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

// Create a function
void myFunction() {

cout << "I just got executed!";
}

int main() {

myFunction(); // call the function
  return 0;
}

// Outputs
"I just got executed!"

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!

Function Declaration and Definition

A C++ function consist of two parts:

• Declaration: the return type, the name of the function, 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:

Example

int main() {

myFunction();
  return 0;
}

void myFunction() {

cout << "I just got executed!";
}

// Error

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!";
}