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C++ 对象的内存布局—— 虚继承下的虚函数

2024-07-18 09:08:19   227人阅读

C++ 对象的内存布局(下)这篇文章的“单一虚拟继承”和“钻石型虚拟继承”时的类内存布局讲得不太清楚，我有一处疑问，我用的是VS2005，因此记录一下。

 

类继承图如下：


这里：类B被类B1和B2虚拟继承，而B1和B2同时被D继承。
B1的f()、B2的f()覆盖了B的f()；
D的f()覆盖了B1的f(),D的f1()覆盖了B1的f1()
D的f()覆盖了B2的f(),D的f2()覆盖了B2的f2()

类代码如下：

class B
{
    public:
        int ib;
        char cb;
    public:
        B():ib(0),cb('B') {}
 
        virtual void f() { cout << "B::f()" << endl;}
        virtual void Bf() { cout << "B::Bf()" << endl;}
};
class B1 :  virtual public B//虚拟继承，钻石型继承
{
    public:
        int ib1;
        char cb1;
    public:
        B1():ib1(11),cb1('1') {}
 
        virtual void f() { cout << "B1::f()" << endl;}
        virtual void f1() { cout << "B1::f1()" << endl;}
        virtual void Bf1() { cout << "B1::Bf1()" << endl;}
 
};
class B2:  virtual public B
{
    public:
        int ib2;
        char cb2;
    public:
        B2():ib2(12),cb2('2') {}
 
        virtual void f() { cout << "B2::f()" << endl;}
        virtual void f2() { cout << "B2::f2()" << endl;}
        virtual void Bf2() { cout << "B2::Bf2()" << endl;}
       
};
 
class D : public B1, public B2
{
    public:
        int id;
        char cd;
    public:
        D():id(100),cd('D') {}
 
		virtual void f() { cout << "D::f()" << endl;}
		virtual void f1() { cout << "D::f1()" << endl;}
        virtual void f2() { cout << "D::f2()" << endl;}
        virtual void Df() { cout << "D::Df()" << endl;}
};

（1）单一虚拟继承：B1虚拟继承自B

查看类B1的内存布局的代码如下：

{
	int** pVtab = NULL;
	Fun pFun = NULL;

	B1 bb1;

	pVtab = (int**)&bb1;

	cout << "sizeof(B1) = " << sizeof(B1) << endl;

	cout << "[0] B1::_vfptr->" << endl;
	for (int i = 0; ; i++)
	{
		pFun = (Fun)pVtab[0][i];
		cout << '\t' << "[" << i << "] 0x" << (int*)pFun << "\t";
		if (pFun == NULL)
		{
			cout << endl;
			break;
		}
		pFun();
	}

	cout << "[1] B1::_vbptr->" << endl;
	cout << "\t" << "[0] B1dB1bptrB1 " << pVtab[1][0] << endl;
	cout << "\t" << "[1] B1dB1bptrB " << pVtab[1][1] << endl;

	cout << "[2] B1::ib1 = " << (int)pVtab[2] << endl;
	cout << "[3] B1::cb1 = " << (char)pVtab[3] << endl;

	cout << "[4] = 0x" << (int*)pVtab[4]/*[0]*/ << endl;

	cout << "[5] B::_vfptr->" << endl;	//
	cout << '\t' << "[" << 0 << "] thunk = 0x" << (int*)pVtab[5][0] << endl;
	for (int i = 1; ; i++)
	{
		pFun = (Fun)pVtab[5][i];
		cout << '\t' << "[" << i << "] 0x" << (int*)pFun << "\t";
		if (pFun == NULL)
		{
			cout << endl;
			break;
		}
		pFun();
	}
	
	cout << "[6] B::ib = " << (int)pVtab[6] << endl;
	cout << "[7] B::ic = " << (char)pVtab[7] << endl;
}

代码输出了类B1的布局：

sizeof(B1) = 32
[0] B1::_vfptr->
        [0] 0x0041106E  B1::f1()
        [1] 0x004110AF  B1::Bf1()
        [2] 0x00000000
[1] B1::_vbptr->
        [0] B1dB1bptrB1 -4
        [1] B1dB1bptrB 16
[2] B1::ib1 = 11
[3] B1::ic1 = 1
[4] = 0x00000000     //VC++中的一个NULL分隔符把B和B1的布局分开
[5] B::_vfptr->
        [0] 0x00411055  //奇怪的地方：这个不是函数B1::f()的地址，因为执行这个地址的函数会出错，那么整个B1类的B1::f()虚函数在哪里呢？难道是“调整块”的地址？
        [1] 0x004111F4  B::Bf()
        [2] 0x00000000
[6] B::ib = 0
[7] B::ic = B


（2）钻石型虚拟继承

查看类D的内存布局的代码如下：

{
	int** pVtab = NULL;
	Fun pFun = NULL;

	D d;

	pVtab = (int**)&d;

	cout << "sizeof(D) = " << sizeof(D) << endl;
	
	cout << "[0] D::B1::_vfptr->" << endl;
	for (int i = 0; ; i++)
	{
		pFun = (Fun)pVtab[0][i];
		cout << '\t' << "[" << i << "] 0x" << (int*)pFun << "\t";
		if (pFun == NULL)
		{
			cout << endl;
			break;
		}
		pFun();
	}

	cout << "[1] D::B1::_vbptr->" << endl;
	cout << "\t" << "[0] DdB1bptrB1 " << pVtab[1][0] << endl;
	cout << "\t" << "[1] DdB1bptrB " << pVtab[1][1] << endl;

	cout << "[2] B1::ib1 = " << (int)pVtab[2] << endl;
	cout << "[3] B1::cb1 = " << (char)pVtab[3] << endl;

	cout << "[4] D::B2::_vfptr->" << endl;
	for (int i = 0; ; i++)
	{
		pFun = (Fun)pVtab[4][i];
		cout << '\t' << "[" << i << "] 0x" << (int*)pFun << "\t";
		if (pFun == NULL)
		{
			cout << endl;
			break;
		}
		pFun();
	}

	cout << "[5] D::B2::_vbptr->" << endl;
	cout << "\t" << "[0] DdB2bptrB2 " << pVtab[5][0] << endl;
	cout << "\t" << "[1] DdB1bptrB " << pVtab[5][1] << endl;

	cout << "[6] B2::ib2 = " << (int)pVtab[6] << endl;
	cout << "[7] B2::cb2 = " << (char)pVtab[7] << endl;

	cout << "[8] D::id = " << (int)pVtab[8] << endl;
	cout << "[9] D::cd = " << (char)pVtab[9] << endl;

	cout << "[10] = 0x" << (int*)pVtab[10]/*[0]*/ << endl;

	cout << "[11] D::B::_vfptr->" << endl;

	cout << '\t' << "[" << 0 << "] 0x" << (int*)pVtab[11][0] << endl;//奇怪的地方
	for (int i = 1; ; i++)
	{
		pFun = (Fun)pVtab[11][i];
		cout << '\t' << "[" << i << "] 0x" << (int*)pFun << "\t";
		if (pFun == NULL)
		{
			cout << endl;
			break;
		}
		pFun();
	}

	cout << "[12] B::ib = " << (int)pVtab[12] << endl;
	cout << "[13] B::ic = " << (char)pVtab[13] << endl;
}

 代码输出了类D的布局：

sizeof(D) = 56
[0] D::B1::_vfptr->
        [0] 0x00411163  D::f1()
        [1] 0x004110AF  B1::Bf1()
        [2] 0x004110D2  D::Df()
        [3] 0x00000000
[1] D::B1::_vbptr->
        [0] DdB1bptrB1 -4
        [1] DdB1bptrB 40
[2] B1::ib1 = 11
[3] B1::cb1 = 1
[4] D::B2::_vfptr->
        [0] 0x0041101E  D::f2()
        [1] 0x00411159  B2::Bf2()
        [2] 0x00000000
[5] D::B2::_vbptr->
        [0] DdB2bptrB2 -4
        [1] DdB1bptrB 24
[6] B2::ib2 = 12
[7] B2::cb2 = 2
[8] D::id = 100
[9] D::cd = D
[10] = 0x00000000   //VC++中的一个NULL分隔符把B和B1和B2的布局分开
[11] D::B::_vfptr->
        [0] 0x004110C3    //奇怪的地方：这个不是函数D::f()的地址，因为执行这个地址的函数会出错，那么整个D类的D::f()虚函数在哪里呢？难道是“调整块”的地址？
        [1] 0x004111F4  B::Bf()
        [2] 0x00000000
[12] B::ib = 0
[13] B::cb = B


目前我还不清楚是怎么回事，不知道是不是thunk之类的原因。

参考：C++ 对象的内存布局(下)


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