Error handling with assert - ios

Question

Suppose I have a class.
I want to give an error in init if my class doesn't follow the described rule
class Puzzle {
var puzzle_array: [Int]
var zero_index: Int
public init(array: [Int]) {
assert(array.count == 9, "Array should be lenght 9")
assert(array.index(of: 0) != nil, "There should ne 0 in array")
puzzle_array = array
zero_index = puzzle_array.index(of: 0)!
}
}
Then I need to create several instances of this class in a loop. Some of them, won't satisfy the condition, I described in init and complier and I will get an error.
But what I want is to skip creation of this instances without executing an error. And I want to keep checking condition logic inside init.
My initial idea might be wrong, but I'll appreciate if you help me to make it more properly.

Answer 1

You could use a failable initialiser to accomplish this, your object will be nil if you don't satisfy your checks.
class Puzzle {
var puzzle_array: [Int]
var zero_index: Int
public init?(array: [Int]) {
guard array.count == 9, array.index(of: 0) != nil else {
return nil
}
puzzle_array = array
zero_index = puzzle_array.index(of: 0)!
}
}

Answer 2

I disagree with #colmg that you should use failable initialiser, because it's destroys information about what actually went wrong.
Instead you should use throwable initalizer:
class Puzzle {
var puzzle_array: [Int]
var zero_index: Int
public init(array: [Int]) throws {
try assert(array.count == 9, "Array should be lenght 9")
try assert(array.index(of: 0) != nil, "There should ne 0 in array")
puzzle_array = array
zero_index = puzzle_array.index(of: 0)!
}
}
Assuming that assert is not standard version here, but this function:
struct AssertError: Error {
let description: String
}
func assert(_ condition: #autoclosure () -> Bool, _ description: String) throws {
if !condition() {
throw AssertError(description: description)
}
}
Now you can do this:
do {
let puzzle = try Puzzle([0, 1, 2, 3, 4])
} catch let error {
// Here you can handle error, and see what exactly went wrong, instead of just knowing that initialisation failed
}
Or you can use more advanced version:
func assert(_ condition: #autoclosure () -> Bool, _ error: Error) throws {
if !condition() {
throw error
}
}
class Puzzle {
var puzzle_array: [Int]
var zero_index: Int
public init(array: [Int]) throws {
try assert(array.count == 9, PuzzleError.invalidArrayLength)
try assert(array.index(of: 0) != nil, PuzzleError.arrayContainsZero)
puzzle_array = array
zero_index = puzzle_array.index(of: 0)!
}
enum PuzzleError: Error {
case invalidArrayLength
case noZeroInArray
}
}