Swift Nested Functions

In this article, you will learn about nested functions in Swift and how to use them with examples.

If aFunctionIf it exists within the body of another function, it is called a nested function.

Syntax of nested functions

func funcname() {
　　　　//External function statements
　　　　func anotherFuncname() {
　　　　　　　　//Internal function statements
　　　　}
}

Here, the function anotherFuncname is located inside the body of another function funcname.

It should be noted that internal functions can only be called and used within the enclosed function (external function).

Example1Nested functions without return values

func outputMessageByGreeting(_ message: String) {
　　　　
　　　　func addGreetingAndPrint() {
　　　　　　　　print("Hello! \(message)")
　　　　}
　　　　addGreetingAndPrint()
}
outputMessageByGreeting("Jack")

The output when running the program is:

Hello! Jack

In the above program, the nested function addGreetingAndPrint() was called from the enclosed function outputMessageByGreeting().

The statement outputMessageByGreeting("Jack") calls the external function. The statement addGreetingAndPrint() inside the external function calls to output Hello Jack!

You cannot call the function addGreetingAndPrint outside the function outputMessageByGreeting.

Example2Nested functions with parameters and return values

Nested functions can contain functions with parameters and return values.

func operate(with symbol: String)　-> (Int, Int)　-> Int {
　　　　
　　　　func add(num1: Int, num2: Int)　-> Int {
　　　　　　　　return num1　+　num)2
　　　　}
　　　　
　　　　func subtract(num1: Int, num2: Int)　-> Int {
　　　　　　　　return num1　-　num)2
　　　　}
　　　　let operation = (symbol == "+)) ? add : subtract
　　　　return operation
}
let operation = operate(with: "+))
let result = operation(2,　3)
print(result)

The output when running the program is:

5

In the above program,

• The external function is operate(), returning a Function type (Int, Int) -> Int.

• The internal (nested) functions are add() and subtract().

The way nested functions add() and subtract() are being used is outside the enclosed function operate(). This is possible because the external function returns one of these functions.

We have used internal functions outside of the enclosed function operate() as operation(2,3) Called internally by add(2,3) Output to the console5.