为什么由两个空class导出的空class的大小是2？

Question

我知道空class的大小为1只是为了符合不允许对象（及其classes）大小为0的标准。这里，我我得到派生 class D 的大小为 2。为什么在这种情况下行为不同假设没有从class B和C继承到D的数据成员或虚拟指针？

#include<iostream>
using namespace std;

class A {};
class B : public A {};
class C : public A {};
class D : public B, public C {};

int main() {
    cout<<"Size of D is: "<<sizeof(D)<<endl;
    return 0;
}

Answer 1

对我来说，空碱基优化是否可以应用在这里，取决于人们如何解释 [intro.object/8]:

Unless an object is a bit-field or a base class subobject of zero size, the address of that object is the address of the first byte it occupies. Two objects a and b with overlapping lifetimes that are not bit-fields may have the same address if one is nested within the other, or if at least one is a base class subobject of zero size and they are of different types; otherwise, they have distinct addresses.

B和C是不同的类型吗？他们都是A。实际上是两个不同的 A 对象。编译器编写者可以在此处停止分别为 B 和 C 分配存储空间，而无需检查 A 是否为空。

值得注意的是，如果您有 B 和 C 继承自 separate 基础，则使用 g++ 时大小会返回 1：

Live Example

#include<iostream>

class A  {};
class A1 {};
class B : public A {};
class C : public A1 {};
class D : public B, public C {};

int main() {
    std::cout << "Size of D is: " << sizeof(D) << std::endl;
    return 0;
}

Answer 2

这是因为您继承自两个基 class，它们本身派生自相同的基 class A，看看当您更改程序时输出如何变化对此

#include<iostream>
using namespace std;

class A {};
class Z {};
class B : public A {};
class C : public Z {};
class D : public B, public C {};

int main() {
    cout<<"Size of D is: "<<sizeof(D)<<endl;
    return 0;
}

如您所见，D 的大小为 1

该问题与您可能知道的可怕钻石的问题类似。编译器试图消除示例中 D 中 A 的两个实例的歧义。并且这里没有应用空基优化，标准没有要求，编译器也没有实现（见下文）

该标准明确允许编译器不对多继承情况应用空基优化。相关引用自标准 here

Allowing standard-layout classes to have base classes forces compilers to implement the empty base optimization for standard-layout classes, and this could break a compiler's application binary interface (ABI). See 9.2/18 above. This is believed not to be a concern for modern compilers, except possibly in the case of multiple inheritance. Since multiple inheritance is not central to this proposal, allowing standard-layout classes or their bases to use multiple inheritance will be eliminated from the proposal if it proves contentious.

为什么由两个空class导出的空class的大小是2？

Why the size of empty class that is derived from two empty classes is 2?

c++

inheritance

class

multiple-inheritance