C++类内存分布
来源：https://www.cnblogs.com/jerry19880126/p/3616999.html
书上类继承相关章节到这里就结束了，这里不妨说下C++内存分布结构，我们来看看编译器是怎么处理类成员内存分布的，特别是在继承、虚函数存在的情况下。
32bit程序：
工欲善其事，必先利其器，我们先用好Visual Studio工具，像下面这样一步一步来：
先选择左侧的C/C++->命令行，然后在其他选项这里写上/d1 reportAllClassLayout，它可以看到所有相关类的内存布局，如果写上/d1 reportSingleClassLayoutXXX（XXX为类名），则只会打出指定类XXX的内存布局。近期的VS版本都支持这样配置。
下面可以定义一个类，像下面这样：
1 class Base
2 {
3 int a;
4 int b;
5 public:
6 void CommonFunction();
7 };
然后编译一下，可以看到输出框里面有这样的排布：
这里不想花精力在内存对齐因素上，所以成员变量都设为int型。
从这里可以看到普通类的排布方式，成员变量依据声明的顺序进行排列（类内偏移为0开始），成员函数不占内存空间。
再看下继承，往后面添加如下代码：
1 class DerivedClass: public Base
2 {
3 int c;
4 public:
5 void DerivedCommonFunction();
6 };
编译，然后看到如下的内存分布（父类的内存分布不变，这里只讨论子类成员变量的内存分布）：
可以看到子类继承了父类的成员变量，在内存排布上，先是排布了父类的成员变量，接着排布子类的成员变量，同样，成员函数不占字节。
下面给基类加上虚函数，暂时注释掉DerivedClass，看一下这时的内存排布：
1 class Base
2 {
3 int a;
4 int b;
5 public:
6 void CommonFunction();
7 void virtual VirtualFunction();
8 };
这个内存结构图分成了两个部分，上面是内存分布，下面是虚表，我们逐个看。VS所带编译器是把虚表指针放在了内存的开始处（0地址偏移），然后再是成员变量；下面生成了虚表，紧跟在&Base1_meta后面的0表示，这张虚表对应的虚指针在内存中的分布，下面列出了虚函数，左侧的0是这个虚函数的序号，这里只有一个虚函数，所以只有一项，如果有多个虚函数，会有序号为1，为2的虚函数列出来。
编译器是在构造函数创建这个虚表指针以及虚表的。
那么编译器是如何利用虚表指针与虚表来实现多态的呢？是这样的，当创建一个含有虚函数的父类的对象时，编译器在对象构造时将虚表指针指向父类的虚函数；同样，当创建子类的对象时，编译器在构造函数里将虚表指针（子类只有一个虚表指针，它来自父类）指向子类的虚表（这个虚表里面的虚函数入口地址是子类的）。
所以，如果是调用Base *p = new Derived();生成的是子类的对象，在构造时，子类对象的虚指针指向的是子类的虚表，接着由Derived*到Base*的转换并没有改变虚表指针，所以这时候p->VirtualFunction，实际上是p->vfptr->VirtualFunction，它在构造的时候就已经指向了子类的VirtualFunction，所以调用的是子类的虚函数，这就是多态了。
下面加上子类，并在子类中添加虚函数，像下面这样：
1 class DerivedClass: public Base
2 {
3 int c;
4 public:
5 void DerivedCommonFunction();
6 void virtual VirtualFunction();
7 };
可以看到子类内存的排布如下：
上半部是内存分布，可以看到，虚表指针被继承了，且仍位于内存排布的起始处，下面是父类的成员变量a和b，最后是子类的成员变量c，注意虚表指针只有一个，子类并没有再生成虚表指针了；下半部的虚表情况与父类是一样的。
我们把子类换个代码，像这样：
1 class DerivedClass1 : public Base
2 {
3 int c;
4 public:
5 void DerivedCommonFunction();
6 void virtual VirtualFunction2();
7 };
注意到这时我们并没有覆写父类的虚方法，而是重声明了一个新的子类虚方法，内存分布如下：
还是只有一个虚表指针，但是下方虚表的内容变化了，虚表的0号是父类的VirtualFunction，而1号放的是子类的VirtualFunction2。也就是说，如果定义了DerivedClass的对象，那么在构造时，虚表指针就会指向这个虚表，以后如果调用的是VirtualFunction，那么会从父类中寻找对应的虚函数，如果调用的是VirtualFunction2，那么会从子类中寻找对应的虚函数。
我们再改造一下子类，像这样：
1 class DerivedClass1 : public Base
2 {
3 int c;
4 public:
5 void DerivedCommonFunction();
6 void virtual VirtualFunction();
7 void virtual VirtualFunction2();
8 };
我们既覆写父类的虚函数，也有新添的虚函数，那么可以料想的到，是下面的这种内存分布：
下面来讨论多重继承，代码如下：
1 class Base
2 {
3 int a;
4 int b;
5 public:
6 void CommonFunction();
7 void virtual VirtualFunction();
8 };
9
10
11 class DerivedClass1: public Base
12 {
13 int c;
14 public:
15 void DerivedCommonFunction();
16 void virtual VirtualFunction();
17 };
18
19 class DerivedClass2 : public Base
20 {
21 int d;
22 public:
23 void DerivedCommonFunction();
24 void virtual VirtualFunction();
25 };
26
27 class DerivedDerivedClass : public DerivedClass1, public DerivedClass2
28 {
29 int e;
30 public:
31 void DerivedDerivedCommonFunction();
32 void virtual VirtualFunction();
33 };
内存分布从父类到子类，依次如下：
Base中有一个虚表指针，地址偏移为0
DerivedClass1继承了Base，内存排布是先父类后子类。
DerivedClass2的情况是类似于DerivedClass1的。
下面我们重点看看这个类DerivedDerivedClass，由外向内看，它并列地排布着继承而来的两个父类DerivedClass1与DerivedClass2，还有自身的成员变量e。DerivedClass1包含了它的成员变量c，以及Base，Base有一个0地址偏移的虚表指针，然后是成员变量a和b；DerivedClass2的内存排布类似于DerivedClass1，注意到DerivedClass2里面竟然也有一份Base。
这里有两份虚表了，分别针对DerivedClass1与DerivedClass2，在&DerivedDericedClass_meta下方的数字是首地址偏移量，靠下面的虚表的那个-16表示指向这个虚表的虚指针的内存偏移，这正是DerivedClass2中的{vfptr}在DerivedDerivedClass的内存偏移。
如果采用虚继承，像下面这样：
1 class DerivedClass1: virtual public Base
2 {
3 int c;
4 public:
5 void DerivedCommonFunction();
6 void virtual VirtualFunction();
7 };
8
9 class DerivedClass2 : virtual public Base
10 {
11 int d;
12 public:
13 void DerivedCommonFunction();
14 void virtual VirtualFunction();
15 };
16
17 class DerivedDerivedClass : public DerivedClass1, public DerivedClass2
18 {
19 int e;
20 public:
21 void DerivedDerivedCommonFunction();
22 void virtual VirtualFunction();
23 };
Base类没有变化，但往下看：
DerivedClass1就已经有变化了，原来是先排虚表指针与Base成员变量，vfptr位于0地址偏移处；但现在有两个虚表指针了，一个是vbptr，另一个是vfptr。vbptr是这个DerivedClass1对应的虚表指针，它指向DerivedClass1的虚表vbtable，另一个vfptr是虚基类表对应的虚指针，它指向vftable。
下面列出了两张虚表，第一张表是vbptr指向的表，8表示{vbptr}与{vfptr}的偏移；第二张表是vfptr指向的表，-8指明了这张表所对应的虚指针位于内存的偏移量。
DerivedClass2的内存分布类似于DerivedClass1，同样会有两个虚指针，分别指向两张虚表（第二张是虚基类表）。
下面来仔细看一下DerivedDerivedClass的内存分布，这里面有三个虚指针了，但base却只有一份。第一张虚表是内含DerivedClass1的，20表示它的虚指针{vfptr}离虚基表指针{vbptr}的距离，第二张虚表是内含DerivedClass2的，12表示它的虚指针{vfptr}离虚基表指针{vbptr}的距离，最后一张表是虚基表，-20指明了它对应的虚指针{vfptr}在内存中的偏移。
虚继承的作用是减少了对基类的重复，代价是增加了虚表指针的负担（更多的虚表指针）。
下面总结一下（当基类有虚函数时）：
1. 每个'有virtual函数的类'都有'虚函数表指针'和'虚函数表'；
2. 如果不是虚继承：那么子类将父类的'虚函数表指针'继承下来，并指向自身的'虚函数表'（发生在对象构造时）。有多少个虚函数，'虚函数表'里面的项就会有多少。多重继承时，基类'虚函数表'与'虚函数表指针'存在多个
3. 如果是虚继承：那么子类会有两种虚指针，'虚函数表指针'指向自己的'虚函数表'；'虚基类表指针'指向'虚基类表'。多重继承时，'虚函数表'与'虚函数表指针'有且只有一份，'虚基类表指针'与'虚基类表'存在多个。
64bit程序：
一般继承
1 class Base 2 { 3 int a; 4 int b; 5 public: 6 void CommonFunction(); 7 void virtual VirtualFunction(); 8 }; 9 10 class DerivedClass1 : public Base 11 { 12 int c; 13 public: 14 void DerivedCommonFunction(); 15 void virtual VirtualFunction(); 16 }; 17 18 class DerivedClass2 : public Base 19 { 20 int d; 21 public: 22 void DerivedCommonFunction(); 23 void virtual VirtualFunction(); 24 }; 25 26 class DerivedDerivedClass : public DerivedClass1, public DerivedClass2 27 { 28 int e; 29 public: 30 void DerivedDerivedCommonFunction(); 31 void virtual VirtualFunction(); 32 };
class Base size(16): +--- 0 | {vfptr} 8 | a 12 | b +--- Base::$vftable@: | &Base_meta | 0 0 | &Base::VirtualFunction Base::VirtualFunction this adjustor: 0 只要有虚函数，就有这条 class DerivedClass1 size(24): +--- 0 | +--- (base class Base) 0 | | {vfptr} 8 | | a 12 | | b | +--- 16 | c | <alignment member> (size=4) +--- DerivedClass1::$vftable@: | &DerivedClass1_meta | 0 0 | &DerivedClass1::VirtualFunction DerivedClass1::VirtualFunction this adjustor: 0 class DerivedClass2 size(24): +--- 0 | +--- (base class Base) 0 | | {vfptr} 8 | | a 12 | | b | +--- 16 | d | <alignment member> (size=4) +--- DerivedClass2::$vftable@: | &DerivedClass2_meta | 0 0 | &DerivedClass2::VirtualFunction DerivedClass2::VirtualFunction this adjustor: 0 class DerivedDerivedClass size(56): +--- 0 | +--- (base class DerivedClass1) 0 | | +--- (base class Base) 0 | | | {vfptr} 8 | | | a 12 | | | b | | +--- 16 | | c | | <alignment member> (size=4) | +--- 24 | +--- (base class DerivedClass2) 24 | | +--- (base class Base) 24 | | | {vfptr} 32 | | | a 36 | | | b | | +--- 40 | | d | | <alignment member> (size=4) | +--- 48 | e | <alignment member> (size=4) +--- DerivedDerivedClass::$vftable@DerivedClass1@: 表示DerivedDerivedClass的一个$vftable相对于DerivedClass1来说在0处 | &DerivedDerivedClass_meta | 0 0 | &DerivedDerivedClass::VirtualFunction DerivedDerivedClass::$vftable@DerivedClass2@: 表示DerivedDerivedClass的一个$vftable相对于DerivedClass2来说在-24处 | -24 0 | &thunk: this-=24; goto DerivedDerivedClass::VirtualFunction DerivedDerivedClass::VirtualFunction this adjustor: 0
虚继承
1 class Base 2 { 3 int a; 4 int b; 5 public: 6 void CommonFunction(); 7 void virtual VirtualFunction(); 8 }; 9 10 class DerivedClass1 : virtual public Base 11 { 12 int c; 13 public: 14 void DerivedCommonFunction(); 15 void virtual VirtualFunction(); 16 }; 17 18 class DerivedClass2 : virtual public Base 19 { 20 int d; 21 public: 22 void DerivedCommonFunction(); 23 void virtual VirtualFunction(); 24 }; 25 26 class DerivedDerivedClass : public DerivedClass1, public DerivedClass2 27 { 28 int e; 29 public: 30 void DerivedDerivedCommonFunction(); 31 void virtual VirtualFunction(); 32 };
class Base size(16): +--- 0 | {vfptr} 8 | a 12 | b +--- Base::$vftable@: | &Base_meta | 0 0 | &Base::VirtualFunction Base::VirtualFunction this adjustor: 0 class DerivedClass1 size(32): +--- 0 | {vbptr} 8 | c | <alignment member> (size=4) +--- +--- (virtual base Base) 16 | {vfptr} 24 | a 28 | b +--- DerivedClass1::$vbtable@: 0 | 0 1 | 16 (DerivedClass1d(DerivedClass1+0)Base) DerivedClass1::$vftable@: | -16 0 | &DerivedClass1::VirtualFunction DerivedClass1::VirtualFunction this adjustor: 16 vbi: class offset o.vbptr o.vbte fVtorDisp 只要有虚继承（包括非直接继承中包含虚继承），就要这条 Base 16 0 4 0 class DerivedClass2 size(32): +--- 0 | {vbptr} 8 | d | <alignment member> (size=4) +--- +--- (virtual base Base) 16 | {vfptr} 24 | a 28 | b +--- DerivedClass2::$vbtable@: 0 | 0 1 | 16 (DerivedClass2d(DerivedClass2+0)Base) DerivedClass2::$vftable@: | -16 0 | &DerivedClass2::VirtualFunction DerivedClass2::VirtualFunction this adjustor: 16 vbi: class offset o.vbptr o.vbte fVtorDisp Base 16 0 4 0 class DerivedDerivedClass size(56): +--- 0 | +--- (base class DerivedClass1) 0 | | {vbptr} 8 | | c | | <alignment member> (size=4) | +--- 16 | +--- (base class DerivedClass2) 16 | | {vbptr} 24 | | d | | <alignment member> (size=4) | +--- 32 | e | <alignment member> (size=4) +--- +--- (virtual base Base) 40 | {vfptr} 48 | a 52 | b +--- DerivedDerivedClass::$vbtable@DerivedClass1@: 表示DerivedDerivedClass的一个$vbtable相对于DerivedClass1来说 0 | 0 1 | 40 (DerivedDerivedClassd(DerivedClass1+0)Base) 其中40 = {vfptr} - {vbptr} = 40 - 0 DerivedDerivedClass::$vbtable@DerivedClass2@: 表示DerivedDerivedClass的一个$vbtable相对于DerivedClass2来说 0 | 0 1 | 24 (DerivedDerivedClassd(DerivedClass2+0)Base) 其中24 = {vfptr} - {vbptr} = 40 - 16 DerivedDerivedClass::$vftable@: | -40 0 | &DerivedDerivedClass::VirtualFunction DerivedDerivedClass::VirtualFunction this adjustor: 40 vbi: class offset o.vbptr o.vbte fVtorDisp 只要有虚继承（包括非直接继承中包含虚继承），就要这条 Base 40 0 4 0
一般继承+虚继承
1 class Base 2 { 3 int a; 4 int b; 5 public: 6 void CommonFunction(); 7 void virtual VirtualFunction(); 8 }; 9 10 class DerivedClass1 : public Base 11 { 12 int c; 13 public: 14 void DerivedCommonFunction(); 15 void virtual VirtualFunction(); 16 }; 17 18 class DerivedClass2 : public Base 19 { 20 int d; 21 public: 22 void DerivedCommonFunction(); 23 void virtual VirtualFunction(); 24 }; 25 26 class DerivedClass3 : virtual public Base 27 { 28 int m; 29 public: 30 void DerivedCommonFunction(); 31 void virtual VirtualFunction(); 32 }; 33 34 class DerivedClass4 : virtual public Base 35 { 36 int n; 37 public: 38 void DerivedCommonFunction(); 39 void virtual VirtualFunction(); 40 }; 41 42 class DerivedDerivedClass : public DerivedClass1, public DerivedClass2, public DerivedClass3, public DerivedClass4 43 { 44 int e; 45 public: 46 void DerivedDerivedCommonFunction(); 47 void virtual VirtualFunction(); 48 };
class Base size(16): +--- 0 | {vfptr} 8 | a 12 | b +--- Base::$vftable@: | &Base_meta | 0 0 | &Base::VirtualFunction Base::VirtualFunction this adjustor: 0 class DerivedClass1 size(24): +--- 0 | +--- (base class Base) 0 | | {vfptr} 8 | | a 12 | | b | +--- 16 | c | <alignment member> (size=4) +--- DerivedClass1::$vftable@: | &DerivedClass1_meta | 0 0 | &DerivedClass1::VirtualFunction DerivedClass1::VirtualFunction this adjustor: 0 class DerivedClass2 size(24): +--- 0 | +--- (base class Base) 0 | | {vfptr} 8 | | a 12 | | b | +--- 16 | d | <alignment member> (size=4) +--- DerivedClass2::$vftable@: | &DerivedClass2_meta | 0 0 | &DerivedClass2::VirtualFunction DerivedClass2::VirtualFunction this adjustor: 0 class DerivedClass3 size(32): +--- 0 | {vbptr} 8 | m | <alignment member> (size=4) +--- +--- (virtual base Base) 16 | {vfptr} 24 | a 28 | b +--- DerivedClass3::$vbtable@: 0 | 0 1 | 16 (DerivedClass3d(DerivedClass3+0)Base) DerivedClass3::$vftable@: | -16 0 | &DerivedClass3::VirtualFunction DerivedClass3::VirtualFunction this adjustor: 16 vbi: class offset o.vbptr o.vbte fVtorDisp Base 16 0 4 0 class DerivedClass4 size(32): +--- 0 | {vbptr} 8 | n | <alignment member> (size=4) +--- +--- (virtual base Base) 16 | {vfptr} 24 | a 28 | b +--- DerivedClass4::$vbtable@: 0 | 0 1 | 16 (DerivedClass4d(DerivedClass4+0)Base) DerivedClass4::$vftable@: | -16 0 | &DerivedClass4::VirtualFunction DerivedClass4::VirtualFunction this adjustor: 16 vbi: class offset o.vbptr o.vbte fVtorDisp Base 16 0 4 0 class DerivedDerivedClass size(104): +--- 0 | +--- (base class DerivedClass1) 0 | | +--- (base class Base) 0 | | | {vfptr} 8 | | | a 12 | | | b | | +--- 16 | | c | | <alignment member> (size=4) | +--- 24 | +--- (base class DerivedClass2) 24 | | +--- (base class Base) 24 | | | {vfptr} 32 | | | a 36 | | | b | | +--- 40 | | d | | <alignment member> (size=4) | +--- 48 | +--- (base class DerivedClass3) 48 | | {vbptr} 56 | | m | | <alignment member> (size=4) | +--- 64 | +--- (base class DerivedClass4) 64 | | {vbptr} 72 | | n | | <alignment member> (size=4) | +--- 80 | e | <alignment member> (size=4) +--- +--- (virtual base Base) 88 | {vfptr} 96 | a 100 | b +--- DerivedDerivedClass::$vftable@DerivedClass1@: | &DerivedDerivedClass_meta | 0 0 | &DerivedDerivedClass::VirtualFunction DerivedDerivedClass::$vftable@DerivedClass2@: | -24 0 | &thunk: this-=24; goto DerivedDerivedClass::VirtualFunction DerivedDerivedClass::$vbtable@DerivedClass3@: 0 | 0 1 | 40 (DerivedDerivedClassd(DerivedClass3+0)Base) DerivedDerivedClass::$vbtable@DerivedClass4@: 0 | 0 1 | 24 (DerivedDerivedClassd(DerivedClass4+0)Base) DerivedDerivedClass::$vftable@DerivedClass3@: | -88 0 | &thunk: this-=88; goto DerivedDerivedClass::VirtualFunction DerivedDerivedClass::VirtualFunction this adjustor: 0 vbi: class offset o.vbptr o.vbte fVtorDisp 只要有虚继承（包括非直接继承中包含虚继承），就要这条 offset表示vftable所在偏移 o.vbptr表示选中的vbtable的指针（这边指向DerivedClass3） Base 88 48 4 0 这边的Base是指我们申明的Base积累
虚继承+一般继承
1 class DerivedDerivedClass : public DerivedClass3, public DerivedClass4, public DerivedClass1, public DerivedClass2
结果同 一般继承+虚继承 表明结果跟继承顺序没关系
一般继承和虚继承混用1
1 class Base 2 { 3 int a; 4 int b; 5 public: 6 void CommonFunction(); 7 void virtual VirtualFunction(); 8 }; 9 10 class DerivedClass1 : public Base 11 { 12 int c; 13 public: 14 void DerivedCommonFunction(); 15 void virtual VirtualFunction(); 16 }; 17 18 class DerivedClass2 : public Base 19 { 20 int d; 21 public: 22 void DerivedCommonFunction(); 23 void virtual VirtualFunction(); 24 }; 25 26 class DerivedClass3 : virtual public Base 27 { 28 int m; 29 public: 30 void DerivedCommonFunction(); 31 void virtual VirtualFunction(); 32 }; 33 34 class DerivedClass4 : virtual public Base 35 { 36 int n; 37 public: 38 void DerivedCommonFunction(); 39 void virtual VirtualFunction(); 40 }; 41 42 class DerivedDerivedClass : virtual public DerivedClass1, virtual public DerivedClass2, virtual public DerivedClass3, virtual public DerivedClass4 43 { 44 int e; 45 public: 46 void DerivedDerivedCommonFunction(); 47 void virtual VirtualFunction(); 48 };
class Base size(16): +--- 0 | {vfptr} 8 | a 12 | b +--- Base::$vftable@: | &Base_meta | 0 0 | &Base::VirtualFunction Base::VirtualFunction this adjustor: 0 class DerivedClass1 size(24): +--- 0 | +--- (base class Base) 0 | | {vfptr} 8 | | a 12 | | b | +--- 16 | c | <alignment member> (size=4) +--- DerivedClass1::$vftable@: | &DerivedClass1_meta | 0 0 | &DerivedClass1::VirtualFunction DerivedClass1::VirtualFunction this adjustor: 0 class DerivedClass2 size(24): +--- 0 | +--- (base class Base) 0 | | {vfptr} 8 | | a 12 | | b | +--- 16 | d | <alignment member> (size=4) +--- DerivedClass2::$vftable@: | &DerivedClass2_meta | 0 0 | &DerivedClass2::VirtualFunction DerivedClass2::VirtualFunction this adjustor: 0 class DerivedClass3 size(32): +--- 0 | {vbptr} 8 | m | <alignment member> (size=4) +--- +--- (virtual base Base) 16 | {vfptr} 24 | a 28 | b +--- DerivedClass3::$vbtable@: 0 | 0 1 | 16 (DerivedClass3d(DerivedClass3+0)Base) DerivedClass3::$vftable@: | -16 0 | &DerivedClass3::VirtualFunction DerivedClass3::VirtualFunction this adjustor: 16 vbi: class offset o.vbptr o.vbte fVtorDisp Base 16 0 4 0 class DerivedClass4 size(32): +--- 0 | {vbptr} 8 | n | <alignment member> (size=4) +--- +--- (virtual base Base) 16 | {vfptr} 24 | a 28 | b +--- DerivedClass4::$vbtable@: 0 | 0 1 | 16 (DerivedClass4d(DerivedClass4+0)Base) DerivedClass4::$vftable@: | -16 0 | &DerivedClass4::VirtualFunction DerivedClass4::VirtualFunction this adjustor: 16 vbi: class offset o.vbptr o.vbte fVtorDisp Base 16 0 4 0 class DerivedDerivedClass size(112): +--- 0 | {vbptr} 8 | e | <alignment member> (size=4) +--- +--- (virtual base DerivedClass1) 16 | +--- (base class Base) 16 | | {vfptr} 24 | | a 28 | | b | +--- 32 | c | <alignment member> (size=4) +--- +--- (virtual base DerivedClass2) 40 | +--- (base class Base) 40 | | {vfptr} 48 | | a 52 | | b | +--- 56 | d | <alignment member> (size=4) +--- +--- (virtual base Base) 64 | {vfptr} 72 | a 76 | b +--- +--- (virtual base DerivedClass3) 80 | {vbptr} 88 | m | <alignment member> (size=4) +--- +--- (virtual base DerivedClass4) 96 | {vbptr} 104 | n | <alignment member> (size=4) +--- DerivedDerivedClass::$vbtable@DerivedDerivedClass@: 表示有5个虚继承类（包括非直接继承中包含虚继承类） 第二列指明类所在位置 0 | 0 1 | 16 (DerivedDerivedClassd(DerivedDerivedClass+0)DerivedClass1) 2 | 40 (DerivedDerivedClassd(DerivedDerivedClass+0)DerivedClass2) 3 | 64 (DerivedDerivedClassd(DerivedDerivedClass+0)Base) 4 | 80 (DerivedDerivedClassd(DerivedDerivedClass+0)DerivedClass3) 5 | 96 (DerivedDerivedClassd(DerivedDerivedClass+0)DerivedClass4) DerivedDerivedClass::$vftable@DerivedClass1@: DerivedClass1类本身申明为非虚继承，所以才有这个 | -16 0 | &DerivedDerivedClass::VirtualFunction DerivedDerivedClass::$vftable@DerivedClass2@: DerivedClass2类本身申明为非虚继承，所以才有这个 | -40 0 | &thunk: this-=24; goto DerivedDerivedClass::VirtualFunction DerivedDerivedClass::$vftable@DerivedClass3@: DerivedClass3类本身和DerivedClass4类本身申明为虚继承，所以才公用一个 | -64 0 | &thunk: this-=48; goto DerivedDerivedClass::VirtualFunction DerivedDerivedClass::$vbtable@DerivedClass3@: DerivedClass3类本身申明为虚继承，所以才有这个 0 | 0 1 | -16 (DerivedDerivedClassd(DerivedClass3+0)Base) DerivedDerivedClass::$vbtable@DerivedClass4@: DerivedClass4类本身申明为虚继承，所以才有这个 0 | 0 1 | -32 (DerivedDerivedClassd(DerivedClass4+0)Base) DerivedDerivedClass::VirtualFunction this adjustor: 16 vbi: class offset o.vbptr o.vbte fVtorDisp o.vbte表示vb的table entry，参见DerivedDerivedClass::$vbtable@DerivedDerivedClass@ DerivedClass1 16 0 4 0 DerivedClass2 40 0 8 0 Base 64 0 12 0 DerivedClass3 80 0 16 0 DerivedClass4 96 0 20 0
一般继承和虚继承混用2
1 class Base 2 { 3 int a; 4 int b; 5 public: 6 void CommonFunction(); 7 void virtual VirtualFunction(); 8 }; 9 10 class DerivedClass1 : public Base 11 { 12 int c; 13 public: 14 void DerivedCommonFunction(); 15 void virtual VirtualFunction(); 16 }; 17 18 class DerivedClass2 : public Base 19 { 20 int d; 21 public: 22 void DerivedCommonFunction(); 23 void virtual VirtualFunction(); 24 }; 25 26 class DerivedClass3 : virtual public Base 27 { 28 int m; 29 public: 30 void DerivedCommonFunction(); 31 void virtual VirtualFunction(); 32 }; 33 34 class DerivedClass4 : virtual public Base 35 { 36 int n; 37 public: 38 void DerivedCommonFunction(); 39 void virtual VirtualFunction(); 40 }; 41 42 class DerivedDerivedClass : virtual public DerivedClass1, virtual public DerivedClass2, public DerivedClass3, public DerivedClass4 43 { 44 int e; 45 public: 46 void DerivedDerivedCommonFunction(); 47 void virtual VirtualFunction(); 48 };
class Base size(16): +--- 0 | {vfptr} 8 | a 12 | b +--- Base::$vftable@: | &Base_meta | 0 0 | &Base::VirtualFunction Base::VirtualFunction this adjustor: 0 class DerivedClass1 size(24): +--- 0 | +--- (base class Base) 0 | | {vfptr} 8 | | a 12 | | b | +--- 16 | c | <alignment member> (size=4) +--- DerivedClass1::$vftable@: | &DerivedClass1_meta | 0 0 | &DerivedClass1::VirtualFunction DerivedClass1::VirtualFunction this adjustor: 0 class DerivedClass2 size(24): +--- 0 | +--- (base class Base) 0 | | {vfptr} 8 | | a 12 | | b | +--- 16 | d | <alignment member> (size=4) +--- DerivedClass2::$vftable@: | &DerivedClass2_meta | 0 0 | &DerivedClass2::VirtualFunction DerivedClass2::VirtualFunction this adjustor: 0 class DerivedClass3 size(32): +--- 0 | {vbptr} 8 | m | <alignment member> (size=4) +--- +--- (virtual base Base) 16 | {vfptr} 24 | a 28 | b +--- DerivedClass3::$vbtable@: 0 | 0 1 | 16 (DerivedClass3d(DerivedClass3+0)Base) DerivedClass3::$vftable@: | -16 0 | &DerivedClass3::VirtualFunction DerivedClass3::VirtualFunction this adjustor: 16 vbi: class offset o.vbptr o.vbte fVtorDisp Base 16 0 4 0 class DerivedClass4 size(32): +--- 0 | {vbptr} 8 | n | <alignment member> (size=4) +--- +--- (virtual base Base) 16 | {vfptr} 24 | a 28 | b +--- DerivedClass4::$vbtable@: 0 | 0 1 | 16 (DerivedClass4d(DerivedClass4+0)Base) DerivedClass4::$vftable@: | -16 0 | &DerivedClass4::VirtualFunction DerivedClass4::VirtualFunction this adjustor: 16 vbi: class offset o.vbptr o.vbte fVtorDisp Base 16 0 4 0 class DerivedDerivedClass size(104): +--- 0 | +--- (base class DerivedClass3) 0 | | {vbptr} 8 | | m | | <alignment member> (size=4) | +--- 16 | +--- (base class DerivedClass4) 16 | | {vbptr} 24 | | n | | <alignment member> (size=4) | +--- 32 | e | <alignment member> (size=4) +--- +--- (virtual base DerivedClass1) 40 | +--- (base class Base) 40 | | {vfptr} 48 | | a 52 | | b | +--- 56 | c | <alignment member> (size=4) +--- +--- (virtual base DerivedClass2) 64 | +--- (base class Base) 64 | | {vfptr} 72 | | a 76 | | b | +--- 80 | d | <alignment member> (size=4) +--- +--- (virtual base Base) 88 | {vfptr} 96 | a 100 | b +--- DerivedDerivedClass::$vbtable@DerivedClass3@: 0 | 0 1 | 88 (DerivedDerivedClassd(DerivedClass3+0)Base) 2 | 40 (DerivedDerivedClassd(DerivedDerivedClass+0)DerivedClass1) 3 | 64 (DerivedDerivedClassd(DerivedDerivedClass+0)DerivedClass2) DerivedDerivedClass::$vbtable@DerivedClass4@: 0 | 0 1 | 72 (DerivedDerivedClassd(DerivedClass4+0)Base) DerivedDerivedClass::$vftable@DerivedClass1@: | -40 0 | &DerivedDerivedClass::VirtualFunction DerivedDerivedClass::$vftable@DerivedClass2@: | -64 0 | &thunk: this-=24; goto DerivedDerivedClass::VirtualFunction DerivedDerivedClass::$vftable@DerivedClass3@: | -88 0 | &thunk: this-=48; goto DerivedDerivedClass::VirtualFunction DerivedDerivedClass::VirtualFunction this adjustor: 40 vbi: class offset o.vbptr o.vbte fVtorDisp DerivedClass1 40 0 8 0 DerivedClass2 64 0 12 0 Base 88 0 4 0
https://www.cnblogs.com/jlxuexijidi-2015/p/5293617.html virtual成员函数
