Is there any difference between "mutating" function and "inout" parameters in Swift?

Question

As per Swift documentation both mutating and inout keywords are used to modify the value types from within a function. Is there any difference between "mutating" and "inout" and any special case where we need to use either of them.

Answer 1

mutating marks a method. inout marks a parameter. They are completely different things.

Methods marked with mutating can mutate self i.e. set properties of self, reassign self etc.

struct Foo {
    var foo: Int

    mutating func mutate() {
        foo += 1 // this is mutating self
    }
}

Parameters marked with inout basically become var variables, as opposed to let constants. You can change them, and the changes will also reflect on the caller's side.

func f(_ x: inout Int) {
    x = 10
}

var a = 1
f(&a)
print(a) // 10