C++对象内存分布(3) - 菱形继承(virtual)

1.前言

本篇文章的全部代码样例。假设是windows上编译执行。则使用的是visual studio 2013。假设是RHEL6.5平台(linux kernal: 2.6.32-431.el6.i686)上编译执行，则其gcc版本号为4.4.7，例如以下所看到的：

[root@MiWiFi-R1CM ~]# gcc --version

gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-4)

2.菱形继承类的内存分布

本篇文章主要讨论的是虚继承(virtual)下的内存分布。

2.1.类的结构

2.2.实现代码windows版本号

以下的代码执行在windows 7+visual studio 2013平台上。

分别打印类Base1对象的内存分布。和类Derive对象的内存分布。

#include <iostream>
using namespace std;
class Base
{
public:
int _iBase;
char _cBase;
public:
Base() : _iBase(1111), _cBase('A')
{
}
virtual void func()
{
cout << "Base::func()" << endl;
}
virtual void baseFunc()
{
cout << "Base::baseFunc()" << endl;
}
};

class Base1 : public virtual Base
{
public:
int _iBase1;
char _cBase1;
public:
Base1() : _iBase1(2222), _cBase1('B')
{
}
virtual void func()
{
cout << "Base1::func()" << endl;
}
virtual void func1()
{
cout << "Base1::func1()" << endl;
}
virtual void baseFunc1()
{
cout << "Base1::baseFunc1()" << endl;
}
};

class Base2 : public virtual Base
{
public:
int _iBase2;
char _cBase2;
public:
Base2() : _iBase2(3333), _cBase2('C')
{
}
virtual void func()
{
cout << "Base2::func()" << endl;
}
virtual void func2()
{
cout << "Base2::func2()" << endl;
}
virtual void baseFunc2()
{
cout << "Base2::baseFunc2()" << endl;
}
};

class Derive : public virtual Base1, public virtual Base2
{
public:
int _iDerive;
char _cDerive;
public:
Derive() : _iDerive(4444), _cDerive('D')
{
}
virtual void func()
{
cout << "Derive::func()" << endl;
}
virtual void func1()
{
cout << "Derive::func1()" << endl;
}
virtual void func2()
{
cout << "Derive::func2()" << endl;
}
virtual void deriveFunc()
{
cout << "Derive::deriveFunc()" << endl;
}
};

typedef void(*Fun)();

int main() {
int **pVtab = NULL;
Fun pFun = NULL;

Base1 bb1;

cout << "sizeof(Base)" << sizeof(Base) << endl;
cout << "sizeof(Base1)" << sizeof(Base1) << endl;

//Base1 layout
pVtab = (int**)&bb1;
cout << "[0] Base1::_vptr->" << "  addr:" << &pVtab[0] << endl;
pFun = (Fun)pVtab[0][0];
cout << "    [0]";
pFun(); //Base1::func1()
pFun = (Fun)pVtab[0][1];
cout << "    [1]";
pFun(); //Base1::baseFunc1()
cout << "    [2]";
cout << pVtab[0][2] << endl;

cout << "[1] Base1::_vbptr->" << "  addr:" << &pVtab[1] << endl;
cout << "    [0]";
cout << pVtab[1][0] << endl;
cout << "    [1]offset->";
cout << pVtab[1][1] << endl;
cout << "    [2]";
cout << pVtab[1][2] << endl;

cout << "[2] Base1:_iBase1 -> ";
cout << (int)pVtab[2] << endl; //Base1:_iBase1

cout << "[3] Base1:_cBase1 -> ";
cout << (char)(int)pVtab[3] << endl; //Base1:_cBase1

cout << "[4] unknown ";
cout << (int)pVtab[4] << endl;

cout << "[5] Base::_vfptr->" << "  addr:" << &pVtab[4] << endl;
cout << "    [0]";
pFun = (Fun)pVtab[5][0];
pFun();
cout << "    [1]";
pFun = (Fun)pVtab[5][1];
pFun();
cout << "    [2]";
cout << (int)pVtab[5][2] << endl;

cout << "[6] Base:_iBase -> ";
cout << (int)pVtab[6] << endl; //Base:_iBase

cout << "[7] Base:_cBase -> ";
cout << (char)(int)pVtab[7] << endl; //Base:_cBase

cout << endl;
cout << "###########################" << endl;
cout << endl;

//Derive layout
// sub Derive vtbl layout
Derive d;
cout << "sizeof(Derive)" << sizeof(Derive) << endl;

pVtab = (int**)&d;
cout << "[0] Derive::_vfptr->" << "  addr:" << &pVtab[0] << endl;
cout << "     [0] ";
pFun = (Fun)pVtab[0][0];
pFun(); //Derive::func1();
cout << "     [1] ";
cout << pVtab[0][1] << endl;

cout << "[1] Derive::_vbptr->" << "  addr:" << &pVtab[1] << endl;
cout << "     [0] ";
cout << pVtab[1][0] << endl;
cout << "     [1]offset-> ";
cout << pVtab[1][1] << endl;
cout << "     [2] ";
cout << pVtab[1][2] << endl;
cout << "     [3] ";
cout << pVtab[1][3] << endl;
cout << "     [4] ";
cout << pVtab[1][4] << endl;

cout << "[2] Derive::_iDerive -> ";
cout << (int)pVtab[2] << endl; //Derive:_iDerive

cout << "[3] Derive::_cDerive -> ";
cout << (char)(int)pVtab[3] << endl; //Derive::_cDerive

cout << "[4] unknown-> " << (int)pVtab[4] << endl;

// sub Base vtbl layout
cout << "[5] Base::_vfptr->" << "  addr:" << &pVtab[5] << endl;
cout << "     [0] ";
pFun = (Fun)pVtab[5][0];
pFun();
pFun = (Fun)pVtab[5][1];
cout << "     [1] ";
pFun();
cout << "     [2] ";
cout << pVtab[5][2] << endl;

cout << "[6] Base::_iBase -> ";
cout << (int)pVtab[6] << endl;

cout << "[7] Base::_cBase -> ";
cout << (char)(int)pVtab[7] << endl;

cout << "[8] unknown-> ";
cout << (int)pVtab[8] << endl;

// sub Base1 vtbl layout
cout << "[9] Base1::_vfptr->" << "  addr:" << &pVtab[9] << endl;
cout << "     [0] ";
cout << pVtab[9][0] << endl;
pFun = (Fun)pVtab[9][1];
cout << "     [1] ";
pFun();
pFun = (Fun)pVtab[9][2];
cout << "     [2] ";
cout << (int)pFun << endl;
pFun = (Fun)pVtab[9][3];
cout << "     [3] ";
cout << (int)pFun << endl;

cout << "[10] unknown-> ";
cout << (int)pVtab[10] << endl;

cout << "[11] Base1::_iBase1 -> ";
cout << (int)pVtab[11] << endl;

cout << "[12] Base1::_cBase1 -> ";
cout << (char)(int)pVtab[12] << endl;

cout << "[13] unknown-> ";
cout << (int)pVtab[13] << endl;

// sub Base2 vtbl layout
cout << "[14] Base2::_vfptr->" << "  addr:" << &pVtab[14] << endl;
cout << "     [0] ";
cout << pVtab[14][0] << endl;
pFun = (Fun)pVtab[14][1];
cout << "     [1] ";
pFun();
cout << "     [2] ";
cout << pVtab[14][2] << endl;
cout << "     [3] ";
cout << pVtab[14][3] << endl;

cout << "[15] unknown-> ";
cout << (int)pVtab[15] << endl;

cout << "[16] Base2::_iBase2 -> ";
cout << (int)pVtab[16] << endl;

cout << "[17] Base2::_cBase2 -> ";
cout << (char)(int)pVtab[17] << endl;

return 0;
}

Base1对象的内存分布为：



Derive对象的内存分布为：

2.3.实现代码linux版本号

以下代码中，对于这4个类的定义，与前面的windows代码中对类的定义是全然一样的，所以这里省略掉了。仅仅列出用于输出内存分布的代码：

int main()
{
int **pVtab = NULL;
Fun pFun = NULL;

Base1 bb1;

cout << "sizeof(Base)" << sizeof(Base) << endl;
cout << "sizeof(Base1)" << sizeof(Base1) << endl;

//Base1 layout
pVtab = (int**)&bb1;
cout << "[0] Base1::_vptr->" << "  addr:" << &pVtab[0] << endl;
pFun = (Fun)pVtab[0][0];
cout << "    [0]";
pFun(); //Base1::func1()
pFun = (Fun)pVtab[0][1];
cout << "    [1]";
pFun(); //Base1::baseFunc1()
pFun = (Fun)pVtab[0][2];
cout << "    [2]";
pFun();
cout << "    [3]";
cout << pVtab[0][3];

cout << "[1] Base1:_iBase1 -> ";
cout << (int)pVtab[1] << endl;  //Base1:_iBase1

cout << "[2] Base1:_cBase1 -> ";
cout << (char)(int)pVtab[2] << endl;  //Base1:_cBase1

// Base layout
cout << "[3] Base::_vptr->" << "  addr:" << &pVtab[3] << endl;
pFun = (Fun)pVtab[3][0];
cout << "    [0]";
pFun(); //Base1::func()
pFun = (Fun)pVtab[3][1];
cout << "    [1]";
pFun(); //Base::baseFunc()
cout << "    [2]";
cout << pVtab[3][2] << endl;

cout << "[4] Base:_iBase -> ";
cout << (int)pVtab[4] << endl; //Base:_iBase
cout << "[5] Base:_cBase -> ";
cout << (char)(int)pVtab[5] << endl; //Base:_cBase

cout << "[6] " << (int)pVtab[6] << "  addr:" << &pVtab[6] << endl; //dirty data

cout << endl;
cout << "###########################" << endl;
cout << endl;

// Derive layout
// sub Derive vtbl layout
Derive d;
cout << "sizeof(Derive)" << sizeof(Derive) << endl;

pVtab = (int**)&d;
cout << "[0] Derive::_vfptr->" << "  addr:" << &pVtab[0] << endl;
cout << "     [0] ";
pFun(); //Derive::func1();
pFun = (Fun)pVtab[0][1];
cout << "     [1] ";
pFun(); //Base1::baseFunc1();
pFun = (Fun)pVtab[0][2];
cout << "     [2] ";
pFun(); //Derive::deriveFunc();
pFun = (Fun)pVtab[0][3];
cout << "     [3] ";
pFun();
cout << "     [4] ";
cout << pVtab[0][4] << endl;

cout << "[1] Derive:_iDerive -> ";
cout << (int)pVtab[1] << endl; //Derive:_iDerive

cout << "[2] Derive:_cDerive -> ";
cout << (char)(int)pVtab[2] << endl; //Derive:_cDerive

// sub Base1 vtbl layout
cout << "[3] Base1::_vptr->" << "  addr:" << &pVtab[3] << endl;
pFun = (Fun)pVtab[3][0];
cout << "     [0] ";
pFun();
pFun = (Fun)pVtab[3][1];
cout << "     [1] ";
pFun();
pFun = (Fun)pVtab[3][2];
cout << "     [2] ";
pFun();
cout << "     [3] ";
cout << pVtab[3][3] << endl;

cout << "[4] Base1::_iBase1 -> ";
cout << (int)pVtab[4] << endl;

cout << "[5] Base1::_cBase1 -> ";
cout << (char)(int)pVtab[5] << endl;

// sub Base vtbl layout
cout << "[6] Base::_vptr->" << "  addr:" << &pVtab[6] << endl;
pFun = (Fun)pVtab[6][0];
cout << "     [0] ";
pFun();
pFun = (Fun)pVtab[6][1];
cout << "     [1] ";
pFun();
pFun = (Fun)pVtab[6][2];
cout << "     [2] ";
cout << (int)pFun << endl;

cout << "[7] Base::_iBase -> ";
cout << (int)pVtab[7] << endl;

cout << "[8] Base::_cBase -> ";
cout << (char)(int)pVtab[8] << endl;

// sub Base2 vtbl layout
cout << "[9] Base2::_vptr->" << "  addr:" << &pVtab[9] << endl;
pFun = (Fun)pVtab[9][0];
cout << "     [0] ";
pFun();
pFun = (Fun)pVtab[9][1];
cout << "     [1] ";
pFun();
pFun = (Fun)pVtab[9][2];
cout << "     [2] ";
pFun();
cout << "     [3] ";
cout << pVtab[9][3] << endl;

cout << "[10] Base2::_iBase2 -> ";
cout << (int)pVtab[10] << endl;

cout << "[11] Base2::_cBase2 -> ";
cout << (char)(int)pVtab[11] << endl;

return 0;
}

Base1对象的内存分布为：



Derive对象的内存分布为：

2.4.Base1类的内存分布对照

Windows 7 + Visual studio 2013 Linux RHEL6.5 + gcc 4.4.7

2.5.Derive类的内存分布对照

Windows 7 + Visual studio 2013 Linux RHEL6.5 + gcc 4.4.7

2.6.Derive类在虚继承与非虚继承中的内存对照

在前面的章节(2.5)中，已经列出了Derive在虚继承(virtual)的情况下，在windows以及linux平台下的具体内存分布。

在前一篇文章中，我们获得了非虚继承(non virtual)的情况下，Derive类的内存分布。

这里将它与本篇文章中获得的虚继承的分布。进行一个对照。

下图是windows平台下。非虚继承与虚继承对类的内存分布的影响对照。

非虚继承(non virtual) 虚继承(virtual)



下图是linux平台下。非虚继承与虚继承对类的内存分布的影响对照。

非虚继承(non virtual) 虚继承(virtual)