point of declaration (POD), introduces entity 'i' int j = i; // POD, introduces entity 'j' // point of look-up (POL), names visible entity 'i' int k = l; // POD, introduces entity declared // (relative invisibility) int l; // point of declaration (POD), introduces entity 'l' int m = n; // POD, introduces entity 'm' // POL, names invisible entity 'n' invisibility) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 // translation unit 2 int n; // POD, introduces entity 'n' 1 2 3 STARTING SIMPLE global scope Lookup of entities at global scope

constructor, destructor, or copy assignment operator… This definition is essentially the same as a C++03 POD type. armv7-a System V x86 System V x86 Microsoft A Composite Type not larger than 4 bytes is returned 16 bytes x86 System V fundamental only x86-64 Microsoft 1,2,4,8 bytes, C++03 POD x86 Microsoft 1,2,4,8 bytes, C++03 POD Composite types are required to be “trivial” to get into registers!C.20: If you ___$ReturnUdt$[esp-4] mov DWORD PTR [eax], 60 mov DWORD PTR [eax+4], 0 ret 0 C H R 67 not a POD not a POD size > 4 not fundamentalCan we do something about it? ● std::chrono would have to give up encapsulation

更加明确用的哪一种类型转换（ cast ），从而避免一些像是 static_cast(ptr) 的错误 。 • 虽然作者也经常会忍不住在 zeno 中用 编译器默认生成的构造函数：无参数（小心 POD 陷阱！） • 除了我们自定义的构造函数外，编译器还会自动生成一些构造函数。 • 当一个类没有定义任何构造函数，且所有成员都有无参构造函数时，编译器会自动生成一 个无参构造函数 Pig() ，他会调用每个成员的无参构造函数。 这些类型被称为 POD （ plain-old-data ）。 • POD 的存在是出于兼容性和性能的考虑。 << 取决于内存的随机值 编译器默认生成的构造函数：无参数（ POD 陷阱解决方案） • 不过我们可以手动指定初始化 weight 为 0 。 • 通过 {} 语法指定的初始化值，会在编译器自 动生成的构造函数里执行。 编译器默认生成的构造函数：无参数（ POD 陷阱解决方案，续）

14 Aside: POD Types  Standard-layout types are defined in C++11.  The 2003 Standard didn’t mention standard-layout types — it describes layout guarantees in terms of POD types.  POD stands for “Plain (There’s also a POF.)  A POD is a standard-layout class that also lacks:  private or protected non-static data members  user-declared constructors  base classes  In practice, POD classes are too restrictive

Implementation The C++ implementation will create expressive API calls through utilizing combination of : ▪POD structs -- structs with data members only ▪designated initializers -- initializing a struct with: { the road state -- There is only one. // the .= syntax is a designated initializer used to assign POD values auto result = processCarStateTransition({ .car = Car::FastSportsCar, .startingRoadState Why was there ambiguity? The RoadState type had 1. A default value when unspecified 2. Implicit POD construction could not differ between the three structs {.type = RoadType::Highway, .speed = Speed::FAST}

unified! value/ Regular SI base class “struct” … functor/ callable COM class CORBA interface POD value traits type enum class expr. template MI base class flag enum property variant dynamic unified! value/ Regular SI base class “struct” … functor/ callable COM class CORBA interface POD value traits type enum class expr. template MI base class flag enum property variant dynamic unified! value/ Regular SI base class “struct” … functor/ callable COM class CORBA interface POD value traits type enum class expr. template MI base class flag enum property variant dynamic

v=I8UvQKvOSSw Fertig, Andreas (2020) “C++20: Aggregate, POD, trivial type, standard layout class, what is what.” https://andreasfertig.blog/2021/01/cpp20-aggregate-pod-trivial-type-standard-layout-class-what-is- what/

怎么样，是不是和 Go 语言很像？ 初始化列表 初始化是一个非常重要的语言特性，最常见的就是在对象进行初始化时进行使用。在传统 C++ 中， 不同的对象有着不同的初始化方法，例如普通数组、POD （Plain Old Data，即没有构造、析构和虚函 数的类或结构体）类型都可以使用 {} 进行初始化，也就是我们所说的初始化列表。而对于类对象的初始 化，要么需要通过拷贝构造、要么就需要使用 为解决这个问题，C++11 首先把初始化列表的概念绑定到类型上，称其为 std::initializer_list， 允许构造函数或其他函数像参数一样使用初始化列表，这就为类对象的初始化与普通数组和 POD 的初 始化方法提供了统一的桥梁，例如： #include #include #include class MagicFoo

de-virtualization • every impl ptr to member being equally sized • But MSVC... • PtrToMember = 4 bytes for POD. • 12 for forward decl 6 2Alternative Method invoke impl 9 / 2 4 / 2 0 2 4 6 3 template

buf[1000]; // classic C memcpy(&buf[0], src.data(), src.size()); db_format data; // any large fixed-layout POD read_next_chunk(&data); // e.g., from disk or network while (/*...*/) read_next_chunk(&data); Artificial