一种直接访问C++对象的protected成员的通用方法

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Motivation：

通过一个C++对象的指针直接访问该对象的protected成员变量，无需修改该类的实现，这是我们使用第三方C++类库时常见的需求。

My Approch：

#define PROTECTED_CAST_DECL (CLASS_TYPE ,MEMBER_TYPE ,MEMBER_NAME) /

template <typenameClassType , typenameMemberType > /

class C ##MEMBER_NAME ##Accessor /

{ /

class CAccessor: public ClassType /

{ /

friend class C ##MEMBER_NAME##Accessor; /

}; /

public: /

static MemberType& GetMember (ClassType* pClass) /

{ /

return ((CAccessor*)pClass )->MEMBER_NAME; /

} /

};

#define PROTECTED_CAST (CLASS_TYPE ,CLASS_OBJECT_PTR ,MEMBER_TYPE,MEMBER_NAME ) (C##MEMBER_NAME ##Accessor<CLASS_TYPE , MEMBER_TYPE>::GetMember (CLASS_OBJECT_PTR))

Sample：

以下示例代码展示了PROTECTED_CAST的使用方法，代码在VC++ 2008下测试通过。

class CPrivateMemberWrapper

{

protected :

int m_iValue ;

double m_dValue ;

string m_sValue;

public :

CPrivateMemberWrapper (inti , double d , const char * lps ):

m_iValue (i), m_dValue (d), m_sValue (lps)

{}

~CPrivateMemberWrapper () {}

};

PROTECTED_CAST_DECL (CPrivateMemberWrapper, int , m_iValue)

PROTECTED_CAST_DECL (CPrivateMemberWrapper, double , m_dValue)

PROTECTED_CAST_DECL (CPrivateMemberWrapper, string , m_sValue)

int _tmain (int argc, _TCHAR * argv[])

{

CPrivateMemberWrapper Wrapper(1979, 3.14, "CPrivateMemberWrapper");

cout << PROTECTED_CAST(CPrivateMemberWrapper , &Wrapper , int , m_iValue ) << endl;

cout << PROTECTED_CAST(CPrivateMemberWrapper , &Wrapper , double ,m_dValue ) << endl;

cout << PROTECTED_CAST(CPrivateMemberWrapper , &Wrapper , string, m_sValue ) << endl ;

return 0;

}