CPP Tutorials

Inline Functions in C++

Hello, young coders! Today, we’re diving into the fascinating realm of inline functions in C++. Even if you’re a beginner, don’t fret! Our exploration is designed just for you. So, get ready, and let’s start this thrilling journey together!

What are Inline Functions?

C++ offers inline functions to make function calls more efficient. An inline function is a function that’s directly inserted or substituted at the place where it’s called. This happens during the compilation process, and it helps reduce the overhead of function calls. Inline functions are useful for small code snippets, and they can make the program run faster by avoiding the usual function call process.


inline return-type function-name(parameters)
    // function code

Remember, inlining is only a request to the compiler, not a command. The compiler can ignore the request for inlining. 

The compiler may not perform inlining in such circumstances as: 

  • If a function includes a loop, such as a ‘for‘, ‘while‘, or ‘do-while‘ loop.
  • If a function uses static variables, which retain their values between function calls.
  • If a function calls itself in a recursive manner.
  • If a function has a return type other than ‘void‘ but lacks a ‘return‘ statement within its body.
  • If a function includes a ‘switch‘ statement or uses the ‘goto‘ statement.

Why Inline Functions are Used?

When a program runs and encounters a function call, the CPU remembers where it left off, saves the function’s arguments in a specific place in memory, and starts executing the called function’s code. Once the called function is done, it stores the result in a memory location and returns control back to where it left off in the original function. This process involves some time overhead.

For large or complex functions, the time spent on this process is usually minor compared to the time the function itself takes to run. But in the case of small, frequently used functions, the time taken for the function call overhead can be more than the time it takes to actually do the function’s task. This occurs because the time needed to switch between functions becomes significant when the function itself is very quick to execute.

Inline Functions in Real Life

Imagine you’re a chef in a busy kitchen. You have a go-to recipe that you use frequently. Instead of searching for the recipe every time you need it, you’ve memorized it. Now, you can prepare the dish much faster because you don’t waste time searching for instructions. This is similar to how inline functions work in programming. They allow us to include the code of a function directly where it’s called, eliminating the time it takes to jump to the function. It’s like having the recipe right at your fingertips, making your code run faster and more efficiently.

Inline function and classes

You can also create inline functions within a class. When you define a function inside a class, it’s automatically considered as an inline function. This means that the usual rules and benefits of inline functions apply here too. If you want to specifically declare an inline function within a class, just write the function’s declaration within the class and define it outside the class using the ‘inline‘ keyword. This way, you can make sure the function is treated as inline and follows the associated rules for optimization.

Here are the syntax examples for inline functions and classes in C++:

Inline Function:

#include <iostream>
using namespace std;

// Inline function declaration
inline int add(int a, int b) {
    return a + b;

int main() {
    int x = 5, y = 3;

    // Calling the inline function
    int result = add(x, y);

    cout << "Sum: " << result << endl;

    return 0;


Sum: 8

Inline Class:

#include <iostream>
using namespace std;

// Inline class definition
class Circle {
    double radius;

    // Constructor
    inline Circle(double r) : radius(r) {}

    // Inline member function to calculate the area
    inline double calculateArea() {
        return 3.14159 * radius * radius;

int main() {
    Circle circle(5.0);

    // Calling the inline member function
    double area = circle.calculateArea();

    cout << "Area of the circle: " << area << endl;

    return 0;


Area of the circle: 78.5397

Problem Statement and Uses

Problem Statement:

You are designing a calculator application that will perform basic arithmetic operations such as addition, subtraction, multiplication, and division. In order to ensure that the program runs as efficiently as possible for these basic operations, you decide to use inline functions.

Your task:

  • Design inline functions for each of the following arithmetic operations:
    • Addition
    • Subtraction
    • Multiplication
    • Division (assume non-zero denominator)
  • In your main function, allow the user to choose an operation by entering its name (e.g., “addition”) and then prompt them to enter two numbers. Use the appropriate inline function to perform the operation and then display the result.


  • Use the ‘inline‘ keyword before the function declaration to make a function inline.
  • Inline functions are a suggestion to the compiler to replace the function call with the actual code of the function, making the program potentially faster. However, the compiler may choose to ignore the ‘inline‘ keyword in certain situations (e.g., for very large functions).

Example of an inline function for addition:

inline double add(double a, double b) {
    return a + b;

Expected Output:

Here’s an example of how the program should run:

Choose an operation: addition, subtraction, multiplication, division
> addition
Enter first number: 5
Enter second number: 3
Result: 8

Example of Inline Function with Multiple Inheritance

Let’s look at some examples to understand inline functions better. We’ll start with a simple example and gradually add more complexity.

Inline Function with Multiple Inheritance

#include <iostream>
using namespace std;

class Base1 {
    inline void show() {
        cout << "Base1 show" << endl;

class Base2 {
    inline void display() {
        cout << "Base2 display" << endl;

class Derived : public Base1, public Base2 {};

int main() {
    Derived d;
    return 0;


Base1 show
Base2 display


  • The example involves three classes: ‘Derived‘, ‘Base1‘, and ‘Base2‘.
  • The class ‘Derived‘ inherits from both ‘Base1‘ and ‘Base2‘.
  • The ‘Derived‘ class can access the inline functions ‘show‘ and ‘display‘ from ‘Base1‘ and ‘Base2‘, respectively.

The Pros and Cons of Inline Functions

Improved performanceIncreased executable size
Eliminates function call overheadIncreased compilation time
Faster code executionLimited to small and simple functions
Can be used to optimize frequently called functionsMay result in code duplication
Better integration with the surrounding codeNot suitable for recursive functions
The Pros and Cons of Inline Functions

Key Takeaways

  • Optimization for Speed: The ‘inline‘ keyword is used to optimize functions for speed. It suggests the compiler replace the function call directly with the function code during compilation, reducing the overhead of the function call.
  • Small Functions:inline‘ is most effective for small functions. It’s suitable for functions with a relatively small number of statements. For larger functions, the benefit of inlining might be outweighed by increased code size.
  • Performance Boost: Inline functions can improve performance by reducing the function call overhead. However, this improvement might vary based on the compiler and the context of usage.
  • Header Files: Often, inline functions are defined in header files because their code needs to be visible to the compiler at the point of the function call.
  • Use with Caution: While ‘inline‘ can provide speed improvements, using it excessively can bloat the code and actually slow down the program due to increased memory usage.
  • Compiler Decision: The decision to inline a function ultimately lies with the compiler. It can choose to ignore the inline keyword and treat the function as a regular one.
  • The balance between Size and Speed: Using ‘inline‘ is a balance between execution speed and code size. It’s important to measure and profile your code to determine if using ‘inline‘ is beneficial.


In this article, we’ve explored the power of inline functions in C++. We’ve learned what they are, why they’re needed, and how they’re used. We’ve also looked at some real-life scenarios, explored the concept of multiple inheritance, and worked through several examples. Whether you’re a beginner just starting out or an experienced programmer looking to brush up on your skills, understanding inline functions is a crucial step in mastering C++.

Frequently Asked Questions

1. What is an inline function in C++?

In C++, an inline function is a function that is expanded in line when it is called. When the inline function is called whole code of the inline function gets inserted or substituted at the point of inline function call.

2. Why do we use inline functions in C++?

Inline functions are used to reduce the function call overhead. Inline function in C++ is an optimization technique used by the compilers especially to reduce the execution time.

3. How do we declare an inline function in C++?

An inline function in C++ is declared using the keyword inline before the function name.

4. What are the advantages and disadvantages of using inline functions in C++?

Advantages of inline functions include increased execution speed and improved readability. Disadvantages can include increased memory usage and the potential for increased complexity in the code.

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