In-Out 파라미터(Parameters)

In-Out Parameters

Function parameters are constants by default. Trying to change the value of a function parameter from within the body of that function results in a compile-time error. This means that you can’t change the value of a parameter by mistake. If you want a function to modify a parameter’s value, and you want those changes to persist after the function call has ended, define that parameter as an in-out parameter instead.

함수 밖에서 선언한 변수를 함수 안에서 수정 가능하도록 하는 방법

You write an in-out parameter by placing the inout keyword right before a parameter’s type. An in-out parameter has a value that’s passed in to the function, is modified by the function, and is passed back out of the function to replace the original value. For a detailed discussion of the behavior of in-out parameters and associated compiler optimizations, see In-Out Parameters.

You can only pass a variable as the argument for an in-out parameter. You can’t pass a constant or a literal value as the argument, because constants and literals can’t be modified. You place an ampersand (&) directly before a variable’s name when you pass it as an argument to an in-out parameter, to indicate that it can be modified by the function.

NOTE

In-out parameters can’t have default values, and variadic parameters can’t be marked as inout.

Here’s an example of a function called swapTwoInts(_:_:), which has two in-out integer parameters called a and b:

1. func swapTwoInts(_ a: inout Int, _ b: inout Int) {
2. let temporaryA = a
3. a = b
4. b = temporaryA
5. }

The swapTwoInts(_:_:) function simply swaps the value of b into a, and the value of a into b. The function performs this swap by storing the value of a in a temporary constant called temporaryA, assigning the value of b to a, and then assigning temporaryA to b.

You can call the swapTwoInts(_:_:) function with two variables of type Int to swap their values. Note that the names of someInt and anotherInt are prefixed with an ampersand when they’re passed to the swapTwoInts(_:_:) function:

1. var someInt = 3
2. var anotherInt = 107
3. swapTwoInts(&someInt, &anotherInt)
4. print("someInt is now \(someInt), and anotherInt is now \(anotherInt)")
5. // Prints "someInt is now 107, and anotherInt is now 3"

The example above shows that the original values of someInt and anotherInt are modified by the swapTwoInts(_:_:) function, even though they were originally defined outside of the function.

NOTE

In-out parameters aren’t the same as returning a value from a function. The swapTwoInts example above doesn’t define a return type or return a value, but it still modifies the values of someInt and anotherInt. In-out parameters are an alternative way for a function to have an effect outside of the scope of its function body.