Casting base class to derived through std iterator in C++ - c++

Question

in my program I have base GeneralHeader, MacHeader that derived from GeneralHeader and NetworkPacket with member Headers that is std list of GeneralHeader:
//Packet.h
enum HeaderType_t {General_Header_type, MAC_Header_type};
class GeneralHeader {
public:
bool Valid;
HeaderType_t HeaderType;
void PrintMe();
};
struct MACHeader: public GeneralHeader {
long unsigned DestAddr:48;
long unsigned SourceAddr:48;
void PrintMe();
};
struct Packet_t {
list<GeneralHeader> Headers;//TODO GeneralHeader
list<GeneralHeader>::iterator it_Header; //TODO GeneralHeader
void PrintMe();
};
While implementing the PrintMe() of Packet_t, that supposed to print all Headers according to HeaderType: if there is a GeneralHeader - it will use GeneralHeader.PrintMe() and if it is MACHeader in the list - it will print MACHeader.PrintMe())
I'm struggling to cast it_Header iterator from base GeneralHeader to derived MACHeader inside Packet_t method PrintMe():
//Packet.cpp
void GeneralHeader::PrintMe() {
std::cout << "Valid " << GeneralHeader::Valid << endl;
std::cout << "Header Type " << GeneralHeader::HeaderType << endl;
};
void HW_MACHeader::PrintMe() {
std::cout << "------------------------ " << endl;
std::cout << "---- MAC HEADER --- " << endl;
std::cout << "------------------------ " << endl;
GeneralHeader::PrintMe();
};
void NetworkPacket_t::PrintMe() {
std::cout << "Valid Packet " << NetworkPacket_t::ValidPacket << endl;
for (it_Header = Headers.begin(); it_Header != Headers.end(); it_Header++) {
switch (it_Header->HeaderType) {
case MAC_Header_type:
static_cast<HW_MACHeader*>(it_Header)->PrintMe();
break;
default:
std::cout << "default" << endl;
};
it_Header++;
};
};
The error: invalid static_cast from type 'std::_List_iterator' to type 'MACHeader*'
Thank you for any help.

Answer 1

The desired/normal polymorphic way would be:
Redefine PrintMe() to a virtual function so that cast is not necessary:
class GeneralHeader {
public:
bool Valid;
HeaderType_t HeaderType;
virtual void PrintMe();
};
class MACHeader: public GeneralHeader {
long unsigned DestAddr:48;
long unsigned SourceAddr:48;
public:
void PrintMe();
};
Also use vector of pointers to GeneralHeader:
list<GeneralHeader*>::iterator it_Header;
Then you can:
(*it_Header)->printMe();
The for loop will be simpler:
for (it_Header = Headers.begin(); it_Header != Headers.end();++it_Header)
(*it_Header)->PrintMe();
I don't know why you need the it_Header to be a member of the class? Can't it just be local to the loop?

Answer 2

You need to dereference it_Header to access the "underlying" object to address the compiler error :
static_cast<HW_MACHeader*>(*it_Header)->PrintMe();
Hoever, that will not solve your problem: you have a list of GeneralHeader; therefore because you want to downcast to an instance of HW_MACHeader, you need to use dynamic_cast; this has to be done either on a reference or a pointer:
dynamic_cast<HW_MACHeader&>(*it_Header).PrintMe();
The line above takes the object "referenced" by it_Header, and tells the compiler to cast it dynamically to a reference of type HW_MACHeader.
Note that dynamic_cast will return a null pointer if it cannot cast down to the type you want.
However, this is not a proper way to do this. You should follow user2672165's advice, and use virtual functions.