Who can explain this code for me?about virtual function

///////////////////////////////////////////////
// CIOMessageMap.h
class __declspec(novtable) CIOMessageMap
{
public:
virtual bool ProcessIOMessage(IOType clientIO, ClientContext*
pContext, DWORD dwSize) = 0;
};

#define BEGIN_IO_MSG_MAP() \
public: \
bool ProcessIOMessage(IOType clientIO, ClientContext* pContext, DWORD
dwSize = 0) \
{ \
bool bRet = false;

#define IO_MESSAGE_HANDLER(msg, func) \
if (msg == clientIO) \
bRet = func(pContext, dwSize);

#define END_IO_MSG_MAP() \
return bRet; \
}


Why class CIOMessageMap should be declared by declspec(novtable)?
The macros implements the member function of CIOMessageMap --
ProcessIOMessage();But in other places of project,there no any
inheritances of CIOMessageMap.
Who can tell me the advantages about this coding style?
There are will best in detail.

Alex
Thu Oct 25 01:55:38 PDT 2007

"IOCPer" wrote:
>
> Why class CIOMessageMap should be declared by
> declspec(novtable)?

You don't have to declare it without vtable. The
`__declspec(novtable)' modifier is an optimization. When you
know for sure that some class will never be instantiated,
then you can cancel the generation of [redundant] vtable. It
can reduce the resulting code size.

> The macros implements the member function of
> CIOMessageMap --
> ProcessIOMessage();But in other places of project,there no
> any
> inheritances of CIOMessageMap.

Either you didn't find all occurrences of `CIOMessageMap' or
it's not used anywhere.

> Who can tell me the advantages about this coding style?

The advantage is reduced code size.

> There are will best in detail.

For more details see here:

"novtable"
http://msdn2.microsoft.com/en-us/library/k13k85ky(VS.80).aspx


Alex

Alexander
Thu Oct 25 11:46:39 PDT 2007

The class has a pure virtual method thus it cannot be
instantiated. Hence the __declspec(novtable) optimization.

--
=====================================
Alexander Nickolov
Microsoft MVP [VC], MCSD
email: agnickolov@mvps.org
MVP VC FAQ: http://vcfaq.mvps.org
=====================================

"IOCPer" <kuibing@gmail.com> wrote in message
news:1193299090.466208.52350@e34g2000pro.googlegroups.com...
> ///////////////////////////////////////////////
> // CIOMessageMap.h
> class __declspec(novtable) CIOMessageMap
> {
> public:
> virtual bool ProcessIOMessage(IOType clientIO, ClientContext*
> pContext, DWORD dwSize) = 0;
> };
>
> #define BEGIN_IO_MSG_MAP() \
> public: \
> bool ProcessIOMessage(IOType clientIO, ClientContext* pContext, DWORD
> dwSize = 0) \
> { \
> bool bRet = false;
>
> #define IO_MESSAGE_HANDLER(msg, func) \
> if (msg == clientIO) \
> bRet = func(pContext, dwSize);
>
> #define END_IO_MSG_MAP() \
> return bRet; \
> }
>
>
> Why class CIOMessageMap should be declared by declspec(novtable)?
> The macros implements the member function of CIOMessageMap --
> ProcessIOMessage();But in other places of project,there no any
> inheritances of CIOMessageMap.
> Who can tell me the advantages about this coding style?
> There are will best in detail.
>