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.

Example

char foo() {
    static char id = 'A';
    cout << "foo: returning " << id << "\n";
    return id++;
}

auto glob = foo();

int main() {
    cout << "glob = " << glob << '\n';
    for (int i=0; i<5; i++) {
        if (i>10) {
            static char zip = foo();
            cout << "zip = " << zip << '\n';
        }
        static char bar = foo();
        cout << "bar = " << bar << '\n';
    }
    return 0;
}

foo: returning A
glob = A
foo: returning B
bar = B
bar = B
bar = B
bar = B
bar = B

CS253: Software Development with C++

Spring 2018