Functions and collections · Lesson 7 of 21

References: by value or by reference?

Does a function get a copy or the original, and why const& is everywhere.

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When you pass a variable to a function, C++ passes a copy (by value). Changes inside the function hit the copy; the original stays the same. To let the function change the original, pass it by reference with &.

C++

#include <iostream>

void addBonusCopy(int xp) { xp += 50; }
void addBonusRef(int& xp) { xp += 50; }

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

int main() {
    int xp = 100;
    addBonusCopy(xp);
    std::cout << "after copy: " << xp << "\n";
    addBonusRef(xp);
    std::cout << "after ref:  " << xp << "\n";

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

xp (in main)100int

xp (addBonusCopy)100 → 150a separate copy

xp (addBonusRef)→ main's xpint& (another name)

A reference isn't a new variable: it's another name for the same memory.

Copying an int is cheap; copying a long std::string or a million-element vector isn't. So when a function only reads, pass a `const` reference: no copy, and no accidental changes.

C++

#include <iostream>
#include <string>
#include <vector>

// Reads only: const& avoids copying the whole vector
int total(const std::vector<int>& scores) {
    int sum = 0;
    for (int s : scores) sum += s;
    return sum;
}

int main() {
    std::vector<int> scores = {90, 75, 88};
    std::cout << total(scores) << "\n";

    int lives = 3;
    int& alias = lives;    // alias and lives are the same int
    alias--;
    std::cout << lives << "\n";
    return 0;
}

What's the output?

#include <iostream>
void f(int a, int& b) {
    a = a * 10;
    b = b * 10;
}
int main() {
    int x = 2, y = 3;
    f(x, y);
    std::cout << x << " " << y << "\n";
}

What will this code print?

Exercise

Write void doubleAll(std::vector<int>& v) that doubles every element in place. The program should print 2 4 6.

Expected output: 2 4 6

C++

#include <iostream>
#include <vector>

void doubleAll(std::vector<int>& v) {
    for (int x : v) {
        x *= 2;
    }
}

int main() {
    std::vector<int> v = {1, 2, 3};
    doubleAll(v);
    for (int x : v) std::cout << x << " ";
    std::cout << "\n";
    return 0;
}

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#include <iostream> void addBonusCopy(int xp) { xp += 50; } void addBonusRef(int& xp) { xp += 50; } void swapValues(int& a, int& b) { int temp = a; a = b; b = temp; } int main() { int xp = 100; addBonusCopy(xp); std::cout << "after copy: " << xp << "\n"; addBonusRef(xp); std::cout << "after ref: " << xp << "\n"; int x = 1, y = 2; swapValues(x, y); std::cout << x << " " << y << "\n"; return 0; }

#include <iostream> #include <string> #include <vector> // Reads only: const& avoids copying the whole vector int total(const std::vector<int>& scores) { int sum = 0; for (int s : scores) sum += s; return sum; } int main() { std::vector<int> scores = {90, 75, 88}; std::cout << total(scores) << "\n"; int lives = 3; int& alias = lives; // alias and lives are the same int alias--; std::cout << lives << "\n"; return 0; }

#include <iostream> #include <vector> void doubleAll(std::vector<int>& v) { for (int x : v) { x *= 2; } } int main() { std::vector<int> v = {1, 2, 3}; doubleAll(v); for (int x : v) std::cout << x << " "; std::cout << "\n"; return 0; }