一个班级的VTT是什么？ 最近遇到了一个对我来说是新的C++链接错误。 libfoo.so: undefined reference to `VTT for Foo' libfoo.so: undefined reference to `vtable for Foo'
我认识到了错误并解决了问题，但我仍然有一个困扰的问题：VTT到底是什么 旁白：对于那些感兴趣的人来说，当您忘记定义类中声明的第一个虚拟函数时，就会出现问题。vtable进入类的第一个虚拟函数的编译单元。如果忘记定义该函数，则会出现链接器错误，它找不到vtable，而对开发人员友好得多的用户找不到该函数。该页面现在处于脱机状态，并且没有存档它。所以，我找到了一份存档的文本副本 在圣路易斯华盛顿大学计算机科学系的分布式对象计算实验室，有原始注释的全文，作为研究生Morgan Deters的“（2005年秋季）”的一部分在线发布。一个班级的VTT是什么？ 最近遇到了一个对我来说是新的C++链接错误。 libfoo.so: undefined reference to `VTT for Foo' libfoo.so: undefined reference to `vtable for Foo',c++,gcc,vtable,virtual-inheritance,vtt,C++,Gcc,Vtable,Virtual Inheritance,Vtt,我认识到了错误并解决了问题，但我仍然有一个困扰的问题：VTT到底是什么 旁白：对于那些感兴趣的人来说，当您忘记定义类中声明的第一个虚拟函数时，就会出现问题。vtable进入类的第一个虚拟函数的编译单元。如果忘记定义该函数，则会出现链接器错误，它找不到vtable，而对开发人员友好得多的用户找不到该函数。该页面现在处于脱机状态，并且没有存档它。所以，我找到了一份存档的文本副本 在圣路易斯华盛顿大学计算机科学系的分布式对象计算实验室，有原始注释的全文，作为研究生Morgan Deters的“（200
: 摩根威慑网页：
- ,
- 他的传记
- 谷歌学者简介：
- 最近的网页
- 2015年讣告：“Morgan G.Deters博士，博士，35岁，前保龄球场，最近在纽约布鲁克林，于2015年1月17日在特立尼达的多巴哥去世。”
ABclass A {
public:
int a;
class B : public A {
public:
int b;
};
Bclass A {
public:
int a;
virtual void v();
};
class B : public A {
public:
int b;
};
+-----------------------+
| 0 (top_offset) |
+-----------------------+
b --> +----------+ | ptr to typeinfo for B |
| vtable |-------> +-----------------------+
+----------+ | A::v() |
| a | +-----------------------+
+----------+
| b |
+----------+
top\u offset现在考虑多重继承：
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;
};
+-----------------------+
| 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 | +-----------------------+
+----------+
+-----------------------+
| 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() |
+-----------------------+
B-in-D
+-----------------------+
| 20 (vbase_offset) |
+-----------------------+
| 0 (top_offset) |
+-----------------------+
d --> +----------+ | ptr to typeinfo for B |
| vtable |------> +-----------------------+
+----------+ | B::w() |
| b | +-----------------------+
+----------+ | 0 (vbase_offset) |
| | +-----------------------+
+----------+ | -20 (top_offset) |
| | +-----------------------+
+----------+ | ptr to typeinfo for B |
| | +--> +-----------------------+
+----------+ | | A::v() |
| vtable |---+ +-----------------------+
+----------+
| a |
+----------+
B-in-D
+-----------------------+
| 20 (vbase_offset) |
VTT for D +-----------------------+
+-------------------+ | 0 (top_offset) |
| vtable for D |-------------+ +-----------------------+
+-------------------+ | | ptr to typeinfo for B |
| vtable for B-in-D |-------------|----------> +-----------------------+
+-------------------+ | | B::w() |
| vtable for B-in-D |-------------|--------+ +-----------------------+
+-------------------+ | | | 0 (vbase_offset) |
| vtable for C-in-D |-------------|-----+ | +-----------------------+
+-------------------+ | | | | -20 (top_offset) |
| vtable for C-in-D |-------------|--+ | | +-----------------------+
+-------------------+ | | | | | ptr to typeinfo for B |
| vtable for D |----------+ | | | +-> +-----------------------+
+-------------------+ | | | | | A::v() |
| vtable for D |-------+ | | | | +-----------------------+
+-------------------+ | | | | |
| | | | | C-in-D
| | | | | +-----------------------+
| | | | | | 12 (vbase_offset) |
| | | | | +-----------------------+
| | | | | | 0 (top_offset) |
| | | | | +-----------------------+
| | | | | | ptr to typeinfo for C |
| | | | +----> +-----------------------+
| | | | | C::x() |
| | | | +-----------------------+
| | | | | 0 (vbase_offset) |
| | | | +-----------------------+
| | | | | -12 (top_offset) |
| | | | +-----------------------+
| | | | | ptr to typeinfo for C |
| | | +-------> +-----------------------+
| | | | A::v() |
| | | +-----------------------+
| | |
| | | D
| | | +-----------------------+
| | | | 20 (vbase_offset) |
| | | +-----------------------+
| | | | 0 (top_offset) |
| | | +-----------------------+
| | | | ptr to typeinfo for D |
| | +----------> +-----------------------+
| | | B::w() |
| | +-----------------------+
| | | D::y() |
| | +-----------------------+
| | | 12 (vbase_offset) |
| | +-----------------------+
| | | -8 (top_offset) |
| | +-----------------------+
| | | ptr to typeinfo for D |
+----------------> +-----------------------+
| | C::x() |
| +-----------------------+
| | 0 (vbase_offset) |
| +-----------------------+
| | -20 (top_offset) |
| +-----------------------+
| | ptr to typeinfo for D |
+-------------> +-----------------------+
| A::v() |
+-----------------------+
B-in-D
+-----------------------+
| 20 (vbase_offset) |
+-----------------------+
| 0 (top_offset) |
+-----------------------+
| ptr to typeinfo for B |
+---------------------------------> +-----------------------+
| | B::w() |
| +-----------------------+
| C-in-D | 0 (vbase_offset) |
| +-----------------------+ +-----------------------+
d --> +----------+ | | 12 (vbase_offset) | | -20 (top_offset) |
| vtable |--+ +-----------------------+ +-----------------------+
+----------+ | 0 (top_offset) | | ptr to typeinfo for B |
| b | +-----------------------+ +-----------------------+
+----------+ | ptr to typeinfo for C | | A::v() |
| vtable |--------> +-----------------------+ +-----------------------+
+----------+ | C::x() |
| c | +-----------------------+
+----------+ | 0 (vbase_offset) |
| | +-----------------------+
+----------+ | -12 (top_offset) |
| vtable |--+ +-----------------------+
+----------+ | | ptr to typeinfo for C |
| a | +-----> +-----------------------+
+----------+ | A::v() |
+-----------------------+
A:：v（）cB:：w（）CB:：w（）。当通过指向C的指针调用w（）
时，需要调整指针（成为w（）
内部的this指针）。这通常称为指针调整
在某些情况下，编译器将生成一个THUBN来修复地址。考虑与上面相同的代码，但是这个时间<代码> c>代码>重写<代码> B/CODE>的成员函数<代码> We（）（<代码> >：
C
的对象布局和vtable现在如下所示：
+-----------------------+
| 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 | +-----------------------+
+----------+
+-----------------------+
| 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() |
+-----------------------+
B-in-D
+-----------------------+
| 20 (vbase_offset) |
+-----------------------+
| 0 (top_offset) |
+-----------------------+
d --> +----------+ | ptr to typeinfo for B |
| vtable |------> +-----------------------+
+----------+ | B::w() |
| b | +-----------------------+
+----------+ | 0 (vbase_offset) |
| | +-----------------------+
+----------+ | -20 (top_offset) |
| | +-----------------------+
+----------+ | ptr to typeinfo for B |
| | +--> +-----------------------+
+----------+ | | A::v() |
| vtable |---+ +-----------------------+
+----------+
| a |
+----------+
B-in-D
+-----------------------+
| 20 (vbase_offset) |
VTT for D +-----------------------+
+-------------------+ | 0 (top_offset) |
| vtable for D |-------------+ +-----------------------+
+-------------------+ | | ptr to typeinfo for B |
| vtable for B-in-D |-------------|----------> +-----------------------+
+-------------------+ | | B::w() |
| vtable for B-in-D |-------------|--------+ +-----------------------+
+-------------------+ | | | 0 (vbase_offset) |
| vtable for C-in-D |-------------|-----+ | +-----------------------+
+-------------------+ | | | | -20 (top_offset) |
| vtable for C-in-D |-------------|--+ | | +-----------------------+
+-------------------+ | | | | | ptr to typeinfo for B |
| vtable for D |----------+ | | | +-> +-----------------------+
+-------------------+ | | | | | A::v() |
| vtable for D |-------+ | | | | +-----------------------+
+-------------------+ | | | | |
| | | | | C-in-D
| | | | | +-----------------------+
| | | | | | 12 (vbase_offset) |
| | | | | +-----------------------+
| | | | | | 0 (top_offset) |
| | | | | +-----------------------+
| | | | | | ptr to typeinfo for C |
| | | | +----> +-----------------------+
| | | | | C::x() |
| | | | +-----------------------+
| | | | | 0 (vbase_offset) |
| | | | +-----------------------+
| | | | | -12 (top_offset) |
| | | | +-----------------------+
| | | | | ptr to typeinfo for C |
| | | +-------> +-----------------------+
| | | | A::v() |
| | | +-----------------------+
| | |
| | | D
| | | +-----------------------+
| | | | 20 (vbase_offset) |
| | | +-----------------------+
| | | | 0 (top_offset) |
| | | +-----------------------+
| | | | ptr to typeinfo for D |
| | +----------> +-----------------------+
| | | B::w() |
| | +-----------------------+
| | | D::y() |
| | +-----------------------+
| | | 12 (vbase_offset) |
| | +-----------------------+
| | | -8 (top_offset) |
| | +-----------------------+
| | | ptr to typeinfo for D |
+----------------> +-----------------------+
| | C::x() |
| +-----------------------+
| | 0 (vbase_offset) |
| +-----------------------+
| | -20 (top_offset) |
| +-----------------------+
| | ptr to typeinfo for D |
+-------------> +-----------------------+
| A::v() |
+-----------------------+
B-in-D
+-----------------------+
| 20 (vbase_offset) |
+-----------------------+
| 0 (top_offset) |
+-----------------------+
| ptr to typeinfo for B |
+---------------------------------> +-----------------------+
| | B::w() |
| +-----------------------+
| C-in-D | 0 (vbase_offset) |
| +-----------------------+ +-----------------------+
d --> +----------+ | | 12 (vbase_offset) | | -20 (top_offset) |
| vtable |--+ +-----------------------+ +-----------------------+
+----------+ | 0 (top_offset) | | ptr to typeinfo for B |
| b | +-----------------------+ +-----------------------+
+----------+ | ptr to typeinfo for C | | A::v() |
| vtable |--------> +-----------------------+ +-----------------------+
+----------+ | C::x() |
| c | +-----------------------+
+----------+ | 0 (vbase_offset) |
| | +-----------------------+
+----------+ | -12 (top_offset) |
| vtable |--+ +-----------------------+
+----------+ | | ptr to typeinfo for C |
| a | +-----> +-----------------------+
+----------+ | A::v() |
+-----------------------+
现在，当通过指向-B的指针对C
的实例调用w（）
时，将调用thunk。thunk做什么？让我们将其分解（在这里，使用gdb
）：
请注意，D
继承自B
和C
，B
和C
都继承自A
。这意味着D
中有两个A
的副本。对象布局和vtable嵌入是我们从前面几节中所期望的：
+-----------------------+
| 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() |
+-----------------------+
当然，我们期望A
的数据（成员A
）在D
的对象布局中存在两次（事实上是这样），并且我们期望A
的虚拟成员函数在vtable中表示两次（并且A:：v（）
确实存在）。好的，这里没有什么新内容
钻石：虚拟基地的单一副本
但是，如果我们使用虚拟继承，C++虚拟继承允许我们指定一个钻石层次结构，但只保证一个继承的基本继承基。所以，让我们用这样的方式编写代码：
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();
};
突然间，事情变得复杂多了。如果我们在D
的表示中只能有一个a
的副本，那么我们就再也无法摆脱将aC
嵌入aD
（并将D
的C
部分的vtable嵌入D
的vtable）的“把戏”.但如果我们不能做到这一点，我们如何处理通常的类型替换
让我们尝试绘制布局图：
+-----------------------+
| 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() |
+-----------------------+
好的。你看，A
现在嵌入到D
中，基本上和其他基一样。但是它嵌入到D中，而不是inits直接派生的类中
摩根威慑网页：
- ,
- 他的传记
- 谷歌学者简介：
- 最近的网页
- 2015年讣告：“Morgan G.Deters博士，博士，35岁，前保龄球场，最近在纽约布鲁克林，于2015年1月17日在特立尼达的多巴哥去世。”
第2部分：
存在多重继承时的构造/破坏
怎么
B-in-D
+-----------------------+
| 20 (vbase_offset) |
VTT for D +-----------------------+
+-------------------+ | 0 (top_offset) |
| vtable for D |-------------+ +-----------------------+
+-------------------+ | | ptr to typeinfo for B |
| vtable for B-in-D |-------------|----------> +-----------------------+
+-------------------+ | | B::w() |
| vtable for B-in-D |-------------|--------+ +-----------------------+
+-------------------+ | | | 0 (vbase_offset) |
| vtable for C-in-D |-------------|-----+ | +-----------------------+
+-------------------+ | | | | -20 (top_offset) |
| vtable for C-in-D |-------------|--+ | | +-----------------------+
+-------------------+ | | | | | ptr to typeinfo for B |
| vtable for D |----------+ | | | +-> +-----------------------+
+-------------------+ | | | | | A::v() |
| vtable for D |-------+ | | | | +-----------------------+
+-------------------+ | | | | |
| | | | | C-in-D
| | | | | +-----------------------+
| | | | | | 12 (vbase_offset) |
| | | | | +-----------------------+
| | | | | | 0 (top_offset) |
| | | | | +-----------------------+
| | | | | | ptr to typeinfo for C |
| | | | +----> +-----------------------+
| | | | | C::x() |
| | | | +-----------------------+
| | | | | 0 (vbase_offset) |
| | | | +-----------------------+
| | | | | -12 (top_offset) |
| | | | +-----------------------+
| | | | | ptr to typeinfo for C |
| | | +-------> +-----------------------+
| | | | A::v() |
| | | +-----------------------+
| | |
| | | D
| | | +-----------------------+
| | | | 20 (vbase_offset) |
| | | +-----------------------+
| | | | 0 (top_offset) |
| | | +-----------------------+
| | | | ptr to typeinfo for D |
| | +----------> +-----------------------+
| | | B::w() |
| | +-----------------------+
| | | D::y() |
| | +-----------------------+
| | | 12 (vbase_offset) |
| | +-----------------------+
| | | -8 (top_offset) |
| | +-----------------------+
| | | ptr to typeinfo for D |
+----------------> +-----------------------+
| | C::x() |
| +-----------------------+
| | 0 (vbase_offset) |
| +-----------------------+
| | -20 (top_offset) |
| +-----------------------+
| | ptr to typeinfo for D |
+-------------> +-----------------------+
| A::v() |
+-----------------------+
B-in-D
+-----------------------+
| 20 (vbase_offset) |
+-----------------------+
| 0 (top_offset) |
+-----------------------+
| ptr to typeinfo for B |
+---------------------------------> +-----------------------+
| | B::w() |
| +-----------------------+
| C-in-D | 0 (vbase_offset) |
| +-----------------------+ +-----------------------+
d --> +----------+ | | 12 (vbase_offset) | | -20 (top_offset) |
| vtable |--+ +-----------------------+ +-----------------------+
+----------+ | 0 (top_offset) | | ptr to typeinfo for B |
| b | +-----------------------+ +-----------------------+
+----------+ | ptr to typeinfo for C | | A::v() |
| vtable |--------> +-----------------------+ +-----------------------+
+----------+ | C::x() |
| c | +-----------------------+
+----------+ | 0 (vbase_offset) |
| | +-----------------------+
+----------+ | -12 (top_offset) |
| vtable |--+ +-----------------------+
+----------+ | | ptr to typeinfo for C |
| a | +-----> +-----------------------+
+----------+ | A::v() |
+-----------------------+
+-----------------------+
| 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() |
+-----------------------+
D d; // allocates a D on the stack and constructs it
D *pd = new D; // allocates a D in the heap and constructs it
/* ... */
delete pd; // calls "in-charge deleting" destructor for D
return; // calls "in-charge" destructor for stack-allocated D
D *pd = new D; // allocates a D in the heap and constructs it
C *pc = d; // we have a pointer-to-C that points to our heap-allocated D
/* ... */
delete pc; // call destructor thunk through vtable, but what about delete?
class A {
public:
int a;
};
class B : public virtual A {
public:
int b;
virtual void w();
};
class C : public virtual A {
public:
int c;
};
class D : public B, public C {
public:
int d;
virtual void y();
};
+-----------------------+
| 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 | +-----> +-----------------------+
+----------+
| a |
+----------+