Static Variables

Static variables are often a source of confusion for beginning programmers. In part, this is due to being overloaded with too many details all at once. Consider this issue, we study static variables by first considering a relatable problem. Consider the following code:

#include <iostream>
	using namespace std;

	int x = 1;

	void func1() {
		x++;
		cout << x << endl;
	}

	int main() {
		func1();
		func1();
		func1();
		return 0;
	}

1
2
3

In the code above, we have a global variable, int x = 1. We then have a function, func1(), which increments the global variable, x by 1. Thus, when we go into main(), each call to func1() results in an increment of x.

Now, suppose we don't want that x to be a global variable. There are many reasons for why might not want x to be a global variable (in fact, we generally do not want global variables). Perhaps x is a variable that should only be mutated by func1(); no other function or module should be able to touch it. How do we achieve this? Well, why not just place x inside the body of func1()? Let's try that:

#include <iostream>
using namespace std;

void func1() {
	int x = 0;
	x++;
	cout << x << endl;
}

int main() {
	func1();
	func1();
	func1();
	return 0;
}

1
1
1

Nope, that didn't work. Remember: After a function has finished executing, the stack where its variables are allocated is popped off. Thus, we never see 1 2 3 because each function call starts with int x = 0. Static variables are intended to address this problem:

#include <iostream>
using namespace std;

void func1() {
	static int x = 0;
	x++;
	cout << x << endl;
}

int main() {
	func1();
	func1();
	func1();
	return 0;
}

1
2
3

By appending the static keyword to int x = 0, we've ensured that x is treated as a static variable of the function func1(). This effectively makes x a variable akin to being “global” to func1(). Thus, for every call to func1(), func1() uses the current value of x.

Like global variables, static variables exist in memory so long as our program runs. The different: Static variables are only accessible to their respective modules. In this case, x is only accessible to func1(); nowhere else.