在类上使用#pragma pack [英] Use of #pragma pack on a class

问题描述

我们可以在类之前使用#pragma pack（）吗？

Can we use #pragma pack() before a class?

pragma在这里的意义是什么？我知道它用于向编译器提供有关实现的信息，但是如果我们使用它与一个类怎么办？

What is the significance of pragma here? I know it is used for giving information to compiler regarding implementation, but what if we use it with a class?

推荐答案

对类在结构上的精确效果相同，影响数据成员的对齐。

It has the exact same effect on a class as it does on a struct, affecting the alignment of data members.

实际上在类上使用它是非常不寻常的，几乎总是一个错误。 C ++类对象的布局是严格实现定义的。 C ++编译器通常会努力优化该布局，在可能时删除v表指针。并且当类使用多继承时，可能添加一个。因此，类声明的一个小改动，比如创建一个方法 virtual 或添加一个基类可以显着改变对象布局。这当然会打破依赖于该pragma伪指令的代码。像一个对象序列化为二进制文件不会反序列化正确。一般来说，一个坏的做法，但也发生了所有的时间。不要使用它。

Actually using it on a class is very unusual and almost always a mistake. The layout of a C++ class object is heavily implementation defined. A C++ compiler usually makes an effort to optimize that layout, dropping the v-table pointer when it can. And potentially adding one when the class uses multiple inheritance. So a minor change to the class declaration, like making a method virtual or adding a base class can significantly alter the object layout. This will then of course break the code that depends on that pragma. Like an object serialized to a binary file won't deserialize properly anymore. In general a bad practice too but happens all the time anyway. Don't use it.

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