Lesson 167 +10 XP

Global Variables

A global variable is declared outside any function. It is born when the program starts and lives until the program ends.

How to declare one

int playerScore = 0;      // global

int addPoints(int n) {
    playerScore += n;     // any function can touch it
    return playerScore;
}

Because it's global, playerScore is visible and writable from every function in the file. Any function, any time.

The lifespan

Globals have static storage duration: they are created before main runs and destroyed only when the program exits. They never "go away" mid-program the way locals do.

Why (non-const) globals are discouraged

They're often called evil in C++ circles, and there are real reasons:

Hard to reason about: any function in the file can change a global. To understand its value, you must check every function - not just the code in front of you.
Hidden connections: functions stop being independent. Two functions that don't call each other can still secretly depend on the same global.
Reordering bugs: the order functions run changes the outcome, so moving a call around changes behavior.

int counter = 0;

void a() { counter += 1; }   // who knows what a() does to the rest of the program?
void b() { counter *= 2; }

When globals are okay

Constants are fine: const int MAX_PLAYERS = 4; can't be changed, so it can't cause surprises.
Small amounts of program-wide state that never changes are acceptable.
For mutable shared state, prefer passing values into functions as parameters.

TL;DR

A global is declared outside all functions and lives for the whole program.
Its lifespan: created before main, destroyed at exit (static storage duration).
Mutable globals are discouraged: hard to reason about, hidden connections.
Functions sharing globals secretly depend on each other.
Constants as globals are great; mutable globals are not.

Global Variables