Re: How to force overloaded call in derived classes?
Marcin Gil wrote:
[re-ordered the message, to have the question before the code]
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 PROBLEM
An overloaded ProcessMessage() for CTextMessage in CAnotherProcessor
and CDerivedProcessor is called only when appropriate ProcessMessage
is defined in CMessageProcessor.
Pedantically speaking, CAnotherProcessor::ProcessMessage(CTextMessage)
is not an overload for CProcessor::ProcessMessage(CMessage).
Keeping this clear is important for understanding why the compiler does
not call the function you like it to call.
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::ProcessMessage(CTextMessage) is called
instead of CMessageProcessor::ProcessMessage(CMessage)?
You can't do it directly, but this is one of those situations where RTTI
(in the form of dynamic_cast<>()) is a good solution.
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:
Add:
virtual void ProcessMessage(CMessage& m) {
if (CTextMessage* p = dynamic_cast<CTextMessage*>(&m))
{
ProcessMessage(*p);
}
else
{
// call parent
CMessageProcessor::ProcessMessage(m);
}
}
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; };
};
<snip>
This way, the first processor that adds handling for a new specific
message needs to override a (more) generic ProcessMessage() function to
redirect the handling of the message to the more specialised handler.
The other processors do not need to be changed.
Thank you
-Marcin
Bart v Ingen Schenau
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