swift class的缺省基类（SwiftObject）与内存模型
Hard Constraints on Resilience
The root of a class hierarchy must remain stable, at pain of invalidating the metaclass hierarchy. Note that a Swift class without an explicit base class is implicitly rooted in the SwiftObject Objective-C class.
https://github.com/apple/swift/blob/master/docs/ABI/TypeLayout.rst
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// |
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// This implements the Objective-C root class that provides basic `id`- |
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// compatibility and `NSObject` protocol conformance for pure Swift classes. |
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// |
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//===----------------------------------------------------------------------===// |
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SWIFT_RUNTIME_EXPORT @interface SwiftObject<NSObject> { |
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@private |
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Class isa; |
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SWIFT_HEAPOBJECT_NON_OBJC_MEMBERS; |
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}
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https://github.com/apple/swift/blob/f4db1dd7a4abba2685247e1a7415d4fcb91f640d/stdlib/public/runtime/SwiftObject.h
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// The members of the HeapObject header that are not shared by a |
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// standard Objective-C instance |
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#define SWIFT_HEAPOBJECT_NON_OBJC_MEMBERS \ |
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InlineRefCounts refCounts |
oc端的函数可以使用oc调用约定进行调用；
swift本地端的函数可以使用swift调用约定进行调用；
https://github.com/apple/swift/blob/master/stdlib/public/SwiftShims/HeapObject.h
接下来是 SWIFT_HEAPOBJECT_NON_OBJC_MEMBERS，这是一个宏定义，展开后即：
RefCounts<InlineRefCountBits> refCounts;
这是一个相当重要东西，引用计数、弱引用、unowned 引用都与它有关，同时它也是 Swift 对象（文中后续的 Swift 对象均指引用类型，即 class 的实例）中较为复杂的一个结构。
http://www.manongjc.com/article/64837.html
Swift是苹果公司最新推出的编程语言，据很多人说，是用来”代替“Objective-C。但是没有确切的证据。我花了一些时间对Swift二进制和运行环境实施逆向工程技术，然后我对Swift有些少许的发现。目前为止，结论就是：Swift是没有消息机制的Objective-C。
对象
信不信由你，Swift中的对象就是Objective-C的对象。在Mach-O二进制文件中，__objc_classlist包含每个二进制文件中类的数据。其结构如下所示：
struct objc_class {
uint64_t isa;
uint64_t superclass;
uint64_t cache;
uint64_t vtable;
uint64_t data;
};
https://www.oschina.net/translate/inside-swift
class Human {
//8 type or isa
//retainCount
var age: Int?//16
var name: String?//16
var nicknames: [String] = [String]()
//返回指向 Human 实例头部的指针
func headPointerOfClass() -> UnsafeMutablePointer<Int8> {
let opaquePointer = Unmanaged.passUnretained(self as AnyObject).toOpaque()
let mutableTypedPointer = opaquePointer.bindMemory(to: Int8.self, capacity: MemoryLayout<Human>.stride)
return UnsafeMutablePointer<Int8>(mutableTypedPointer)
}
}
MemoryLayout<Human>.size //8
let human = Human()
let arrFormJson = ["goudan","zhaosi", "wangwu"]
//拿到指向 human 堆内存的 void * 指针
let humanRawPtr = UnsafeMutableRawPointer(human.headPointerOfClass())
//nicknames 数组在内存中偏移 48byte 的位置(8+8+16+16)
let humanNickNamesPtr = humanRawPtr.advanced(by: 48).assumingMemoryBound(to: Array<String>.self)
print(human.nicknames)
//[]
humanNickNamesPtr.initialize(to: arrFormJson)
print(human.nicknames)
human.nicknames //["goudan","zhaosi", "wangwu"]
let retainCount = humanRawPtr.advanced(by: 8).assumingMemoryBound(to: Int8.self)
print(retainCount.pointee)
参考文献：
https://www.cnblogs.com/doudouyoutang/p/9760603.html
https://www.oschina.net/translate/inside-swift
