Swift cast protocol type to struct and pass as inout

Question

protocol ValueHolder {
}

struct A: ValueHolder {
    var value = 5
}

var x: ValueHolder = A()

func f(a: inout A) {
    a.value = 10
}

I want to use pass x to f. Is it possible?

Edit: I understand all of the staff about value semantics and inout parameters. The problem is that x needs to be casted to A and I wonder if that can be done without copying it ( as makes a typed copy of it) so that I can use f on x .

Answer 1

I want to use pass x (and not its copy) to f. Is it possible?

Not exactly — at least, not if the parameter is an A. An A is a struct. Even with inout , a struct is still a value type. inout allows the original value to be replaced, but what it is replaced with is another A. Simply put, a struct cannot be mutated in place.

That being so, your question seems to lose its meaning. Since a struct cannot be mutated in place, there is no real reason to use inout here at all. You are not doing anything that calls for inout . You might as well just drop the inout and accept the value semantics:

func f(a: A) -> A {
    var a = a
    a.value = 10
    return a
}
x = f(a:x as! A)

If you really want to keep the inout , then type a: as a ValueHolder and cast inside the function, like this:

var x: ValueHolder = A()
func f(a: inout ValueHolder) {
    var b = a as! A
    b.value = 10
    a = b
}
f(a:&x)
(x as! A).value // 10