基类和派生类构造函数在C++中调用

#define DECLARE_NEWMSG(ClassName,ClassID)                
static ClassName* FromMsg(ClassA* psg)               
{                                                       
   return dynamic_cast<ClassName*>(psg);              
}                                                       
static ClassA* newMsg()                            
{                                                       
   return new ClassName;                               
}                                                       
enum {ID = ClassID};
GET_EVENT = 0x41,
typedef ClassA* (*pNewMsg)(void); //function pointer
typedef struct
{
    int         Id;       
    CString     Ascii;       
    pNewMsg    MessageFunc;    //calls new for the particular message
} stStruct;
ClassA
{
//stmts;
};
ClassA::ClassA(int Id,pNewMsg newMsg,const char* Ascii,void* Data,size_t DataSize)
{
    initialises all the variables;
}   
class GetEvent : public ClassA
{
public:
    DECLARE_NEWMSG(GetEvent,GET_EVENT);
    GetEvent();
};

static const GetEvent cGetEvent;
GetEvent::GetEvent():ClassA( ID, newMsg, NULL, NULL, 0 )
{
//no stmts inside;
}

无法理解行 GetEvent：：GetEvent((：ClassA( ID， NewMsg， NULL， NULL， 0 (。他们是否从派生类构造函数传递接收到的值，然后将其分配给基类构造函数。如果是这样，那么 newMsg 将保持未定义状态，因为它在调用基类构造函数时被定义......如果在相反的情况下采用，那么 ID 将保持未定义状态...

无法理解语句的执行，

static const GetEvent cGetEvent;
GetEvent::GetEvent():ClassA( ID, newMsg, NULL, NULL, 0 )
{
//no stmts inside;
}

还有函数指针，

typedef ClassA* (*pNewMsg)(void); 
#define DECLARE_NEWMSG(ClassName,ClassID)                
static ClassName* FromMsg(ClassA* psg)               
{                                                       
   return dynamic_cast<ClassName*>(psg);              
}                                                       
static ClassA* newMsg()                            
{                                                       
   return new ClassName;                               
}