Mirror.swift

import Foundation

class A {
  private var value: Int = 42
}

let a = A()

let mirror = Mirror(reflecting: a)

extension Mirror {
    func firstChild<T>(named name: String) -> T? {
        children.compactMap {
            guard let value = $0.value as? T else { return nil }

            return $0.label == name ? value : nil
        }.first
    }
}

let privatePropertyValue: Int = mirror.firstChild(named: "value")!

print(privatePropertyValue) // prints "42"