See this page as a slide show

Local Static

CS253 Local Static

Try #1

Here’s a program to generate unique student ID numbers:

unsigned long next_id() {
    unsigned long id=800000000UL;
    return ++id;
}

int main() {
    cout << next_id() << '\n';
    cout << next_id() << '\n';
    cout << next_id() << '\n';
}

800000001
800000001
800000001

That wasn’t very good.

Try #2

Let’s move id out of next_id():

unsigned long id=800000000UL;

unsigned long next_id() {
    return ++id;
}

int main() {
    cout << next_id() << '\n';
    cout << next_id() << '\n';
    cout << next_id() << '\n';
}

800000001
800000002
800000003

That’s better, but now we have an evil global variable.

Try #3

Let’s make id static:

static unsigned long id=800000000UL;

unsigned long next_id() {
    return ++id;
}

int main() {
    cout << next_id() << '\n';
    cout << next_id() << '\n';
    cout << next_id() << '\n';
}

800000001
800000002
800000003

That’s better in that id is only visible to this file, but that’s still semi-global. Can we do better?

Try #4

Move id back to next_id(), but leave it static.

unsigned long next_id() {
    static unsigned long id=800000000UL;
    return ++id;
}

int main() {
    cout << next_id() << '\n';
    cout << next_id() << '\n';
    cout << next_id() << '\n';
}

800000001
800000002
800000003

Hooray! Now, id is private and persistent.

Summary

A variable has two aspects, scope and lifetime.

scope

determines which code can access the variable

lifetime

determines when the variable is created, and when it’s destroyed

static changes both of these. It changes the scope of a global variable to be only the current file. It also changes the lifetime of the variable so that it is initialized only once, and persists until the program ends.

A static global is initialized at program start. A static local is initialized upon first use—when the function is first called.