1

Hi everyone, I'm trying to understand what happens with this code

var i: Int?
i? += 1
print(i)     // nil

As I understand, i? is Int? and 1 is Int but somehow += operator still works on operands with 2 different types (Int? and Int).
Could anyone help me explain how everything works under the hood?
Thanks!

2

This is optional chaining, and making this work is the entire purpose of the assignment property in precedencegroup declarations.

It is equivalent to writing something like:

i?.plusEquals(1)
1 Like
3

Yes, I also guessed that it will be equivalent to i?.plusEquals(1). But in this case, i = 1 will be i.plusEquals(1). It makes me quite confused.

Btw, I know i? += 1 will call static func += (left: inout Int, right: Int) function. Does Swift automatically evaluate left parameter and skip the function if left is nil?

4

That is how optional chaining works, yes. I recommend you read about it in the Swift language guide.

1 Like
5

Actually I read Basic Operators, Optional Chaining, Advanced Operators.

I know when I call a function on an optional variable, the variable will be evaluated before function is called. But I can't find any section which mentions about evaluating parameters before calling static function (operators) :sweat_smile:.

I will try to read again. Maybe I missed something.
Thank you very much!!!