Summary - References
- C++ course
- Beginner
- Lesson 76/116
- 5 min
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.
Practice 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.
Starter code
#include <iostream>
// Define the swap function
int main() {
// Declare x and y
// Add a swap call
// Print x and y
return 0;
}Sample 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;
}Expected output
2 1
Practice 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.
Starter code
#include <iostream>
#include <vector>
// Define the printSum function
int main() {
// Declare the vector and add numbers
// Add a printSum call
return 0;
}Sample 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;
}Expected output
9
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 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 Correct answer
- 1 2
- 1 1
- Error
Explanation: swap(x, y) passes by reference, so x and y are exchanged.
2 swap body
void swap(int& a, int& b) { int t = ____; a = b; b = t; }void swap(int& a, int& b) { int t = a; a = b; b = t; }Explanation: int t = a; saves a into a temporary variable.
3 Match the reference tools
| Term | Meaning |
|---|---|
int& r = x |
Alias |
int& n |
Reference parameter |
const int& n |
Read-only |
void swap(int&, int&) |
Two references |