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虚继承

| Tags: C/C++

C++里面的virtual关键字可以用虚函数声明，也可以用于虚继承。上一篇博客讲到了《虚函数》，这篇博客就讲虚继承。

首先来看为什么需要虚继承。C++里面继承关系中有个很有名的继承结构，菱形继承，如下图所示

普通继承，派生类包含了基类所有的非static成员。如果采用普通继承，在上图的iostream类中，实际上会存在两个ios基类。这样会带来很多问题，首先最简单的是空间浪费，iostream类中存在两个相同的ios类,然后是构造效率低，需要构造两个ios类。更严重的是调用基类中的函数时，存在二义性，当iostream调用ios的成员函数时，编译器无法知道是调用istream还是ostream中的ios。

C++的解决方案就是虚拟继承(Virtual Inheritance)。虚拟继承可以说成虚继承，在本文中，这两个词是等价的。 在虚拟继承下，只有一个共享的基类子对象被继承，而无论该基类在派生层次中出现多少次。共享的基类子对象被称为虚拟基类（virtual base class）。在虚拟继承下，基类子对象的复制及由此而引起的二义性都被消除了。

先看看如果没有续集继承的情况下，菱形继承会出现什么情况

普通继承的菱形继承 (inheritance.cc) download
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// 普继承关系下的菱形继承

#include <iostream>

using namespace std;

class top{
    public:
        top(){cout<<"This is top;"<<endl;}
        void printself(){cout << "this is in top::printself" << endl;}
};

class middle1: public top{
};

class middle2: public top{
};

class bottom: public middle1, public middle2{
    public:
        bottom(){cout<<"This is bottom;"<<endl;}
};

int main(void){
    bottom bo;
    bo.printself();
}

没有使用虚继承，那么bottom类在调用printself()就存在二义性，所以在编译的时候会报下面这样的错误。

下面就是使用虚继承的例子

虚继承的菱形继承 (virtual_inheritance.cc) download
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// 使用虚拟继承的菱形继承

#include <iostream>

using namespace std;

class top{
    public:
        top(){cout<<"This is top;"<<endl;}
        void printself(){cout << "this is in top::printself" << endl;}
};

class middle1: virtual public top{
};

class middle2: virtual public top{
};

class bottom: public middle1, public middle2{
    public:
        bottom(){cout<<"This is bottom;"<<endl;}
};

int main(void){
    bottom bo;
    bo.printself();
}

编译和运行结果如下

上面的例子中，采用了虚继承，就没有出现二义性的问题了。虚拟继承声明时，virtual关键字可以放在继承关系的前面也可以放在后面，下面两种方式是等价的。

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class middle1: virtual public top
class middle1: public virtual top

虚拟基类的构造

由虚假继承引发的第一个问题是虚拟基类的构造，例如上面的例子中，构造iostream时，构造了istream和ostream两个基类，如果是虚继承关系，那么只有一个ios虚拟基类，那么谁来构造ios呢？

普通继承关系，基类由派生类构造。虚继承下，虚基类的构造由最终派生类显示调用，即iostream负责构造ios类, 中间类的构造函数将会被抑制，无法完成虚拟基类的构造。看一个虚基类的构造例子

中间类的构造函数被抑制 (constructor.cc) download
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// 虚基类的构造, 中间类的构造函数被抑制

#include <iostream>
#include <string>

using namespace std;

class top{
    public:
        top(string name):_name(name){
            cout<<"This is top(name); name is "<< _name << endl;
        }
        top():_name("top") {
            cout<<"This is top(); name is "<< _name << endl;
        }
        void printself(){cout << "this is in top::printself, name is " << _name << endl;}
    private:
        string _name;
};

class middle1: virtual public top{
    public:
        middle1(string name):top(name){
            cout << "this is middle1, name is " << name << endl;
        };
};

class middle2: virtual public top{
    public:
        middle2(string name):top(name){;
            cout << "this is middle2, name is " << name << endl;
        }
};

class bottom: public middle1, public middle2{
    public:
        bottom():middle1("bottom1"), middle2("bottom2"){cout<<"This is bottom;"<<endl;}
};

int main(void){
    bottom bo;
    bo.printself();
}

运行结果如下

例子中，虽然bottom显示调用了middle1和middle2的构造函数，但是top的构造却不是有这两个中间类完成的, 因为top的成员name的值为“top”，实际上是由最终派生类bottom调用了top的默认构造函数top()。

要想完成虚基类top的构造，必须由最终派生类调用对应的虚基类构造函数。

最终派生类调用虚基类的构造函数 (constructor1.cc) download
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// 最终类显示调用虚基类的构造函数

#include <iostream>
#include <string>

using namespace std;

class top{
    public:
        top(string name):_name(name){
            cout<<"This is top; name is "<< _name << endl;
        }
        top() {};
        void printself(){
            cout << "this is in top::printself, name is " << _name << endl;
        }
    private:
        string _name;
};

class middle1: virtual public top{
    public:
        middle1(string name):top(name){
            cout << "this is middle1, name is " << name << endl;
        }
    protected:
        middle1(){};
};

class middle2: virtual public top{
    public:
        middle2(string name):top(name){
            cout << "this is middle2, name is " << name << endl;
        }
    protected:
        middle2(){};

};

class bottom: public middle1, public middle2{
    public:
        bottom():top("bottom"){cout<<"This is bottom;"<<endl;}
};

class bottom1: public middle1, public middle2{
    public:
        bottom1():middle1("bazinga"),middle2("bazinga"), top("bottom1"){
            cout<<"This is bottom1;"<<endl;
        }
};

int main(void){
    bottom bo;
    bo.printself();
    cout << "------" << endl;
    bottom1 bo1;
    bo1.printself();
}

运行结果如下

在上面的例子中，bottom和bottom1都显示调用了top的构造函数，但前者没有调用了中间类的默认构造函数，后者调用了构造虚基类的构造函数，但结果对于虚基类的构造，都是由最终派生类构造的。

上面是一个中间类构造函数定义方式的好例子，当middle1和middle2做为最终派生类的时候，那么使用带参数的构造函数，做为中间类时，就声明一个为protected的默认构造函数，它仅仅完成类自身的构造和非虚拟继承的基类构造，最终派生类也不需要显示地构造中间类。

构造的顺序

普通继承是按照声明顺序进行构造的，虚继承由于先要进行虚基类的构造，再进行中间类的构造，所以构造顺序是：按照声明顺序构造虚基类，再按照声明顺序构造中间类和普通基类。

先看两个虚基类构造的例子，

top_b类不采用虚继承 (constructor_seq.cc) download
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// 虚基类的构造顺序

#include <iostream>
#include <string>

using namespace std;

class top{
    public:
        top(string name):_name(name){
            cout<<"This is top; name is "<< _name << endl;
        }
        top() {};
        void printself(){
            cout << "this is in top::printself, name is " << _name << endl;
        }
    private:
        string _name;
};

class middle1: virtual public top{
    public:
        middle1(string name):top(name){
            cout << "this is middle1, name is " << name << endl;
        }
    protected:
        middle1(){};
};

class middle2: virtual public top{
    public:
        middle2(string name):top(name){
            cout << "this is middle2, name is " << name << endl;
        }
    protected:
        middle2(){};

};

class top_b{
    public:
        top_b(){ cout<<"This is top_b"<< endl;}
};

class middle_b: public top_b{
    public:
        middle_b(){cout << "this is middle_b" << endl;}
};

class bottom: public middle1, public middle2, public middle_b{
    public:
        bottom():middle1("bazinga"),middle2("bazinga"), top("bottom1"){
            cout<<"This is bottom;"<<endl;
        }
};

int main(void){
    bottom bo;
}

输出结果

另外一个例子

top_b类采用虚继承 (constructor_seq1.cc) download
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// 虚基类的构造顺序

#include <iostream>
#include <string>

using namespace std;

class top{
    public:
        top(string name):_name(name){
            cout<<"This is top; name is "<< _name << endl;
        }
        top() {};
        void printself(){cout << "this is in top::printself, name is " << _name << endl;}
    private:
        string _name;
};

class middle1: virtual public top{
    public:
        middle1(string name):top(name){
            cout << "this is middle1, name is " << name << endl;
        }
    protected:
        middle1(){};
};

class middle2: virtual public top{
    public:
        middle2(string name):top(name){
            cout << "this is middle2, name is " << name << endl;
        }
    protected:
        middle2(){};

};

class top_b{
    public:
        top_b(){ cout<<"This is top_b"<< endl;}
};

class middle_b: virtual public top_b{
    public:
        middle_b(){cout << "this is middle_b" << endl;}
};

class bottom: public middle1, public middle2, public middle_b{
    public:
        bottom():middle1("bazinga"),middle2("bazinga"), top("bottom1"){cout<<"This is bottom;"<<endl;}
};

int main(void){
    bottom bo;
}

输出结果

上面这个两个例子中可以看出top_b的构造顺序是不一样的。第一个例子中，做为普通基类，它放到了middle1和middle2后面构造，但在第二个例子中将它声明为了虚基类，它就放到了middle1和middle2前面构造了。

虚拟基类成员的可视性

派生类从它的基类所继承而来的成员可被分为以下三类：

  • 虚拟基类实例，它们没有被中间类改写，可以直接调用。
  • 存在一个中间类，改写了基类的成员，那么最终派生类，调用时使用的是被中间类改写了的成员。
  • 存在二个或二个以上的中间类，重载了虚基类的成员，那么最终派生类，必须重载这个成员函数。

下面这个例子分别都涉及到了上面三种情况

虚继承中成员的可见性 (members.cc) download
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// 成员函数的调用

#include <iostream>

using namespace std;

class top{
    public:
        void printA(){cout << "this is in top::printA" << endl;}
        void printB(){cout << "this is in top::printB" << endl;}
        void printC(){cout << "this is in top::printC" << endl;}
};

class middle1: virtual public top{
    public:
        void printB(){cout << "this is in middle1::printB" << endl;}
        void printC(){cout << "this is in middle1::printC" << endl;}
};

class middle2: virtual public top{
    public:
        void printC(){cout << "this is in middle2::printC" << endl;}
};

class bottom: public middle1, public middle2{
    public:
        void printC(){cout << "this is in bottom::printC" << endl;}
};

int main(void){
    bottom bo;
    bo.printA();
    bo.printB();
    bo.printC();
}

输出结果

由例子可以看出，上面三点分别对应了printA, printB, printC三个函数。如果不在bottom中重载printC，那么编译是会报错。

虚继承的实现原理

虚继承中，是如何实现只有虚基类的，通过虚继承类的内存分布，可以一探究竟。下面所有关于虚继承内存分布的例子都是和平台相关的:

  • 64位系统
  • 操作系统: ubuntu server 12.04
  • gcc 4.6.3

首先看一个简单的只有一层虚继承关系的例子

一层虚继承 (memory_middle1.cc) download
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// memory layout for one layer virtual inheritance.  

#include <iostream>
#include <stdio.h>

using namespace std;

#define COUT_FUNC(a, b) (cout << a << " | " << b << " | ") 
#define COUT_CNUM(a, b) (cout << a << " | " << b) 
#define COUT_ARG3(a, b, c) (COUT_FUNC(a, b) << c)
#define COMMENT(a) printf("\e[32m%s\e[0m\n", (a));

class top{
    public:
        top(long i):_i(i){}
        top(){}
        virtual void a() {cout << "this is top::a()" << endl;}
        virtual void b() {cout << "this is top::b()" << endl;}
        long _i;
};

class middle1: virtual public top{
    public:
        middle1(long j):top(100),_j(j){}
        virtual void c() {cout << "this is middle1::c()" << endl;}
        virtual void b() {cout << "this is middle1::b()" << endl;}
        long _j;
};

typedef void (*func)(void);

int main(void) {
    long* pobj;
    func pfunc;

    cout << "---memory of middle1---" << endl;

    middle1 m1(101);
    pobj = (long*)&m1;

    pfunc = (func)((long*)*((long*)(pobj[0])));
    COUT_FUNC("middle1[0]:    vtab(0)", (long*)(pobj[0])); pfunc();
    COUT_CNUM("middle1[1]:         _j", (long)pobj[1]) << endl;

    pfunc = (func)((long*)*((long*)(pobj[2])));
    COUT_FUNC("    top[0]:    vtab(0)", (long*)(pobj[2])); pfunc();
    COUT_CNUM("    top[1]:         _i", (long)pobj[3]) << endl;

    cout << endl << "---vtable of middle1---" << endl;

    // class middle1
    COUT_ARG3("middle1[0]: offset(-3)", (long*)pobj[0] - 3, (long)*((long*)pobj[0] - 3)); COMMENT("\t\t\t# offset to vbase");
    COUT_ARG3("            offset(-2)", (long*)pobj[0] - 2, (long)*((long*)pobj[0] - 2)); COMMENT("\t\t\t# offset to begin");
    COUT_ARG3("            offset(-1)", (long*)pobj[0] - 1, (long*)*((long*)pobj[0] - 1)); COMMENT("\t\t# typeinfo for middle1");

    pfunc = (func)((long*)*((long*)(pobj[0])));
    COUT_FUNC("               vtab(0)", (long*)pobj[0]); pfunc();
    pfunc = (func)((long*)*((long*)(pobj[0]) + 1));
    COUT_FUNC("               vtab(1)", (long*)pobj[0] + 1); pfunc();
    COUT_ARG3("               vtab(2)", (long*)pobj[0] + 2, *((long*)(pobj[0])+2)); COMMENT("\t\t\t# next class offset to begin");

    // class top
    COUT_ARG3("    top[0]: offset(-3)", (long*)pobj[2] - 3, (long)*((long*)pobj[2] - 3)); COMMENT("\t\t\t# offset to vbase");
    COUT_ARG3("            offset(-2)", (long*)pobj[2] - 2, (long)*((long*)pobj[2] - 2)); COMMENT("\t\t\t# offset to begin");
    COUT_ARG3("            offset(-1)", (long*)pobj[2] - 1, (long*)*((long*)pobj[2] - 1)); COMMENT("\t\t# typeinfo for middle1");

    pfunc = (func)((long*)*((long*)(pobj[2])));
    COUT_FUNC("               vtab(0)", (long*)pobj[2]); pfunc();
    COUT_ARG3("               vtab(1)", (long*)(pobj[2]) + 1, (long*)*((long*)(pobj[2])+1)); COMMENT("\t\t# virtual thunk to middle1::b()");
    COUT_ARG3("               vtab(2)", (long*)(pobj[2]) + 2, *((long*)(pobj[2])+2)); COMMENT("\t\t\t# end of vtable");

    // VTT for middle1
    COUT_ARG3("      VTT:      vtt(0)", (long*)(pobj[2]) + 3, (long*)*((long*)(pobj[2])+3)); COMMENT("\t\t# for middle1");
    COUT_ARG3("                vtt(1)", (long*)(pobj[2]) + 4, (long*)*((long*)(pobj[2])+4)); COMMENT("\t\t# for top");
}

运行结果如下图

在middle1中有两个vtable，分布指向了各自的虚函数表，而且这两个虚函数表实际是放在一张表，只是分别指向表中不同的位置。vtable的起始地址之前的3个地址分布存放了与虚继承相关的信息。

  • offset(-3)存放的是从middle1对象到虚基类top的偏移。 本例中middle1到top的偏移存放在0x401620为16个字节，top到自身的偏移存放在0x401650为0。
  • offset(-2)存放的是当前这个对象到middle对象内存起始地址的偏移。 本例中middle1到自身的偏移存放在0x401628为0，top到middle起始位置的偏移0x401658为-16
  • offset(-1)存放的是middle1类的typeinfo地址，本例中0x401630, 0x401660都存放的地址0x4016e0。在下面本例memory_middle1的符号列表图中可以看出middle1类的typeinfo地址(图中的红色部分)。

以上信息也有人称为虚继承表，里面存放了虚继承的虚基类地址，在程序寻找虚基类的时候，就是从本表中获取偏移地址，然后找到虚基类的。内存中只有一个虚基类，无论有多个派生类，所有派生类到这个基类，都是通过偏移找到虚基类。

如果派生类重载了虚基类的虚函数函数，在虚基类的虚函数对应的表现中，实际存放的是一个thunk地址（下图中的绿色部分）。例如本地中的重载的 middle1::b()，在地址0x401670存放的就是virtual thunk for middle1::b()。这个thunk仅调整this 指针并跳到middle1::b(), 所以当调用top::b()时，实际上就执行了middle1::b()。

在middle1的虚表结束的时候，放入了一个数值，这个数据与它的虚基类的offset(-2)存放的数字是一样的，都是表示虚基类到类对象内存的其实地址的偏移。而虚基类的虚表结束的地方，则存放的是0。

在虚表结束后，紧跟的是一张VTT表。VTT(Virtual Table Table)是一张记录虚表的表，图中黄色部分色部分标注出来的。它分布存放了middle1类所有的虚表起始地址。VTT表的地址也可以在memory_middle1的符号列表中找到(图中的黄色部分）

使用下面的这个命令可以参看符号列表

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nm -gC memory_middle1

部分输出结果的截图

根据上面的程序分析可以画出middle1的内存结构图如下:

下面是一个菱形结构继承的例子代码，有兴趣的读者可以下载以后，按照上面的方面分析。

菱形虚继承 (memory_bottom.cc) download
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// memory layout for diamond virtual inheritance

#include <iostream>
#include <stdio.h>

using namespace std;

#define COUT_FUNC(a, b) (cout << a << " | " << b << " | ") 
#define COUT_CNUM(a, b) (cout << a << " | " << b) 
#define COUT_ARG3(a, b, c) (COUT_FUNC(a, b) << c)
#define COMMENT(a) printf("\e[32m%s\e[0m\n", (a));

class top{
    public:
        top(long i):_i(i){}
        top(){}
        virtual void a() {cout << "this is top::a()" << endl;}
        virtual void b() {cout << "this is top::b()" << endl;}
        long _i;
};

class middle1: virtual public top{
    public:
        middle1(long j):top(100),_j(j),_l(151){}
        virtual void c() {cout << "this is middle1::c()" << endl;}
        virtual void b() {cout << "this is middle1::b()" << endl;}
        long _j;
        long _l;
};

class middle2: virtual public top{
    public:
        middle2(long k, long m):_k(k),_m(m){}
        virtual void d() {cout << "this is middle2::d()" << endl;}
        long _k;
        long _m;
};

class bottom: virtual public middle1, virtual public middle2{
    public:
        bottom():top(5936l), middle1(5937l), middle2(5938l, 5939l), _n(10000){}
        virtual void b() {cout << "this is bottom::b()" << endl;}
        virtual void e() {cout << "this is bottom::e()" << endl;}
        long _n;
};

typedef void (*func)(void);

int main(void) {
    long* pobj;
    func pfunc;

    cout << "--- member of bottom---" << endl;
    bottom bo;
    pobj = (long*) &bo;

    cout << "bo._n: " << bo._n << endl;
    cout << "   _j: " << bo._j << endl;
    cout << "   _l: " << bo._l << endl;
    cout << "   _i: " << bo._i << endl;
    cout << "   _k: " << bo._k << endl;
    cout << "   _m: " << bo._m << endl;

    cout << endl << endl << "--- memory of bottom---" << endl;

    pfunc = (func)((long*)*((long*)(pobj[0])));
    COUT_FUNC(" bottom[0]:    vtab(0)", (long*)(pobj[0])); pfunc();
    COUT_CNUM(" bottom[1]:         _n", (long)pobj[1]) << endl;

    pfunc = (func)((long*)*((long*)(pobj[2])));
    COUT_FUNC("middle1[0]:    vtab(0)", (long*)(pobj[2])); pfunc();
    COUT_CNUM("middle1[1]:         _j", (long)pobj[3]) << endl;
    COUT_CNUM("middle1[2]:         _l", (long)pobj[4]) << endl;

    pfunc = (func)((long*)*((long*)(pobj[5])));
    COUT_FUNC("    top[0]:    vtab(0)", (long*)(pobj[5])); pfunc();
    COUT_CNUM("    top[1]:         _i", (long)pobj[6]) << endl;

    pfunc = (func)((long*)*((long*)(pobj[7])));
    COUT_FUNC("middle2[0]:    vtab(0)", (long*)(pobj[7])); pfunc();
    COUT_CNUM("middle2[1]:         _k", (long)pobj[8]) << endl;
    COUT_CNUM("middle2[2]:         _m", (long)pobj[9]) << endl;

    cout << endl << endl << "--- vtable of bottom---" << endl;

    // class bottom
    COUT_ARG3(" bottom[0]: offset(-3)", (long*)pobj[0] - 3, (long)*((long*)pobj[0] - 3)); COMMENT("\t\t\t# offset to vbase");
    COUT_ARG3("            offset(-2)", (long*)pobj[0] - 2, (long)*((long*)pobj[0] - 2)); COMMENT("\t\t\t# offset to begin");
    COUT_ARG3("            offset(-1)", (long*)pobj[0] - 1, (long*)*((long*)pobj[0] - 1)); COMMENT("\t\t# typeinfo for bottom");
    pfunc = (func)((long*)*((long*)(pobj[0])));
    COUT_FUNC("               vtab(0)", (long*)(pobj[0])); pfunc();
    pfunc = (func)((long*)*((long*)(pobj[0]) + 1));
    COUT_FUNC("               vtab(1)", (long*)pobj[0] + 1); pfunc();
    COUT_ARG3("               vtab(2)", (long*)pobj[0] + 2, *((long*)(pobj[0])+2)); COMMENT("\t\t\t# next class offset to begin");

    // class middle1
    COUT_ARG3("middle1[0]: offset(-3)", (long*)pobj[2] - 3, (long)*((long*)pobj[2] - 3)); COMMENT("\t\t\t# offset to vbase");
    COUT_ARG3("            offset(-2)", (long*)pobj[2] - 2, (long)*((long*)pobj[2] - 2)); COMMENT("\t\t\t# offset to begin");
    COUT_ARG3("            offset(-1)", (long*)pobj[2] - 1, (long*)*((long*)pobj[2] - 1)); COMMENT("\t\t# typeinfo for bottom");

    pfunc = (func)((long*)*((long*)(pobj[2])));
    COUT_FUNC("               vtab(0)", (long*)pobj[2]); pfunc();
    COUT_ARG3("               vtab(1)", (long*)(pobj[2]) + 1, (long*)*((long*)(pobj[2])+1)); COMMENT("\t\t# virtual thunk to bottom::b()");
    COUT_ARG3("               vtab(2)", (long*)pobj[2] + 2, *((long*)(pobj[2])+2)); COMMENT("\t\t\t# next class offset to begin");

    // class top 
    COUT_ARG3("    top[0]: offset(-3)", (long*)pobj[5] - 3, (long)*((long*)pobj[5] - 3)); COMMENT("\t\t\t# offset to vbase");
    COUT_ARG3("            offset(-2)", (long*)pobj[5] - 2, (long)*((long*)pobj[5] - 2)); COMMENT("\t\t\t# offset to begin");
    COUT_ARG3("            offset(-1)", (long*)pobj[5] - 1, (long*)*((long*)pobj[5] - 1)); COMMENT("\t\t# typeinfo for middle1");

    pfunc = (func)((long*)*((long*)(pobj[5])));
    COUT_FUNC("               vtab(0)", (long*)pobj[5]); pfunc();
    COUT_ARG3("               vtab(1)", (long*)(pobj[5]) + 1, (long*)*((long*)(pobj[5])+1)); COMMENT("\t\t# virtual thunk to bottom::b()");
    COUT_ARG3("               vtab(2)", (long*)(pobj[5]) + 2, *((long*)(pobj[5])+2)); COMMENT("\t\t\t# end of vtable");

    // class middle2
    COUT_ARG3("middle2[0]: offset(-3)", (long*)pobj[7] - 3, (long)*((long*)pobj[7] - 3)); COMMENT("\t\t\t# offset to vbase");
    COUT_ARG3("            offset(-2)", (long*)pobj[7] - 2, (long)*((long*)pobj[7] - 2)); COMMENT("\t\t\t# offset to begin");
    COUT_ARG3("            offset(-1)", (long*)pobj[7] - 1, (long*)*((long*)pobj[7] - 1)); COMMENT("\t\t# typeinfo for bottom");

    pfunc = (func)((long*)*((long*)(pobj[7])));
    COUT_FUNC("               vtab(0)", (long*)pobj[7]); pfunc();
    COUT_ARG3("               vtab(1)", (long*)pobj[7] + 1, *((long*)(pobj[7])+1)); COMMENT("\t\t\t# end of vtable");

    // vtt
    COUT_ARG3("      VTT:      vtt(0)", (long*)(pobj[7]) + 2, (long*)*((long*)(pobj[7])+2)); COMMENT("\t\t\t# address align");
    COUT_ARG3("                vtt(1)", (long*)(pobj[7]) + 3, (long*)*((long*)(pobj[7])+3)); COMMENT("\t\t\t# address align");
    COUT_ARG3("                vtt(2)", (long*)(pobj[7]) + 4, (long*)*((long*)(pobj[7])+4)); COMMENT("\t\t# for bottom");
    COUT_ARG3("                vtt(3)", (long*)(pobj[7]) + 5, (long*)*((long*)(pobj[7])+5)); COMMENT("\t\t# for middle1");
    COUT_ARG3("                vtt(4)", (long*)(pobj[7]) + 6, (long*)*((long*)(pobj[7])+6)); COMMENT("\t\t# for top");
    COUT_ARG3("                vtt(5)", (long*)(pobj[7]) + 7, (long*)*((long*)(pobj[7])+7)); COMMENT("\t\t# for middle2");
}

这里仅仅画出内存的结构图如下

在菱形虚继承的关系下，有下面几点需要注意：

  • 在bottom类的内存中，middle2类是放在了top类后面，相对应的，在虚表和VTT表中，middle2都被放在了top类的后面。
  • 计算到虚基类的内存偏移时，计算的是当前类和虚父类的偏移，例如图中，bottom的偏移是bottom到middle1的偏移，不是到top类的偏移。而且只记录了到middle1的偏移，没有到middle2的偏移，原因应该是在声明继承关系时，middle1在middle2之前。
  • 计算到内存开始的偏移时，所有都是按照bottom的起始地址计算。所以两个偏移量不是对应的。

什么时候使用虚继承

这是否意味着，应该尽可能地以虚拟方式派生我们的基类，以便层次结构中后续的派生类可能会需要虚拟继承，是这样吗？不！我们强烈反对，那样做对性能的影响会很严重（而且增加了后续类派生的复杂性）。

那么，我们从不应该使用虚拟继承吗？不是，在实践中几乎所有成功使用虚拟继承的例子中，凡是需要虚拟继承的整个层次结构子树，如iostream 库或Panda 子树，都是由同一个人或项目设计组一次设计完成的。

一般地，除非虚拟继承为一个眼前的设计问题提供了解决方案，否则建议不要使用它。

参考

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