Call C++ from C

In programming, there’s often a need for C and C++ to work together, typically when updating an old code or combining the modules in various languages. Using the C++ code in C programs is very helpful for programmers, especially when updating an old software or mixing the parts that are written in both languages. This guide will show you how to make the C++ functions work with C with simple and useful examples. Calling on C++ from a C program might seem tricky, but this article explains everything step by step, so it’s easy to follow along and start using the two languages together. This guide is helpful for your projects regardless of your level of programming knowledge.

Understanding the Basics

When it comes to invoking the C++ functions from C and using the C++ objects, several key elements need consideration:

Referencing the C++ Classes in C

A common approach to reference the C++ classes in C involves using pointers to classes that can be passed between the C and C++ code.

Name Mangling

To prevent the name changes that can affect a recognition in C, it’s crucial to keep the names consistent. This ensures that C can correctly identify and utilize the functions and objects.

Header File for Dual Purpose

A header file plays a pivotal role, serving a dual purpose for both C and C++. It includes the class definitions that are recognizable by C++ and encapsulates the accessor functions for C.

How to Call C++ from C

To call the C++ functions from C, the extern “C” declaration is crucial. When applied to C++ functions, it ensures that the function names are treated in the C style (using C-linkage) during the compilation process. C does not recognize the C++ features such as function overloading and name mangling which can occur during C++ compilation. Using extern “C”, the C++ compiler generates the function names that adhere to C conventions, allowing a seamless interoperability. This declaration provides a standardized interface, enabling a C code to access and utilize the C++ functions without encountering the naming conflicts or compatibility issues. The following are the examples of calling C++ from C. To comprehend how this combination functions, let’s look more closely at the instances.

Practical Examples: Calculate the Area of a Circle by Calling C++ from C

In this simple example, we will guide you on how you can calculate the area of a circle by calling a C++ function in C. This code consists of a header file (circle.h) and an implementation file (circle.cpp). The first part of the code involves the extern C declaration in the header file. See the following code. Then, let’s elaborate on each line of code individually:


extern "C" {

  double calculate_CArea(double rad);



double Circle_CArea(double rad) {

  return 3.14159 * rad * rad;


Let’s break down each part:

#ifndef CIRCLE_AREA and #define CIRCLE_AREA

These lines are part of include guards which ensures that the contents of the header file are included only once in the compilation process. If CIRCLE_AREA is not defined, the subsequent code will be included and CIRCLE_AREA will be defined.

extern "C"

This syntax is used to declare that the following function has a C linkage. This is essential when writing a code that will be called from both C and C++.

double calculate_CArea(double rad);

This line declares a function named “calculate_Carea” that takes a double argument (rad) and returns a double.

The extern “C” declaration in the header file informs the compiler to use a C-style linkage for the function, making it callable from the C code. Use this code to find the area of a circle by saving it in a “circle.h” file. Once you save this header file, all you need to do is include this as a header file in the C++ program and perform the desired function. The following is the C code that calculates the area of a circle. The “main.c” file includes the C++ header and calls the “Circle_Carea” directly. See the following code:

#include "circle.h"

int main() {

  double rads = 5.0;

  double area = Circle_CArea(rads);

  printf("The  area of the circle is:  %.2f\n", area);

  return 0;


This simple C program calculates and prints a circle’s area using a function that is defined in the previous file named “circle.h”. Let’s break down the code:

#include "circle.h"

This line includes the content of the “circle.h” header file in the program. The header file likely contains the function declarations or macros that are related to circle calculations.

int main() {

The program’s execution starts with the main function, the entry point for C programs.

double rads = 5.0;

A variable rads of type double is declared and assigned with the value of 5.0. This variable likely represents the radius of the circle.

double area = Circle_CArea(rads);

A function named “Circle_Carea” is called with the radius rads as an argument and the result is stored in the variable area.

printf("The area of the circle is: %.2f\n", area);

The result is printed to the console using “printf”. The “The area of the circle is: %.2f\n” string is a format string with a “%f” placeholder for the area value. The “.2” in “%.2f” specifies that only two decimal places should be displayed.

return 0;

The main function concludes by returning 0 which indicates a successful program execution to the operating system.

In summary, the header file declares a function with C linkage, and the implementation file defines the logic for calculating a circle’s area. This separation allows the code to be used in both C and C++ programs. Refer to the output of the code that is given in the following image:

As you can see in the output, the calculated area of the circle is 78.54 which is the computation of area = 3.14*5*5 = 78.54. The function to calculate the area is defined in the “circle.h” header file which is then called in the C++ file with the help of extern “C”.


We guided you through the process of integrating C++ into your C programs in this guide. Integrating a C++ functionality into C codebases requires careful consideration of pointers, name mangling, and dual-purpose header files. The provided examples illustrate a practical approach to achieving this integration. Now that you’ve seen how to connect these two programming languages, you have more tools. Unlocking the power of C++ while working with C could give your projects a whole new level of functionality.

About the author

Kalsoom Bibi

Hello, I am a freelance writer and usually write for Linux and other technology related content