Unit 3: Functions & References

Two names for one thing.

Unit 3 of 8 in C++ for kids. Its 4 lessons are Making Functions, Copies and Originals, Handy Extras and Function Factory — below is everything each one explains, and a question or two from it to try.

Every sample on this page was compiled and run with g++ before it shipped, and prints exactly what it says it prints.

This unit opens with a fortnight’s trial of everything — no card needed — or with a family plan, bought in the iPhone app. The first two units of every track are free for ever. Try it in the app.

🧰 Making Functions

Exactly like C

Say what comes back, then the name, then what goes in. Nothing runs until you *call* it.

C++

int double_it(int n) {
    return n * 2;
}

int main() {
    cout << double_it(5) << endl;
    return 0;
}

It prints

Functions that hand nothing back

A function that only *does* something has the type void.

C++

void cheer(string who) {
    cout << "Go " + who + "!" << endl;
}

int main() {
    cheer("Ada");
    return 0;
}

It prints

Go Ada!

Try it yourself

What does this print?

C++

int add(int a, int b) {
    return a + b;
}

int main() {
    cout << add(2, 3) + add(1, 1) << endl;
    return 0;
}

Why must a function be written ABOVE main?

It looks tidier
C++ reads top to bottom, so it has to know the function before main uses it
main must be last in the file
It does not matter

Answer them in the app

📄 Copies and Originals

A function gets a copy

Hand a value to a function and it works on its own copy. Change it inside and the original never notices — the same trap you met in C.

C++

void tryToChange(int n) {
    n = 99;
}

int main() {
    int score = 5;
    tryToChange(score);
    cout << score << endl;
    return 0;
}

It prints

A reference is another name

Put & after the type and the parameter stops being a copy — it becomes another name for the very same variable. C needed a pointer and a * for this; C++ just uses the name.

C++

void grow(int& n) {
    n = n + 10;
}

int main() {
    int score = 5;
    grow(score);
    cout << score << endl;
    return 0;
}

It prints

Try it yourself

What does this print?

C++

void reset(int& n) {
    n = 0;
}

int main() {
    int lives = 3;
    reset(lives);
    cout << lives << endl;
    return 0;
}

And this one? Look closely at the & — it is missing.

C++

void reset(int n) {
    n = 0;
}

int main() {
    int lives = 3;
    reset(lives);
    cout << lives << endl;
    return 0;
}

Answer them in the app

🎁 Handy Extras

Values you can leave out

Give a parameter a default and callers may skip it. Handy when there is an obvious usual answer.

C++

int power(int base, int times = 2) {
    int total = 1;
    for (int i = 0; i < times; i++) {
        total = total * base;
    }
    return total;
}

int main() {
    cout << power(3) << endl;
    cout << power(3, 3) << endl;
    return 0;
}

It prints

9
27

Passing big things cheaply

A vector handed in by value gets copied — slow for a big one. A reference avoids the copy, and lets the function change it.

C++

void addOne(vector<int>& v) {
    v.push_back(1);
}

int main() {
    vector<int> nums;
    addOne(nums);
    addOne(nums);
    cout << nums.size() << endl;
    return 0;
}

It prints

Try it yourself

What does this print?

C++

void shout(string word, string mark = "!") {
    cout << word + mark << endl;
}

int main() {
    shout("go");
    shout("stop", "?");
    return 0;
}

What does this print? (No & this time.)

C++

void addOne(vector<int> v) {
    v.push_back(1);
}

int main() {
    vector<int> nums;
    addOne(nums);
    cout << nums.size() << endl;
    return 0;
}

Answer them in the app

🏆 Function Factory

Try it yourself

What does this print?

C++

void swapThem(int& a, int& b) {
    int keep = a;
    a = b;
    b = keep;
}

int main() {
    int x = 1;
    int y = 2;
    swapThem(x, y);
    cout << x << y << endl;
    return 0;
}

What does this print?

C++

int countdown(int from) {
    if (from <= 0) {
        return 0;
    }
    cout << from;
    return countdown(from - 1);
}

int main() {
    countdown(4);
    cout << endl;
    return 0;
}

Answer them in the app