C++ 对象内存布局 (5)
下面来看看虚基类对对象内存布局的影响。虚基类的主要作用就是在所有的派生类中，保留且仅保留一份虚基类的suboject。
a. 一个虚基类的情况
#include <iostream>
using namespace std;
class Base
{
public:
int base_member;
};
class Derived : public virtual Base {};
int main(void)
{
Base b;
Derived d;
cout << sizeof(b) << endl;
cout << sizeof(d) << endl;
return 0;
}
运行结果：
注意使用了虚继承，即class Derived : public virtual Base。这次Derived的对象大小为什么为8 bytes呢？这是因为编译器会给Derived对象安插一个虚基类表指针vbptr，下面给出Derived对象的memory layout：
虚基类表指针vbptr指向Derived类的virtual bass class table(虚基类表)，虚基类表中存放的是Derived类的虚基类表指针到虚基类实例指针的偏移量。
如果Derived有两个虚基类，那么这两个虚基类的实例指针，分别位于虚基类表的第2项和第3项，最后一项为0，意味着虚基类表的结束。虚基类表中的第1项是什么？目前不清楚，但是它的值大部分时候为0(请牛人指教，编译器是VC6)。下面我们来检验之。
在main函数的return前，增加如下语句：
cout << "Derived object d's vbptr = " << (unsigned long*)(&d) << endl;
cout << "Address of virtual base class table = " << (unsigned long*)*(unsigned long*)(&d) << endl;
cout << "Item 1 in virtual base class table = " << *(unsigned long*)*(unsigned long*)(&d) << endl;
cout << "Item 2 in virtual base class table = " << *((unsigned long*)*(unsigned long*)(&d) + 1) << endl;
cout << "Item 3 in virtual base class table = " << *((unsigned long*)*(unsigned long*)(&d) + 2) << endl;
cout << "The address of virtual base class Base = " << (Base*)(&d) << endl;
编译后运行结果：
不难发现，虚基类示例地址 = vbptr + offset，即0x0012FF78 = 0x0012FF74 + 4。图示如下：
b. 我们来看看Derived有两个虚基类的情况：
#include <iostream>
using namespace std;
class Base1
{
public:
int base1_member;
};
class Base2
{
public:
int base2_member;
};
class Derived : public virtual Base1, public virtual Base2 {};
int main(void)
{
Base1 b1;
Base2 b2;
Derived d;
cout << sizeof(b1) << endl;
cout << sizeof(b2) << endl;
cout << sizeof(d) << endl;
cout << "Derived object d's vbptr = " << (unsigned long*)(&d) << endl;
cout << "Address of virtual base class table = " << (unsigned long*)*(unsigned long*)(&d) << endl;
cout << "Item 1 in virtual base class table = " << *((unsigned long*)*(unsigned long*)(&d) + 0) << endl;
cout << "Item 2 in virtual base class table = " << *((unsigned long*)*(unsigned long*)(&d) + 1) << endl;
cout << "Item 3 in virtual base class table = " << *((unsigned long*)*(unsigned long*)(&d) + 2) << endl;
cout << "Item 3 in virtual base class table = " << *((unsigned long*)*(unsigned long*)(&d) + 3) << endl;
cout << "The address of virtual base class: Base1's instance = " << (Base1*)(&d) << endl;
cout << "The address of virtual base class: Base2's instance = " << (Base2*)(&d) << endl;
return 0;
}
编译运行结果如下：
Derived对象的memory layout图解如下：
不管Derived类有多少个虚基类，它只有一个vbptr和一个virtual base class table。
继续探讨虚基类对对象内存布局的影响。几个类的继承关系如下图，这是虚基类最为常见的用法之一：
代码如下：
#include <iostream>
using namespace std;
class Base
{
public:
int base_member;
};
class Derived1 : public virtual Base
{
public:
int derived1_member;
};
class Derived2 : public virtual Base
{
public:
int derived2_member;
};
class ChildDerived : public Derived1, public Derived2
{
public:
int childderived_member;
};
int main(void)
{
ChildDerived cd;
cout << "sizeof(Base) = /t/t" << sizeof(Base) << endl;
cout << "sizeof(Derived1) = /t" << sizeof(Derived1) << endl;
cout << "sizeof(Derived2) = /t" << sizeof(Derived2) << endl;
cout << "sizeof(ChildDerived) = /t" << sizeof(ChildDerived) << endl;
cout << endl;
cout << "Derived1 subobject's vbptr = " << (unsigned long*)&cd << endl;
cout << "Derived2 subobject's vbptr = " << (unsigned long*)&cd + 2 << endl;
cout << endl;
cout << "Address of virtual base class table of Derived1 object = " << (unsigned long*)(*((unsigned long*)&cd)) << endl;
cout << "Item 1 in Derived1's virtual base class table = " << *((unsigned long*)*(unsigned long*)(&cd) + 0) << endl;
cout << "Item 2 in Derived1's virtual base class table = " << *((unsigned long*)*(unsigned long*)(&cd) + 1) << endl;
cout << "Item 3 in Derived1's virtual base class table = " << *((unsigned long*)*(unsigned long*)(&cd) + 2) << endl;
cout << endl;
cout << "Address of virtual base class table of Derived2 object = " << (unsigned long*)*((unsigned long*)&cd + 2) << endl;
cout << "Item 1 in Derived2's virtual base class table = " << *((unsigned long*)*((unsigned long*)&cd + 2) + 0) << endl;
cout << "Item 2 in Derived2's virtual base class table = " << *((unsigned long*)*((unsigned long*)&cd + 2) + 1) << endl;
cout << "Item 3 in Derived2's virtual base class table = " << *((unsigned long*)*((unsigned long*)&cd + 2) + 2) << endl;
cout << endl;
cout << "The address of base class Derived1's instance = " << (Derived1*)&cd << endl;
cout << "The address of base class Derived2's instance = " << (Derived2*)&cd << endl;
cout << "The address of base class Base's instance = /t" << (Base*)&cd << endl;
return 0;
}
运行结果如下：
sizeof(Base) = 4 bytes，这个是很好解释的，因为Base中由一个类型为int的成员变量。
sizeof(Derived1) = 12 bytes和sizeofDerived2) = 12 bytes，图解如下：
两个“?”到底是多少我们且不去管它，等我们讨论ChildDerived对象的memory layout时再进行详细说明。从上图可以看到，Base subobject在最后。
现在我们来看看ChildDerived对象的memory layout的情况：
由于Derived1和Derived2都是从Base虚拟继承而来，因而Base是它们的虚基类，编译器在编译的时候会分别给他们安插vbptr指针；Derived1和Derived2同时被ChildDerived普通继承(非虚继承)，根据C++标准的要求，基类在在派生类中保证其原始的完整性，因此两个vbptr被继承到了ChildDerived类；由于Base被虚继承，可以看到Base suboject只有一份拷贝，而且放在最后。
从下面图解可以很清楚地发现：
sizeof(ChildDerived) = 24 bytes；
虚基类实例地址(0x0012FF7C) = Derived1的vbptr (0x0012FF68) + Derived1的vbptr到虚基类实例地址的偏移量(24)；
虚基类实例地址(0x0012FF7C) = Derived2的vbptr (0x0012FF70) + Derived1的vbptr到虚基类实例地址的偏移量(12)
和对象内存布局 (14)差不多，只是改动了两个继承关系。几个类的继承关系如下图，这种继承方式不是很有实际意义，在此纯粹是探究类的memory layout。
代码如下：
#include <iostream>
using namespace std;
class Base
{
public:
int base_member;
};
class Derived1 : public virtual Base
{
public:
int derived1_member;
};
class Derived2 : public virtual Base
{
public:
int derived2_member;
};
class ChildDerived : public virtual Derived1, public virtual Derived2
{
public:
int childderived_member;
};
int main(void)
{
ChildDerived cd;
cout << "sizeof(Base) = /t/t" << sizeof(Base) << endl;
cout << "sizeof(Derived1) = /t" << sizeof(Derived1) << endl;
cout << "sizeof(Derived2) = /t" << sizeof(Derived2) << endl;
cout << "sizeof(ChildDerived) = /t" << sizeof(ChildDerived) << endl;
cout << endl;
cout << "ChildDerived object's vbptr = " << (int*)&cd << endl;
cout << endl;
cout << "Address of virtual base class table of ChildDerived object = " << (int*)(*((int*)&cd)) << endl;
cout << "Item 1 in ChildDerived's virtual base class table = " << *((int*)*(int*)(&cd) + 0) << endl;
cout << "Item 2 in ChildDerived's virtual base class table = " << *((int*)*(int*)(&cd) + 1) << endl;
cout << "Item 3 in ChildDerived's virtual base class table = " << *((int*)*(int*)(&cd) + 2) << endl;
cout << "Item 4 in ChildDerived's virtual base class table = " << *((int*)*(int*)(&cd) + 3) << endl;
cout << "Item 4 in ChildDerived's virtual base class table = " << *((int*)*(int*)(&cd) + 4) << endl;
cout << endl;
cout << "Address of virtual base class table of Derived1 object = " << (int*)*((int*)&cd + 3) << endl;
cout << "Item 1 in Derived1's virtual base class table = " << *((int*)*((int*)&cd + 3) + 0) << endl;
cout << "Item 2 in Derived1's virtual base class table = " << *((int*)*((int*)&cd + 3) + 1) << endl;
cout << "Item 3 in Derived1's virtual base class table = " << *((int*)*((int*)&cd + 3) + 2) << endl;
cout << endl;
cout << "Address of virtual base class table of Derived2 object = " << (int*)*((int*)&cd + 5) << endl;
cout << "Item 1 in Derived2's virtual base class table = " << *((int*)*((int*)&cd + 5) + 0) << endl;
cout << "Item 2 in Derived2's virtual base class table = " << *((int*)*((int*)&cd + 5) + 1) << endl;
cout << "Item 3 in Derived2's virtual base class table = " << *((int*)*((int*)&cd + 5) + 2) << endl;
cout << endl;
cout << "The address of base class Derived1's instance = " << (Derived1*)&cd << endl;
cout << "The address of base class Derived2's instance = " << (Derived2*)&cd << endl;
cout << "The address of base class Base's instance = /t" << (Base*)&cd << endl;
return 0;
}
注意：因为可能出现负数，所以在上面的程序中，将原来的unsigned long* 改为了int*。运行结果：
图解如下：
下面讨论虚基类和虚函数同时存在的时候，对对象内存布局的影响。
假定各个类之间的关系如下图：
Base中声明了一个虚函数vfBase()和一个整形成员变量；
Derived1 override了Base中声明的虚函数vfBase()，声明了一个虚函数vfDerived1()，另有一个整形成员变量derived1_member；
Derived2 override了Base中声明的虚函数vfBase()，声明了一个虚函数vfDerived2()，另有一个整形成员变量derived2_member；
ChildDerived分别override了Base、Derived1和Derived2中声明的虚函数vfBase()、vfDerived1() 和vfDerived2()，另外有一个整形成员变量childderived_member；
代码如下：
#include <iostream>
using namespace std;
typedef void (*VFun)(void);
template<typename T>
VFun virtualFunctionPointer(T* b, int i)
{
return (VFun)(*((int*)(*(int*)b) + i));
}
template<typename T>
int virtualBaseTableOffset(T* b, int i)
{
return (int)*((int*)*(int*)b + i);
}
class Base
{
public:
int base_member;
inline virtual void vfBase()
{
cout << "This is in Base::vfBase()" << endl;
}
};
class Derived1 : public virtual Base
{
public:
int derived1_member;
inline void vfBase()
{
cout << "This is in Derived1::vfBase()" << endl;
}
inline virtual void vfDerived1()
{
cout << "This is in Derived1::vfDerived1()" << endl;
}
};
class Derived2 : public virtual Base
{
public:
int derived2_member;
inline void vfBase()
{
cout << "This is in Derived2::vfBase()" << endl;
}
inline virtual void vfDerived2()
{
cout << "This is in Derived2::vfDerived2()" << endl;
}
};
class ChildDerived : public Derived1, public Derived2
{
public:
int childderived_member;
inline void vfBase()
{
cout << "This is in ChildDerived::vfBase()" << endl;
}
inline void vfDerived1()
{
cout << "This is in ChildDerived::vfDerived1()" << endl;
}
inline void vfDerived2()
{
cout << "This is in ChildDerived::vfDerived2()" << endl;
}
};
int main(void)
{
ChildDerived cd;
VFun pVF;
int* tmp;
cout << "sizeof(Base) = /t/t" << sizeof(Base) << endl;
cout << "sizeof(Derived1) = /t" << sizeof(Derived1) << endl;
cout << "sizeof(Derived2) = /t" << sizeof(Derived2) << endl;
cout << "sizeof(ChildDerived) = /t" << sizeof(ChildDerived) << endl;
cout << endl;
cout << "address of ChildDerived object:" << endl;
cout << "address = " << (int*)&cd << endl;
cout << endl;
cout << "1st virtual function table: " << endl;
pVF = virtualFunctionPointer(&cd, 0);
pVF();
cout << endl;
cout << "1st virtual base table: " << endl;
tmp = (int*)((int*)&cd + 1);
cout << "address = " << tmp << endl;
cout << virtualBaseTableOffset(tmp, 0) << "/t<- not sure yet, but it doesn't matter here." << endl;
cout << virtualBaseTableOffset(tmp, 1) << "/t<- offset from Derived1 subobject's vbptr to Base subobject." << endl;
cout << virtualBaseTableOffset(tmp, 2) << "/t<- means the end of this virtual base table." << endl;
cout << endl;
tmp = ((int*)&cd) + 3;
cout << "2nd virtual function table: " << endl;
pVF = virtualFunctionPointer(tmp, 0);
pVF();
cout << endl;
cout << "2nd virtual base table: " << endl;
tmp = (int*)((int*)&cd + 4);
cout << "address = " << tmp << endl;
cout << virtualBaseTableOffset(tmp, 0) << "/t<- not sure yet, but it doesn't matter here." << endl;
cout << virtualBaseTableOffset(tmp, 1) << "/t<- offset from Derived2 subobject's vbptr to Base subobject." << endl;
cout << virtualBaseTableOffset(tmp, 2) << "/t<- means the end of this virtual base table." << endl;
cout << endl;
tmp = ((int*)&cd) + 7;
cout << "3rd virtual function table: " << endl;
pVF = virtualFunctionPointer(tmp, 0);
pVF();
cout << endl;
cout << "Derived1 subobject address = /t" << (Derived1*)&cd << endl;
cout << "Derived2 subobject address = /t" << (Derived2*)&cd << endl;
cout << "Base subobject address = /t" << (Base*)&cd << endl;
return 0;
}
运行结果如下：
ChildDerived、Derived1和Derived2对象memory layout分别图解如下：
两个虚基类偏移量图解如下(虚函数表和虚基类表略)：
结论：
其一，只要涉及到虚基类，一切问题就变得复杂起来；
其二，如果同时存在vfptr和vbptr，vfptr居前，vbptr居后；
其三，普通基类居前，虚基类总是尽可能地排列在layout的最后；
其四，两个同一层次的虚基类subobject，先声明者居前，后声明者居后，这点和普通基类是一样的；
其五，两个不同层次的虚基类subobject，层次高者居前，层次低者居后；
其六，Stan Lippman建议，不要在一个virtual base class中声明nonstatic data member，理由是这样做会是问题变得非常复杂。
