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Reference parameters

Learn pass-by-reference and how changes propagate to the caller.

Reference parameters

If you declare a function parameter as a reference, the function can modify the caller's variable directly. The syntax: void addOne(int& n). The & mark says the parameter is a reference, not a copy.

So a call addOne(x) changes x itself, not a copy. Pass by value does not propagate changes; pass by reference does. Reference parameters are also used to avoid copying large objects.

Pass by value versus by reference

In pass by value, the parameter is a copy of the argument; changes inside the function do not reach the caller. In pass by reference, the parameter is an alias bound to the argument itself; changes propagate at once.

void f(int n) is by value, void f(int& n) is by reference. The only visual difference is the & mark, but the behavior is completely different. Pass by reference is also used to return more than one value from a function.

Practice The addOne function

Define a void addOne(int& n) function; in the body write n = n + 1;. In main declare int x = 5;, call addOne(x), and print x with std::cout.

Starter code

main.cpp
#include <iostream>

// Define the addOne function

int main() {
    // Declare x
    // Add an addOne call
    // Print x
    return 0;
}

Sample solution

main.cpp
#include <iostream>

void addOne(int& n) {
    n = n + 1;
}

int main() {
    int x = 5;
    addOne(x);
    std::cout << x << "\n";
    return 0;
}

Expected output

6

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What is the value of x after this code runs?

main.cpp
void addOne(int& n) {
    n = n + 1;
}
int main() {
    int x = 5;
    addOne(x);
    std::cout << x;
    return 0;
}
  • 5
  • 6 Correct answer
  • 0
  • Gives an error

Explanation: addOne(x) passes by reference, so x increases: 5 → 6.

2 Match each expression to its meaning

Term Meaning
int& n Reference parameter
addOne(x) Call
n = n + 1 Body
x Argument

3 Reference parameter

Question 
void addOne(int____ n) { n = n + 1; }
Answer 
void addOne(int& n) { n = n + 1; }

Explanation: void addOne(int& n) is pass by reference.

All lessons in Beginner

1 C++ basics

  1. What is C++?
  2. Your first program
  3. std::cout
  4. std::cin
  5. Comments
  6. Summary - C++ basics

2 Variables

  1. Declaring variables
  2. auto
  3. const
  4. constexpr
  5. Type conversion
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. bool and char
  4. std::string
  5. sizeof
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Bitwise operators
  6. Summary - Operators

6 Strings

  1. std::string basics
  2. String concatenation
  3. String length
  4. Substring
  5. String search
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. The switch statement
  4. The ternary operator
  5. Nested conditionals
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. Range-based for
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding products
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. C-style arrays
  2. Array access
  3. std::array
  4. Multidimensional arrays
  5. Arrays and loops
  6. Summary - Arrays

11 std::vector

  1. Declaring vectors
  2. push_back
  3. size and at
  4. Vector access
  5. Vector loop
  6. Summary - std::vector

12 Functions

  1. Defining functions
  2. Return values
  3. Parameters
  4. Function overloading
  5. Default parameters
  6. Summary - Functions

13 References

  1. What is a reference?
  2. Declaring references
  3. Reference parameters
  4. const references
  5. Dangling references
  6. Summary - References

14 Pointer basics

  1. What is a pointer?
  2. The & address operator
  3. Pointer dereference
  4. nullptr
  5. Pointer arithmetic
  6. Summary - Pointer basics

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string operations

  1. String streams
  2. stoi and stod
  3. to_string
  4. String comparison
  5. String modification
  6. Summary - Advanced strings

17 Structs

  1. Defining structs
  2. Using structs
  3. Struct and function
  4. Array of structs
  5. Nested structs
  6. Summary - Structs

18 Exception basics

  1. try/catch
  2. throw
  3. std::exception
  4. Custom exceptions
  5. catch all
  6. Summary - Exceptions

19 Namespaces

  1. Defining namespaces
  2. using
  3. std namespace
  4. Nested namespaces
  5. Namespace alias
  6. Summary - Namespaces

20 Beginner final

  1. Project design
  2. Data model
  3. Business logic
  4. Error handling
  5. Putting it together