Passing by Value and by Reference

By value: the function gets a copy

Normally a function receives a copy of each argument. Changing the copy doesn't change the original:

#include <iostream>
using namespace std;

void addTen(int n) {
    n += 10;
    cout << "Inside: " << n << "\n";
}

int main() {
    int score = 5;
    addTen(score);
    cout << "Outside: " << score << "\n";   // still 5!
    return 0;
}

Output

Inside: 15
Outside: 5

By reference: the function uses the original

Put an & after the type and the parameter becomes a reference: another name for the caller's variable. Now changes stick:

#include <iostream>
using namespace std;

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

int main() {
    int score = 5;
    addTen(score);
    cout << "Score: " << score << "\n";     // now 15
    return 0;
}

Output

Score: 15

Think of it like this: By value is giving someone a photocopy of your homework: they can scribble on it, your copy stays clean. By reference is giving them your actual notebook.

A classic: swap

#include <iostream>
using namespace std;

void swapValues(int& a, int& b) {
    int temp = a;
    a = b;
    b = temp;
}

int main() {
    int x = 1, y = 2;
    swapValues(x, y);
    cout << "x = " << x << ", y = " << y << "\n";
    return 0;
}

Output

x = 2, y = 1

const references: fast and safe

Copying a big string or list takes time. A const reference passes the original without copying it, and const promises the function won't change it:

#include <iostream>
#include <string>
using namespace std;

void shout(const string& message) {
    cout << message << "!!!\n";
}

int main() {
    shout("I love C++");
    return 0;
}

Output

I love C++!!!

Remember: Small values (int, double, char): pass by value. Big things you only read (string, vectors): pass by const&. Things the function must change: pass by &.