C ++ POD结构继承？对派生成员的内存布局有任何保证 [英] C++ POD struct inheritance? Are there any guarantees about the memory layout of derived members

问题描述

让我们说，我有一个 struct RGB ，我想创建 struct RGBA ，它继承 RGB ：

Let's say, I have a struct RGB and I want to create struct RGBA, which inherits RGB:

struct RGB {
    unsigned char r;
    unsigned char g;
    unsigned char b;
};

struct RGBA: RGB {
    unsigned char a;
};

两者都将用于读取未压缩的图片数据：

Both will be used for reading uncompressed image data:

RGBA *pixel=static_cast<RGBA *>(image->uncompressed_data);

$ b ？是否有人保证：

Question: Is this safe, regarding the memory layout of struct RGBA? Does anyone guarantee, that:



• unsigned char a $ c> RGB struct （不在之前）


• struct RGB 参数从 struct RGBA ？

• unsigned char a comes after the RGB struct (not before)
• There is no padding between struct RGB and the a parameter from struct RGBA?

将 #pragma pack 帮助这里？

推荐答案

不，布局不能保证。唯一的保证是对于标准布局类;并且这种类的条件之一是它

No, the layout is not guaranteed. The only guarantees are for standard-layout classes; and one of the conditions of such a class is that it

在大多数派生类中没有非静态数据成员，具有非静态数据成员的一个基类或不具有非静态数据成员的基类

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

换句话说，所有数据成员必须在同一个类别中，而不能在同一个类别中。

In other words, all data members must be in the same class, not in more than one.

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