如果该类包含基类的成员，则编译器可以利用空的基优化吗? [英] Can the compiler exploit empty base optimisation if the class contains a member of the base class?

问题描述

考虑

struct base {};
struct child : base {};

众所周知，通过应用空基数优化，sizeof(child)可以为1.

It's well-known that sizeof(child) can be 1 by application of the empty base optimisation.

现在，请考虑

struct base {};
struct child : base {base b;};

编译器是否可以立即应用空基优化，或者sizeof(child)必须至少为2?

Can the compiler apply the empty base optimisation now, or must sizeof(child) be at least 2?

参考: http://en.cppreference.com/w/cpp/language/ebo

推荐答案

否，不能.来自同一参考:

No, it cannot. From the same reference:

如果空基类之一是空基优化，则被禁止 也是第一个非静态数据的类型或类型的基础 成员

Empty base optimization is prohibited if one of the empty base classes is also the type or the base of the type of the first non-static data member

因此sizeof(child) >= 2.

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