How to force overloaded call in derived classes?

[Marcin Gil]

First of all thanks for your patience and good pointers
for Visitor and Curiously Recurring Template patterns last time

Now another problem of mine.

THE CODE

#include <iostream>
#include <vector>

struct CMessage;
struct CTextMessage;
struct CServerMessage;
struct CMessageInterface;
struct CMessage;

class CMessageProcessor
{
public:
virtual void ProcessMessage(CMessage& m) { std::cout << "Processing any
message" << std::endl; };
// virtual void ProcessMessage(CTextMessage& a) { std::cout <<
"Processing CTextMessage" << std::endl; }; // (1)
// virtual void ProcessMessage(CServerMessage& c) { std::cout <<
"Processing CServerMessage" << std::endl; };
};

class CAnotherProcessor: public virtual CMessageProcessor
{
public:
virtual void ProcessMessage(CTextMessage& a) { std::cout << "Processing
CTextMessage in Another" << std::endl; };
};

class CDerivedProcessor: public CAnotherProcessor
{
public:
virtual void ProcessMessage(CTextMessage& a) { std::cout << "Processing
CTextMessage in Derived" << std::endl; };
};

struct CMessage
{
virtual void Process(CMessageProcessor& p) = 0;
virtual ~CMessage() {};
};

template<typename Self>
struct CMessageBase: public CMessage
{
virtual void Process(CMessageProcessor& p) {
p.ProcessMessage(*static_cast<Self*>(this)); }; // (2)
};

struct CMessage: public CMessageBase<CMessage>
{
};

struct CTextMessage: public CMessageBase<CTextMessage>
{
};

struct CServerMessage: public CMessageBase<CServerMessage>
{
};


int main()
{
CDerivedProcessor p;
CAnotherProcessor ap;
CMessageProcessor* pP = &p;
CMessageProcessor* pAP = &ap;

std::vector<CMessage*> v;

v.push_back(new CMessage());
v.push_back(new CTextMessage());
v.push_back(new CServerMessage());

for (int i = 0; i < 3; ++i)
{
CMessage& rMsg = *v.at(i);
rMsg.Process(*pP); // (3)
rMsg.Process(*pAP);
//v.at(i)->Process(*pP);
//v.at(i)->Process(*pAP);
}

while (!v.empty())
{
delete v.back();
v.pop_back();
}

return 0;
};


THE PROBLEM

An overloaded ProcessMessage() for CTextMessage in CAnotherProcessor and
CDerivedProcessor is called only when appropriate ProcessMessage is
defined in CMessageProcessor.

Seems obvious since at (3) the virtual function (1) of that definition
is unknown and virtuality doesn't play.
However I missed this since from the beginning I've used
CMessageProcessor with all these functions uncommented.

THE QUESTION

How can I make it that at (3) the call of (2) is done to the function I
want?
Ie: when rMsg at (3) is really CTextMessage and
CDerivedProcessor:rocessMessage(CTextMessage) is called
instead of CMessageProcessor:rocessMessage(CMessage)?

THE NEED

I would like to have several custom message processors that are
able to process custom messages only by defining an overloaded
ProcessMessage(CCustomMessageType) and calling Process() on
CCustomMessageType it will call appropriate ProcessMessage regardles
of what is defined in CMessageProcessor.

So: extending a CCustomProcessor to process new message types
should require only defining a new ProcessMessage(CCustomMessageType)
- at least this is what I would like to have.

Thank you
-Marcin

 

[Ron Natalie]

To use dynamic typing, you'll need to use some RTTI feature.
Overloading isn't going to cut it.

One way is to build some sort of bridge between the CMessage
class heirarchy and the CMessageProcessor (take a look at the
Visitor design pattern).

Another, slightly kludgy but perhaps faster way would be to
do (if I'm understanding you properly:

void CMessageProcessor:rocessGeneric(CMessage& m) {
CServerMessage* csm = dynamic_cast<CServerMessage*)(&m);
if(csm) {
ProcessMessage(*csm);
return;
}
CTextMessage* ctm = dynamic_cast<CTextMessage*>(&m);
if(ctm) {
ProcessMessage(*ctm);
return;
}
ProcessMessage(m);
}

 

[Marcin Gil]

To use dynamic typing, you'll need to use some RTTI feature.
Overloading isn't going to cut it.

One way is to build some sort of bridge between the CMessage
class heirarchy and the CMessageProcessor (take a look at the
Visitor design pattern).

Another, slightly kludgy but perhaps faster way would be to
do (if I'm understanding you properly:

void CMessageProcessor:rocessGeneric(CMessage& m) {
CServerMessage* csm = dynamic_cast<CServerMessage*)(&m);
if(csm) {
ProcessMessage(*csm);
return;
}
CTextMessage* ctm = dynamic_cast<CTextMessage*>(&m);
if(ctm) {
ProcessMessage(*ctm);
return;
}
ProcessMessage(m);
}

I did use Visitor pattern - this is how MessageType->Process(ProcessorType)
works.
However to enable full polymorphism/use overloading I would have to define
a ProcessMessage(MessageType) in CMessageProcessor for each message type
supported (as would uncommenting (1) in my previous post do).

I am also not sure if your "dynamic_cast" solution is better - one has
to add an "if" each time a new type is introduced.

Thank you