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C++
  • 1. C++ Basics
    • 1.1 Input, Output, and Program Structure
      • 1.1.1 Welcome to Data Structures in C++
      • 1.1.2 Hello World
      • 1.1.3 Input and Output
      • 1.1.4 getline and cin
      • 1.1.5 Program Structure
    • 1.2 Basic Data Types
      • 1.2.1 Basic Data Types
        • 1.2.1.1 Differences between C++ and Java Data Types and Variables
      • 1.2.2 Strings and Characters
      • 1.2.3 Numbers
      • 1.2.4 Booleans
    • 1.3 Conditional Statements
      • 1.3.1 Conditional Statements
      • 1.3.2 Basic If/Else Statements
      • 1.3.3 Comparing Strings
      • 1.3.4 Logical Operators
    • 1.4 Loops
      • 1.4.1 Loops
      • 1.4.2 For Loops
      • 1.4.3 While Loops
      • 1.4.4 Searching a String
    • 1.5 Functions in C++
      • 1.5.1 Functions in C++
      • 1.5.2 Defining and Calling Functions
      • 1.5.3 Passing by Reference vs Value
      • 1.5.4 Function Prototypes
  • 2. Going Beyond the Basics
    • 2.1 Vector Basics
      • 2.1.1 Vector Basics
      • 2.1.2 Creating and Accessing Vectors
      • 2.1.3 Inserting into a Vector
      • 2.1.4 Looping Through a Vector
    • 2.2 Function Default Values
      • 2.2.1 Function Default Values
      • 2.2.2 Default Values
      • 2.2.3 Default Values with a Prototype
      • 2.2.4 Example: Splitting a String
    • 2.3 Structs
      • 2.3.1 Structs
      • 2.3.2 Defining and Accessing Structs
      • 2.3.3 Using Structs: Line Length
    • 2.4 File Input/Output
      • 2.4.1 File Input/Output
      • 2.4.2 Reading in a File
      • 2.4.3 Processing a File
      • 2.4.4 Writing to a File
      • 2.4.5 Creating an Input Stream from a String
    • 2.5 Error Handling
      • 2.5.1 Error Handling
      • 2.5.2 Validating a Number
      • 2.5.3 Validating a Vector Index
      • 2.5.4 Throwing Other Values
  • 3. Libraries
    • 3.1 Header Files
      • 3.1.1 Header Files
      • 3.1.2 Header File
      • 3.1.3 Header and Implementation File
      • 3.1.4 Safer Header
    • 3.2 Using Libraries
      • 3.2.1 Using a Utilities Library
      • 3.2.2 The Util Library
  • 4. 2D Vectors, Stacks, and Queues
    • 4.1 2D Vectors
      • 4.1.1 2D Vectors
      • 4.1.2 The 2D Vector
      • 4.1.3 Creating a 2D Vector
    • 4.3 Stacks
      • 4.3.1 Stacks
      • 4.3.2 Basic Stack
      • 4.3.3 Stack Example: Reverse a String
    • 4.5 Queues
      • 4.5.1 Queues
      • 4.5.2 Basic Queues
      • 4.5.3 Queue Example: Next in Line
  • 5. Sets and Maps
    • 5.1 Pairs and Iterators
      • 5.1.1 Pairs and Iterators
      • 5.1.2 Pairs
      • 5.1.3 Iterators
    • 5.3 Sets
      • 5.3.1 Sets
      • 5.3.2 Basic Sets
      • 5.3.3 Iterating Through a Set
      • 5.3.4 Sets of Struct Values
    • 5.4 Maps
      • 5.4.1 Maps
      • 5.4.2 Map Basics
      • 5.4.3 Iterating Through a Map
      • 5.4.4 Updating Maps
  • 6. Recursion
    • 6.1 Functional Recursion
      • 6.1.1 Functional Recursion
      • 6.1.2 Basic Recursive Problem: Exponential
      • 6.1.3 Recursion Example: Reverse String
      • 6.1.4 Recursion Example: Make Sum
    • 6.2 Procedural Recursion
      • 6.2.1 Procedural Recursion
      • 6.2.2 Print Binary
      • 6.2.3 Print Permutations
      • 6.2.4 Depth vs Breadth Search
  • 7. Pointers, Linked Lists, and Graphs
    • 7.1 Pointers
      • 7.1.1 Pointers
      • 7.1.2 Assigning and Updating Pointers
      • 7.1.3 Pointers and Functions
      • 7.1.4 Pointers and Data Structures
    • 7.2 Linked Lists
      • 7.2.1 Linked Lists
      • 7.2.2 Basic Linked List
      • 7.2.3 Linked List and Recursion
      • 7.2.4 Example: Sorted Phone Book
      • 7.2.5 Doubly Linked List
    • 7.3 Graphs
      • 7.3.1 Graphs
      • 7.3.2 Basic Example: Breadth First Search
      • 7.3.3 Application: Connecting Cities
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  • Pass By Value
  • Pass By Reference
  • Why Use Pass by Reference?
  • Try This Example
  1. 1. C++ Basics
  2. 1.5 Functions in C++

1.5.3 Passing by Reference vs Value

Previous1.5.2 Defining and Calling FunctionsNext1.5.4 Function Prototypes

Last updated 3 years ago

By default, parameters are passed by value in C++. This means the value of a parameter is copied into the function. Changes to the parameter do not change the variable that was originally used in the calling function. This is the same as how arguments are passed in Java.

C++ does offer another option called pass by reference where a value in the function points back to the original value. Let’s explore these two options for C++.

Pass By Value

Take a look at the code below as it runs.

Pass by Value

Notice in this example how the value of variable a in the main function does not change when the addFive function adds 5 to the parameter n. That value gets returned and you can see how the original value of a is still 8 and the added value (stored in b) is now 13.

This is the default for a function.

Pass By Reference

In contrast, C++ offers the option to pass by reference. Instead of creating a copy of a variable for the function, C++ uses a pointer back to the variable in the calling function. Any updates to the parameter in the function also updates the variable in the calling function. This is an often used feature.

To pass by reference, add an & before the variable name in the parameter list for the method.

Take a look below at the example as it executes.

If you look at the right hand side, you notice that the parameter n actually points back to the same memory location of a. Instead of making a copy of the variable, it uses the value stored in memory for a. As a result, when the function adds 5 to the parameter n, it is actually adding 5 to the value of a.

When this value is returned to b, you can see that both a and b in the main function are now 13.

Why Use Pass by Reference?

As mentioned above, you will use pass by reference often as you learn to code in C++. There are 2 main use cases for using pass by reference. First, it is very helpful when you want to make changes to several variables in a function. While you can only return one value, if you pass by reference, you can update any number of values in a function and have these updates reflected in the calling function.

The second option is for times when you use large amounts of data, for example when you have a large data structure such as a list. By passing by reference, you can avoid using additional memory to create the copy of the large database.

Try This Example