C++虚表详解

所有结果均为32位系统，指针为4个字节

简单继承

class A {
public:
  int a;
};

class B : public A {
public:
  int b;
};

对象B的内存布局：

b --> +-----------+
      |     a     |
      +-----------+
      |     b     |
      +-----------+

简单继承虚函数

class A {
public:
  int a;
  virtual void v();
};

class B : public A {
public:
  int b;
};

对象B内存布局

/*
                           +-----------------------+
                           |     0 (top_offset)    |
                           +-----------------------+
b --> +----------+         | ptr to typeinfo for B |
      |  vtable  |-------> +-----------------------+
      +----------+         |         A::v()        |
      |     a    |         +-----------------------+
      +----------+
      |     b    |
      +----------+

简单多重继承

class A {
public:
  int a;
  virtual void v();
};

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

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

对象C的内存布局

/*

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

简单多重继承（虚函数）

class A {
public:
  int a;
  virtual void v();
};

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

class C : public A, public B {
public:
  int c;
  void w();
};

对象C的内存布局：

/*

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

菱形继承

class A {
public:
  int a;
  virtual void v();
};

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

class C : public A {
public:
  int c;
  virtual void x();
};

class D : public B, public C {
public:
  int d;
  virtual void y();
};

对象D的内存布局：

/*

                           +-----------------------+
                           |     0 (top_offset)    |
                           +-----------------------+
d --> +----------+         | ptr to typeinfo for D |
      |  vtable  |-------> +-----------------------+
      +----------+         |         A::v()        |
      |     a    |         +-----------------------+
      +----------+         |         B::w()        |
      |     b    |         +-----------------------+
      +----------+         |         D::y()        |
      |  vtable  |---+     +-----------------------+
      +----------+   |     |   -12 (top_offset)    |
      |     a    |   |     +-----------------------+
      +----------+   |     | ptr to typeinfo for D |
      |     c    |   +---> +-----------------------+
      +----------+         |         A::v()        |
      |     d    |         +-----------------------+
      +----------+         |         C::x()        |
                           +-----------------------+

菱形继承(虚继承)

class A {
public:
  int a;
  virtual void v();
};

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

class C : public virtual A {
public:
  int c;
  virtual void x();
};

class D : public B, public C {
public:
  int d;
  virtual void y();
};

对象D的内存布局：

/*

                                   +-----------------------+
                                   |   20 (vbase_offset)   |
                                   +-----------------------+
                                   |     0 (top_offset)    |
                                   +-----------------------+
                                   | ptr to typeinfo for D |
                      +----------> +-----------------------+
d --> +----------+    |            |         B::w()        |
      |  vtable  |----+            +-----------------------+
      +----------+                 |         D::y()        |
      |     b    |                 +-----------------------+
      +----------+                 |   12 (vbase_offset)   |
      |  vtable  |---------+       +-----------------------+
      +----------+         |       |    -8 (top_offset)    |
      |     c    |         |       +-----------------------+
      +----------+         |       | ptr to typeinfo for D |
      |     d    |         +-----> +-----------------------+
      +----------+                 |         C::x()        |
      |  vtable  |----+            +-----------------------+
      +----------+    |            |    0 (vbase_offset)   |
      |     a    |    |            +-----------------------+
      +----------+    |            |   -20 (top_offset)    |
                      |            +-----------------------+
                      |            | ptr to typeinfo for D |
                      +----------> +-----------------------+
                                   |         A::v()        |
                                   +-----------------------+

• 这里需要注意，因为A中有虚函数，所以虚基类A前面必须带有虚表指针。

如果A中没有虚函数

class A {
public:
  int a;
  void v();
};

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

class C : public virtual A {
public:
  int c;
  virtual void x();
};

class D : public B, public C {
public:
  int d;
  virtual void y();
};

对象D的内存布局：

/*

                                   +-----------------------+
                                   |   20 (vbase_offset)   |
                                   +-----------------------+
                                   |     0 (top_offset)    |
                                   +-----------------------+
                                   | ptr to typeinfo for D |
                      +----------> +-----------------------+
d --> +----------+    |            |         B::w()        |
      |  vtable  |----+            +-----------------------+
      +----------+                 |         D::y()        |
      |     b    |                 +-----------------------+
      +----------+                 |   12 (vbase_offset)   |
      |  vtable  |---------+       +-----------------------+
      +----------+         |       |    -8 (top_offset)    |
      |     c    |         |       +-----------------------+
      +----------+         |       | ptr to typeinfo for D |
      |     d    |         +-----> +-----------------------+
      +----------+                 |         C::x()        |
      |     a    |                 +-----------------------+
      +----------+