Thread: Questions on classes reference to single variable

So, I know that this code works, but I have several questions.

1) Is is practical to have multiple classes reference to a single variable or class entity ?

2) When writing a multithreading program, will this technique break?

3) is there a smarter or better approach, than this simple code?

Code:

#include <iostream>
using namespace std;

class Increase
{
	int & numb;
public:
	Increase( int & n ) : numb(n) { }

	void by( unsigned int val) {  numb += val; }

};

class Decrease
{
	int & numb;
public:
	Decrease( int & n) : numb(n) { }

	void by( unsigned int val) { numb -= val; }
};


int main(void)
{
  int number = 20;
  Increase inc(number);
  Decrease dec(number);

  inc.by(20);
  cout<< number << "\n";
  dec.by(10);
  cout<< number <<"\n";
  return 0;
}