问题描述
根据 C ++ 11 9.1 / 7（draft n3376）， standard-layout p>
According to C++11 9.1/7 (draft n3376), a standard-layout class is a class that:
没有虚拟函数（10.3），没有虚拟基类（10.1），
has no virtual functions (10.3) and no virtual base classes (10.1),
对所有非静态数据成员具有相同的访问控制（条款11），
has the same access control (Clause11) for all non-static data members,
没有非标准布局基类，
在大多数派生类中没有非静态数据成员，最多一个基类具有非静态数据成员，或没有基类与非静态数据成员相同，
either has no non-static data members in the most derived class and at most one base class with non-static data members, or has no base classes with non-static data members, and
没有与第一个非静态数据成员相同类型的基类。
has no base classes of the same type as the first non-static data member.
这说明一个空类是一个 standard-layout 并且以空类作为基础的另一个类也是一个 standard-layout 类，只要该类的第一个非静态数据成员与基类的类型不同。
it follows that an empty class is a standard-layout class; and that another class with an empty class as a base is also a standard-layout class provided the first non-static data member of such class is not of the same type as the base.
此外， 9.2 / 19 表示：
这似乎意味着 Empty Base Class Optimization 至少对于标准布局类。我的观点是，如果空基本优化不是强制的，那么 standard-layout 类的布局不是标准的，而是取决于实现是否实现所述优化。我的推理是正确的，还是我错过了什么？
This seems to imply that the Empty Base Class Optimization is now a mandatory optimization, at least for standard-layout classes. My point is that if the empty base optimization isn't mandated, then the layout of a standard-layout class would not be standard but rather depend on whether the implementation implements or not said optimization. Is my reasoning correct, or am I missing something?
推荐答案
是的，你是正确的， PODs revisitedproposals：
Yes, you're correct, that was pointed out in the "PODs revisited" proposals: http://www.open-std.org/jtc1/sc22/WG21/docs/papers/2007/n2342.htm#ABI
Embarcadero编译器文档还声明：
The Embarcadero compiler docs also state it: http://docwiki.embarcadero.com/RADStudio/en/Is_standard_layout
另一个关键点是[class.mem] / 16
Another key point is [class.mem]/16
请注意，只有数据成员会影响布局兼容性，而不是基类，因此这两个标准布局类与布局兼容：
Note that only data members affect layout compatibility, not base classes, so these two standard layout classes are layout-compatible:
struct empty { };
struct stdlayout1 : empty { int i; };
struct stdlayout2 { int j; };
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