What is an inout parameter?

Sources & Resources

Main Source: 🔗 Ace the iOS Interview

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Further Reading:

TL/DR

Whenever you pass a value type into a function, only a copy of the value is passed along. As a result, even if you attempt to change the value of that parameter inside the function, the variable at the calling site will still maintain its original value.

var currentAge = 26

func updateAge(passedInAge: Int){
    var passedInAge = passedInAge
    passedInAge = 42
    print(passedInAge) // 42
    print(currentAge) // 26
}

updateAge(passedInAge: currentAge)

If we want to change the value of the parameter itself instead of just working with a copy of the data, we’ll need to add the inout keyword. This will allow us to make changes directly to the variable that was passed in even if it’s a value type.

We’ll need to use the & symbol when providing an inout parameter:

var currentAge = 26

func updateAgeWithInout(passedInAge: inout Int) {
    passedInAge = 42
    print(passedInAge) // 42
    print(currentAge) // 42
}

// currentAge is 26 before the call and 42 after
updateAgeWithInout(passedInAge: &currentAge)

The inout keyword is used very often in Swift and enables syntactic sugar like the += operator which modifies the value of the variable on the left-hand side of the operator in place.