[gmpi] Re: low level API - Abstract Factory summary
- From: Jeff McClintock <jeffmcc@xxxxxxxxxx>
- To: gmpi@xxxxxxxxxxxxx
- Date: Tue, 08 Feb 2005 08:46:21 +1300
Re the dll Abstract Factory.
Cheers to Tim and Angus, here's a possible implementation.. please rip
this to pieces ( constructively criticize )
Angus said:
The single entry point should return a master object which provides
functions for, at a minimum:-
- Enumerating the plugin-classes this DLL can instantiate. Could be as
simple as passing in an index, the enumerate function returns either a
class-id, or null to mean "no more".
- Instantiating a specific plug-in (pass the class-identifier as an
argument, returns an interface-pointer to the newly created instance)
let's call it:
class GMPI_Plugin_Factory
{
void GetMetaData( int index, gmpi_descriptor *gd );
GMPI_Plugin *Instantiate( int index );
};
..where gmpi_descriptor describes the plugin...
struct gmpi_descriptor
{
char *unique_identifier;
char *name;
//etc
};
Notes: I've modified Tim's idea a little to separate metadata from
instantiation. This is a nod to the external metadata camp, and at
least makes it easy to switch to external metadata later, or for the
plugin to read it from an external file if nesc.
..So the dll has a single exported function..
extern "C" {
__declspec (dllexport)
IGMPI_Plugin_Factory *GetPluginFactory(void)
{
static GMPI_Plugin_Factory factory;
return &factory;
}
}
Notes: factory is implemented as a Meyer's singleton. I've assumed
we're using a COM-like technique whereby we can deal in C++ objects
without worrying too much about the underlying plain C binary interface.
and finally,
GMPI_Plugin's 'interface' is defined as a pure abstract base class.
This is all the host ever 'sees' to hide the internal details..
class IGMPI_Plugin
{
virtual void placeholder1(void) = 0;
virtual void placeholder2(void) = 0;
};
...and the 'real' concrete plugin base class that we base our plugins on..
class GMPI_Plugin : public IGMPI_Plugin
{
virtual void placeholder1(void){};
virtual void placeholder2(void){};
};
Notes: IGMPI_Plugin should probably include the standard 'COM stuff' too..
have at it...
Jeff
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