THE WORLD'S LARGEST WEB DEVELOPER SITE

C++ Functions


A function is a block of code which only runs when it is called.

You can pass data, known as parameters, into a function.

Functions are used to perform certain actions, and they are important for reusing code: Define the code once, and use it many times.


Create a Function

C++ provides some pre-defined functions, such as main(), which is used to execute code. But you can also create your own functions to perform certain actions.

To create (often referred to as declare) a function, specify the name of the function, followed by parentheses ():

Syntax

void myFunction() {
  // code to be executed
}

Example Explained

  • myFunction() is the name of the function
  • void means that the function does not have a return value. You will learn more about return values later in the next chapter
  • inside the function (the body), add code that defines what the function should do

Call a Function

Declared functions are not executed immediately. They are "saved for later use", and will be executed later, when they are called.

To call a function, write the function's name followed by two parentheses () and a semicolon ;

In the following example, myFunction() is used to print a text (the action), when it is called:

Example

Inside main, call myFunction():

// Create a function
void myFunction() {
  cout << "I just got executed!";
}

int main() {
  myFunction(); // call the function
  return 0;
}

// Outputs "I just got executed!"
Run example »

A function can be called multiple times:

Example

void myFunction() {
  cout << "I just got executed!\n";
}

int main() {
  myFunction();
  myFunction();
  myFunction();
  return 0;
}

// I just got executed!
// I just got executed!
// I just got executed!
Run example »

Function Declaration and Definition

A C++ function consist of two parts:

  • Declaration: the function's name, return type, and parameters (if any)
  • Definition: the body of the function (code to be executed)
void myFunction() { // declaration
  // the body of the function (definition)
}

Note: If a user-defined function, such as myFunction() is declared after the main() function, an error will occur. It is because C++ works from top to bottom; which means that if the function is not declared above main(), the program is unaware of it:

Example

int main() {
  myFunction();
  return 0;
}

void myFunction() {
  cout << "I just got executed!";
}

// Error
Run example »

However, it is possible to separate the declaration and the definition of the function - for code optimization.

You will often see C++ programs that have function declaration above main(), and function definition below main(). This will make the code better organized and easier to read:

Example

// Function declaration
void myFunction();

// The main method
int main() {
  myFunction();  // call the function
  return 0;
}

// Function definition
void myFunction() {
  cout << "I just got executed!";
}
Run example »