:

int x = 0;