工具：VS2013

先说一下VS环境下查看类内存分布的方法：

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。

编译后，输出如图的内存布局：

测试代码

#include <iostream>

using namespace std;

typedef void(*FUN)();

class A{

public:

	int a;

	virtual void print(){}

	virtual void print_a(){

		cout << "print_a()" << endl;

	}

}; 

class B : virtual public A{

public:

	int b;

	void print(){}

	virtual void print_b(){

		cout << "print_b()" << endl;

	}

};

class C : virtual public A{

public:

	int c;

	void print(){}

	virtual void print_c(){

		cout << "print_c()" << endl;

	}

};

class D : public B, public C{

public:

	int d;

	void print(){}

	virtual void print_d(){

		cout << "print_d()" << endl;

	}

};

int main()

{

	D derived;

	FUN *pFun = (FUN*)((*(int*)(A*)(&derived) + 4));

	(*pFun)();

	system("Pause");

	return 0;

}

对象内存分布

1>  class A	size(8):

1>  	+---

1>   0	| {vfptr}

1>   4	| a

1>  	+---

1>

1>  A::$vftable@:

1>  	| &A_meta

1>  	|  0

1>   0	| &A::print

1>   1	| &A::print_a

1>

1>  A::print this adjustor: 0

1>  A::print_a this adjustor: 0

1>

1>

1>  class ?$is_error_code_enum@PBD	size(1):

1>  	+---

1>  	| +--- (base class ?$integral_constant@_N$0A@)

1>  	| +---

1>  	+---

1>

1>

1>

1>  class B	size(20):

1>  	+---

1>   0	| {vfptr}

1>   4	| {vbptr}

1>   8	| b

1>  	+---

1>  	+--- (virtual base A)

1>  12	| {vfptr}

1>  16	| a

1>  	+---

1>

1>  B::$vftable@B@:

1>  	| &B_meta

1>  	|  0

1>   0	| &B::print_b

1>

1>  B::$vbtable@:

1>   0	| -4

1>   1	| 8 (Bd(B+4)A)

1>

1>  B::$vftable@A@:

1>  	| -12

1>   0	| &B::print

1>   1	| &A::print_a

1>

1>  B::print this adjustor: 12

1>  B::print_b this adjustor: 0

1>

1>  vbi:	   class  offset o.vbptr  o.vbte fVtorDisp

1>                 A      12       4       4 0

1>

1>

1>  class C	size(20):

1>  	+---

1>   0	| {vfptr}

1>   4	| {vbptr}

1>   8	| c

1>  	+---

1>  	+--- (virtual base A)

1>  12	| {vfptr}

1>  16	| a

1>  	+---

1>

1>  C::$vftable@C@:

1>  	| &C_meta

1>  	|  0

1>   0	| &C::print_c

1>

1>  C::$vbtable@:

1>   0	| -4

1>   1	| 8 (Cd(C+4)A)

1>

1>  C::$vftable@A@:

1>  	| -12

1>   0	| &C::print

1>   1	| &A::print_a

1>

1>  C::print this adjustor: 12

1>  C::print_c this adjustor: 0

1>

1>  vbi:	   class  offset o.vbptr  o.vbte fVtorDisp

1>                 A      12       4       4 0

1>

1>

1>  class D	size(36):

1>  	+---

1>  	| +--- (base class B)

1>   0	| | {vfptr}

1>   4	| | {vbptr}

1>   8	| | b

1>  	| +---

1>  	| +--- (base class C)

1>  12	| | {vfptr}

1>  16	| | {vbptr}

1>  20	| | c

1>  	| +---

1>  24	| d

1>  	+---

1>  	+--- (virtual base A)

1>  28	| {vfptr}

1>  32	| a

1>  	+---

1>

1>  D::$vftable@B@:

1>  	| &D_meta

1>  	|  0

1>   0	| &B::print_b

1>   1	| &D::print_d

1>

1>  D::$vftable@C@:

1>  	| -12

1>   0	| &C::print_c

1>

1>  D::$vbtable@B@:

1>   0	| -4

1>   1	| 24 (Dd(B+4)A)

1>

1>  D::$vbtable@C@:

1>   0	| -4

1>   1	| 12 (Dd(C+4)A)

1>

1>  D::$vftable@A@:

1>  	| -28

1>   0	| &D::print

1>   1	| &A::print_a

1>

1>  D::print this adjustor: 28

1>  D::print_d this adjustor: 0

1>

1>  vbi:	   class  offset o.vbptr  o.vbte fVtorDisp

1>                 A      28       4       4 0

运行结果