为什么gcc在VTT中实现了top_offset? [英] Why is there a top_offset in VTT implemented by gcc?

问题描述

此处是 VTT的详细说明，投票表决的答案.但是答案并不能解释为什么VTT中存在top-offset.

Here is a detailed description of VTT in the top-voted answer.But the answer does not explain why is there a top-offset in the VTT.

从我的角度来看，当我们down_cast一个base指针指向derived指针时，编译器已经知道offset需要在编译时进行调整(是没有虚拟派生的)，因此无需在以下情况下存储top_offset:

From my point of view,when we down_cast a base pointer to derived pointer,the compiler already knows the offset needed to be adjusted in compile time(when there is no virtual derivation) ,so there is no need to store a top_offset in situation below:

class A {
public:
  int a;
};
class B {
public:
  int b;
  virtual void w();
};

class C : public A, public B {
public:
  int c;
};

在这种情况下，类型C的对象的布局如下(数字假设32位指针):

In this case, objects of type C are laid out like this (numbers assuming 32-bit pointers):

                           +-----------------------+
                           |     0 (top_offset)    |//why?
                           +-----------------------+
c --> +----------+         | ptr to typeinfo for C |
      |  vtable  |-------> +-----------------------+
      +----------+         |         A::v()        |
      |     a    |         +-----------------------+
      +----------+         |    -8 (top_offset)    |//why?
      |  vtable  |---+     +-----------------------+
      +----------+   |     | ptr to typeinfo for C |
      |     b    |   +---> +-----------------------+
      +----------+         |         B::w()        |
      |     c    |         +-----------------------+
      +----------+

为什么在这种情况下VTT中会有top_offset?我认为top_offset和virtual base offset仅在虚拟继承中需要.

Why is there a top_offset in VTT under such situation? I think the top_offset and virtual base offset are only needed in virtual inheritance.

推荐答案

void *top(B *b) { return dynamic_cast<void *>(b); }

编译器无法在编译时确定正确的偏移量.可以使用空指针，指向完整的B对象的指针或指向B子对象的指针来调用此函数.这三种情况需要以不同的方式处理. vtable中的偏移量使它可以正常工作.

There is no way for the compiler to determine at compile time what the correct offset is. This function may be called with a null pointer, a pointer to a complete B object, or a pointer to a B subobject. The three cases need to be handled differently. The offset in the vtable is what allows this to work.

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