How to force overloaded call in derived classes?

Marcin Gil <>
Thu, 5 Jul 2007 08:04:15 CST
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First of all thanks for your patience and good pointers
for Visitor and Curiously Recurring Template patterns last time :)

Now another problem of mine.


#include <iostream>
#include <vector>

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

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

class CDerivedProcessor: public CAnotherProcessor
    virtual void ProcessMessage(CTextMessage& a) { std::cout <<
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 = *;
        rMsg.Process(*pP); // (3)

    while (!v.empty())
        delete v.back();

    return 0;


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.


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


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

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