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