C++ Functions

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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!"

Try it now

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!

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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

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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!";

Try it now

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C++ Function Parameters

Information can be passed to functions as a parameter. Parameters act as variables inside the function.

Parameters are specified after the function name, inside the parentheses. You can add as many parameters as you want, just separate them with a comma:

Syntax

void functionName(parameter1, parameter2, parameter3) {
// code to be executed
}

The following example has a function that takes a string called fname as parameter. When the function is called, we pass along a first name, which is used inside the function to print the full name:

Example

void myFunction(string fname) {
cout << fname << " Refsnes\n";
}

int main() {
myFunction("Liam");
myFunction("Jenny");
myFunction("Anja");
return 0;
}

// Liam Refsnes
// Jenny Refsnes
// Anja Refsnes

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When a parameter is passed to the function, it is called an argument. So, from the example above: fname is a parameter, while Liam, Jenny and Anja are arguments.

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C++ Default Parameters

You can also use a default parameter value, by using the equals sign (=).

If we call the function without an argument, it uses the default value ("Norway"):

Example

void myFunction(string country = "Norway") {
cout << country << "\n";
}

int main() {
myFunction("Sweden");
  myFunction("India");
  myFunction();
  myFunction("USA");
  return 0;
}

// Sweden
// India
// Norway
// USA

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A parameter with a default value, is often known as an "optional parameter". From the example above, country is an optional parameter and "Norway" is the default value.

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C++ Multiple Parameters

Inside the function, you can add as many parameters as you want:

Example

void myFunction(string fname, int age) {
cout << fname << " Refsnes. " << age << " years old. \n";
}

int main() {
myFunction("Liam", 3);
myFunction("Jenny", 14);
myFunction("Anja", 30);
return 0;
}

// Liam Refsnes. 3 years old.
// Jenny Refsnes. 14 years old.
// Anja Refsnes. 30 years old.

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Note that when you are working with multiple parameters, the function call must have the same number of arguments as there are parameters, and the arguments must be passed in the same order.

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C++ The Return Keyword

The void keyword, used in the previous examples, indicates that the function should not return a value. If you want the function to return a value, you can use a data type (such as int, string, etc.) instead of void, and use the return keyword inside the function:

Example

int myFunction(int x) {
return 5 + x;
}

int main() {
cout << myFunction(3);
return 0;
}

// Outputs 8 (5 + 3)

Try it now

This example returns the sum of a function with two parameters:

Example

int myFunction(int x, int y) {
return x + y;
}

int main() {
cout << myFunction(5, 3);
return 0;
}

// Outputs 8 (5 + 3)

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You can also store the result in a variable:

Example

int myFunction(int x, int y) {
  return x + y;
}

int main() {
  int z = myFunction(5, 3);
cout << z;
  return 0;
}
// Outputs 8 (5 + 3)

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C++ Functions - Pass By Reference

In the examples from the previous page, we used normal variables when we passed parameters to a function. You can also pass a reference to the function. This can be useful when you need to change the value of the arguments:

Example

void swapNums(int &x, int &y) {
  int z = x;
x = y;
  y = z;
}

int main() {
  int firstNum = 10;
  int secondNum = 20;

cout << "Before swap: " << "\n";
cout << firstNum << secondNum << "\n";

  // Call the function, which will change the values of firstNum and secondNum
swapNums(firstNum, secondNum);

cout << "After swap: " << "\n";
cout << firstNum << secondNum << "\n";

  return 0;
}

Try it now

Practice Excercise Practice now

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C++ Functions

C++ Functions

Create a Function

Syntax

Example Explained

Call a Function

Example

Example

Function Declaration and Definition

Example

C++ Function Parameters

Syntax

Example

C++ Default Parameters

Example

C++ Multiple Parameters

Example

C++ The Return Keyword

Example

Example

Example

C++ Functions - Pass By Reference

Example

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