结构调整的C / C ++ [英] struct alignment C/C++

问题描述

在C / C ++（我假设他们在这方面是相同的），如果我有以下几点：

In c/c++ (I am assuming they are the same in this regard), if I have the following:

struct S {
  T a;
  .
  .
  .
} s;

在下面的保证是真的吗？

Is the following guaranteed to be true?

(void*)&s == (void*)&s.a;

或者换句话说，没有任何种类的保证会有没有填充的前的第一个成员？

推荐答案

在C，是的，他们是同一个地址。简单，明了。

In C, yes, they're the same address. Simple, and straightforward.

在C ++中，没有，他们不是同一地址。基类可以（而且我怀疑，千万）所有成员来之前，和虚成员函数通常隐藏数据的struct地方补充。更令人困惑的，C ++编译器还可以随意重新排列成员，除非类是一个标准的布局类型（虽然我不知道任何编译器这样做）

In C++, no, they're not the same address. Base classes can (and I would suspect, do) come before all members, and virtual member functions usually add hidden data to the struct somewhere. Even more confusing, a C++ compiler may also rearrange members at will, unless the class is a standard layout type (though I don't know that any compiler does so)

最后，如果C ++结构组成的标准布局类型的的，不包含任何的基类，也不虚函数，所有成员都具有相同的知名度，以及其他可能的限制我忘了，的然后的它落回到在C的规则，并且需要第一构件是在相同的地址对象本身。

Finally, if the C++ struct is composed of standard layout types, contains no base classes nor virtual functions and all members have the same visibility, and possibly other limitations I forgot, then it falls back on the C rules, and requires the first member to be at the same address as the object itself.

§9.2 / 7

一个标准的布局类是一类：结果
   - 有型非标准布局类（或这些类型的数组）或引用，结果没有非静态数据成员
   - 有没有虚函数（10.3），没有虚基类（10.1），结果
   - 对所有非静态数据成员相同的访问控制（第11条），结果
   - 没有非标准布局基类，结果
   - 无论是在最派生类，并在与非静态数据成员最多有一个基类没有非静态数据成员，或具有与非静态数据成员，和结果，没有基类
   - 具有类型相同的第一个非静态数据成员的无基类

A standard-layout class is a class that:
— has no non-static data members of type non-standard-layout class (or array of such types) or reference,
— has no virtual functions (10.3) and no virtual base classes (10.1),
— has the same access control (Clause 11) for all non-static data members,
— has no non-standard-layout base classes,
— 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
— has no base classes of the same type as the first non-static data member.

§9.2 / 20

一个指针标准布局结构对象，采用reinter pret_cast适当的转换，指向其初始成员（如该成员是一个位域，然后在它所在的单位）和副反之亦然。 [注：因此，有可能是一个标准的布局结构对象内无名填充，但不是在它的开头，如要达到适当的对准。末端注意事项】

A pointer to a standard-layout struct object, suitably converted using a reinterpret_cast, points to its initial member (or if that member is a bit-field, then to the unit in which it resides) and vice versa. [ Note: There might therefore be unnamed padding within a standard-layout struct object, but not at its beginning, as necessary to achieve appropriate alignment. —end note ]

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