Swift 中的弱与无主。内部差异是什么

Swift 中的弱引用（weak）和无主引用（unowned）是用来解决循环引用问题的。循环引用是指两个或多个对象互相持有对方的强引用，导致它们无法被释放，从而造成内存泄漏的问题。弱引用和无主引用的内部差异主要体现在它们在内存管理上的行为和使用场景。

swift
class Person {
    let name: String
    weak var car: Car?
    
    init(name: String) {
        self.name = name
    }
    
    deinit {
        print("\(name) is being deinitialized")
    }
}
class Car {
    let brand: String
    var owner: Person?
    
    init(brand: String) {
        self.brand = brand
    }
    
    deinit {
        print("Car \(brand) is being deinitialized")
    }
}
var john: Person?
var bmw: Car?
john = Person(name: "John")
bmw = Car(brand: "BMW")
john?.car = bmw
bmw?.owner = john
john = nil
bmw = nil

swift
class Country {
    let name: String
    var capital: City?
    
    init(name: String, capitalName: String) {
        self.name = name
        self.capital = City(name: capitalName, country: self)
    }
    
    deinit {
        print("Country \(name) is being deinitialized")
    }
}
class City {
    let name: String
    unowned let country: Country
    
    init(name: String, country: Country) {
        self.name = name
        self.country = country
    }
    
    deinit {
        print("City \(name) is being deinitialized")
    }
}
var china: Country?
china = Country(name: "China", capitalName: "Beijing")
china?.capital = nil
china = nil