Custom COM Implementation?

Question

I'm looking to implement a custom implementation of COM in C++ on a UNIX type platform to allow me to dynamically load and link object oriented code. I'm thinking this would be based on a similar set of functionality that POSIX provides to load and call dll's ie dlopen, dlsym and dlclose.

I understand that the general idea of COM is that you link to a few functions ie QueryInterface, AddRef and Release in a common dll (Kernel32.dll) which then allows you to access interfaces which are just a table of function pointers encapsulated with a pointer to the object for which the function pointers should be called with. These functions are exposed through IUnknown which you must inherit off of.

So how does this all work? Is there a better way to dynamically link and load to object oriented code? How does inheritance from a dll work - does every call to the base class have to be to an exposed member function i.e private/protected/public is simply ignored?

I'm quite well versed in C++ and template meta-programming and already have a fully reflective C++ system i.e member properties, member functions and global/static functions that uses boost.

Answer 1

I'm looking to implement a custom implementation of COM in C++ on a UNIX type platform to allow me to dynamically load and link object oriented code. I'm thinking this would be based on a similar set of functionality that POSIX provides to load and call dll's ie dlopen, dlsym and dlclose.

At its simplest level, COM is implemented with interfaces. In c++, if you are comfortable with the idea of pure virtual, or abstract base classes, then you already know how to define an interface in c++

struct IMyInterface {
  void Method1() =0;
  void Method2() =0;
};

The COM runtime provides a lot of extra services that apply to the windows environment but arn't really needed when implementing "mini" COM in a single application as a means to dynamically link to a more OO interface than traditionally allowed by dlopen, dlsym, etc.

COM objects are implemented in .dll, .so or .dylib files depending on your platform. These files need to export at least one function that is standardized: DllGetClassObject

In your own environment you can prototype it however you want but to interop with the COM runtime on windows obviously the name and parameters need to conform to the com standard.

The basic idea is, this is passed a pointer to a GUID - 16 bytes that uniquely are assigned to a particular object, and it creates (based on the GUID) and returns the IClassFactory* of a factory object.

The factory object is then used, by the COM runtime, to create instances of the object when the IClassFactory::CreateInstance method is called.

So, so far you have

• a dynamic library exporting at least one symbol, named "DllGetClassObject" (or some variant thereof)
• A DllGetClassObject method that checks the passed in GUID to see if and which object is being requested, and then performs a "new CSomeObjectClassFactory"
• A CSomeObjectClassFactory implementation that implements (derives from) IClassFactory, and implements the CreateInstance method to "new" instances of CSupportedObject.
• CSomeSupportedObject that implements a custom, or COM defined interface that derives from IUnknown. This is important because IClassFactory::CreateInstance is passed an IID (again, a 16byte unique id defining an interface this time) that it will need to QueryInterface on the object for.

I understand that the general idea of COM is that you link to a few functions ie QueryInterface, AddRef and Release in a common dll (Kernel32.dll) which then allows you to access interfaces which are just a table of function pointers encapsulated with a pointer to the object for which the function pointers should be called with. These functions are exposed through IUnknown which you must inherit off of.

Actually, COM is implemented by OLE32.dll which exposes a "c" api called CoCreateInstance. The app passed CoCreateInstance a GUID, which it looks up in the windows registry - which has a DB of GUID -> "path to dll" mappings. OLE/COM then loads (dlopen) the dll, calls its DllGetClassObject (dlsym) method, passing in the GUID again, presuming that succeeds, OLE/COM then calls the CreateInstance and returns the resulting interface to app.

So how does this all work? Is there a better way to dynamically link and load to object oriented code? How does inheritance from a dll work - does every call to the base class have to be to an exposed member function i.e private/protected/public is simply ignored?

implicit inheritance of c++ code from a dll/so/dylib works by exporting every method in the class as a "decorated" symbol. The method name is decorated with the class, and type of every parameter. This is the same way the symbols are exported from static libraries (.a or .lib files iirc). Static or dynamic libraries, "private, protected etc." are always enforced by the compiler, parsing the header files, never the linker.

I'm quite well versed in C++ and template meta-programming and already have a fully reflective C++ system i.e member properties, member functions and global/static functions that uses boost.

c++ classes can typically only be exported from dlls with static linkage - dlls that are loaded at load, not via dlopen at runtime. COM allows c++ interfaces to be dynamically loaded by ensuring that all datatypes used in COM are either pod types, or are pure virtual interfaces. If you break this rule, by defining an interface that tries to pass a boost or any other type of object you will quickly get into a situation where the compiler/linker will need more than just the header file to figure out whats going on and your carefully prepared "com" dll will have to be statically or implicitly linked in order to function.

The other rule of COM is, never pass ownership of an object accross a dynamic library boundary. i.e. never return an interface or data from a dll, and require the app to delete it. Interfaces all need to implement IUnknown, or at least a Release() method, that allows the object to perform a delete this. Any returned data types likewise must have a well known de-allocator - if you have an interface with a method called "CreateBlob", there should probably be a buddy method called "DeleteBlob".

Answer 2

To really understand how COM works, I suggest reading "Essential COM" by Don Box.

Answer 3

A couple of things to keep in mind:

• The power of COM comes largely from the IDL and the midl compiler. It allows a verry succint definition of the objects and interfaces with all the C/C++ boilerplate generated for you.

• COM registration. On Windows the class IDs (CLSID) are recorded in the registry where they are associated with the executable. You must provide similar functionality in the UNIX environment.

• The whole IUnknown implementation is fairly trivial, except for QueryInterface which works when implemented in C (i.e. no RTTI).

• A whole another aspect of COM is IDispatch - i.e. late bound method invocation and discovery (read only reflection).

Have a look at XPCOM as it is a multi-platform COM like environment. This is really one of those things you are better off leveraging other technologies. It can suck up a lot of the time better spent elsewhere.

Answer 4

Look at the CORBA documentation, at System.ComponentModel in the sscli, the XPCOM parts of the Mozilla codebase. Miguel de Icaza implemented something like OLE in GNOME called Bonobo which might be useful as well.

Depending on what you're doing with C++ though, you might want to look at plugin frameworks for C++ like Yehia. I believe Boost also has something similar.

Edit: pugg seems better maintained than Yehia at the moment. I have not tried it though.