Summary - References

Combine reference, const reference and safety to write a swap function.

Summary - References

In this group you learned the core tools of references: declaring, reference parameters, const references, and the dangling reference danger. References are a key tool for writing safe and efficient code in C++.

There are two tasks. First, you will write a swap function with two reference parameters. Second, you will define a printSum function with a const std::vector<int>& parameter. Both combine what you have learned.

Exercise The swap function

Define a void swap(int& a, int& b) function; in the body exchange a and b using an int t temporary variable. In main declare int x = 1, y = 2;, call swap(x, y), and print x and y on the same line separated by a space.

Show example solution

#include <iostream>

void swap(int& a, int& b) {
    int t = a;
    a = b;
    b = t;
}

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

Exercise The printSum function

Define a void printSum(const std::vector<int>& v) function; in the body start with int sum = 0;, use a range-based for to add each element to sum, and print sum. In main add 3 and 6 to a std::vector<int>, then call printSum.

Show example solution

#include <iostream>
#include <vector>

void printSum(const std::vector<int>& v) {
    int sum = 0;
    for (int n : v) {
        sum += n;
    }
    std::cout << sum << "\n";
}

int main() {
    std::vector<int> nums;
    nums.push_back(3);
    nums.push_back(6);
    printSum(nums);
    return 0;
}

Test yourself

What does this code print?

void swap(int& a, int& b) {
    int t = a;
    a = b;
    b = t;
}
int main() {
    int x = 1, y = 2;
    swap(x, y);
    std::cout << x << " " << y;
    return 0;
}

2 1
1 2
1 1
Error

Show answer

2 1 — swap(x, y) passes by reference, so x and y are exchanged.

swap body

void swap(int& a, int& b) { int t = ____; a = b; b = t; }

Show answer

void swap(int& a, int& b) { int t = a; a = b; b = t; }

Match the reference tools

int& r = x

Alias
int& n

Reference parameter
const int& n

Read-only
void swap(int&, int&)

Two references

Next lesson: What is a pointer?

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies