Initialization and bounded state

The idea

An initializer establishes the stored properties needed for an instance to be valid. A failable initializer uses init? to return nil when its input cannot create an acceptable value. A throwing initializer can instead preserve a specific error. Choose the failure channel from the caller’s needs, then keep the same invariant intact through every later mutating operation.

A bounded model should reject invalid changes before modifying its fields. A read-only public view with controlled mutation prevents arbitrary assignments from bypassing those checks. Arithmetic deserves attention too: checking current + amount against a limit can overflow before the comparison is useful. When current is already within a nonnegative limit, comparing amount with the remaining capacity avoids that premature addition.

struct Stock {private(set) var count:Int;init?(_ count:Int){guard count>=0 else{return nil};self.count=count}
mutating func take(_ n:Int)->Bool {guard n>=0 && n<=count else{return false};count -= n;return true}}
var item=Stock(4)!
print(item.take(5))
print(item.count)

false
4

Construction establishes a nonnegative count, and the failed withdrawal preserves that state.