记录：C++类内存分布(虚继承与虚函数)

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