c++ - 内存中的虚拟表布局？

Question

虚拟表是如何存储在内存中的？他们的布局？

例如

class A{
    public:
         virtual void doSomeWork();
};

class B : public A{
    public:
         virtual void doSomeWork();
};

A类和B类的虚表在内存中会如何布局？

Answer 1

对于 Linux 中的 GCC 编译器，运行:

g++ -fdump-class-hierarchy example.h

输出是:

Vtable for A
A::_ZTV1A: 3u entries
0     (int (*)(...))0
8     (int (*)(...))(& _ZTI1A)
16    (int (*)(...))A::doSomeWork

Class A
   size=8 align=8
   base size=8 base align=8
A (0x7fb76785a4e0) 0 nearly-empty
    vptr=((& A::_ZTV1A) + 16u)

Vtable for B
B::_ZTV1B: 3u entries
0     (int (*)(...))0
8     (int (*)(...))(& _ZTI1B)
16    (int (*)(...))B::doSomeWork

Class B
   size=8 align=8
   base size=8 base align=8
B (0x7fb7678510d0) 0 nearly-empty
    vptr=((& B::_ZTV1B) + 16u)
  A (0x7fb76785a540) 0 nearly-empty
      primary-for B (0x7fb7678510d0)

Also I've created the vtable-dumper tool to list contents of virtual tables in the shared objects. With this tool you don't need to compile headers, just run it on the object:

vtable-dumper SHLIB