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  • 1. COM: A Brief Introduction Dan Berger [email_address]
  • 2. Outline
    • A Brief History of COM
    • Objects vs. Components
    • COM as a (C++)++
    • The What and How of COM
    • COM as CORBA-light
    • The Definitive References
  • 3. A Brief History of COM
    • The Brain Child of Anthony Williams – outlined in two (internal) MS papers:
      • Object Architecture: Dealing with the Unknown or Type Safety in a Dynamically Extensible Class (1988)
      • On Inheritance: What It Means and How To Use it (1990)
  • 4. History (Cont.)
    • Origins of COM were OLE (Object Linking and Embedding) 1 that shipped with Windows 3.1 (1992)
    • The first public version of COM shipped in OLE 2.0 (1993).
    • DCOM (Distributed COM) was released in 1996 in answer to CORBA. (We’ll ignore it.)
    • COM+ was released along with Windows 2000 and was primarily concerned with MTS. The DCOM moniker was dropped.
  • 5. Objects vs. Components
    • “ Object Oriented Programming = Polymorphism + (Some) Late Binding + (Some) Encapsulation + Inheritance
    • Component Oriented Programming = Polymorphism + (Really) Late Binding + (Real, Enforced) Encapsulation + Interface Inheritance + Binary Reuse”
        • Charlie Kindel “COM Guy” Microsoft Corp. 9/97
  • 6. COM as (C++)++ (Adapted From [2])
    • If you “get” this, it’s all down hill from here.
    • In C++, in particular, the linkage model makes binary distribution and reuse difficult.
    • Consider: You’ve written a class that’s part of a C++ class library.
  • 7. Challenges of Distribution
    • Imagine we distribute the source for the class (as is common in C++ class libraries).
      • If each application that uses it statically links it, it gets duplicated (waste) and is impossible to update/fix in the field without redistributing a new application.
      • If it’s packaged as a shared library/object, the lack of binary standardization moves the problem to one of interoperation.
        • The DLL model (but not the .so model) can actually deal with this lack of standardization, but it’s not pretty.
  • 8. Challenges of Encapsulation
    • Assume we side-step the compiler/linker trouble. The coast still isn’t clear. The C++ standard also lacks any standard definition for binary encapsulation.
    • So changes to the internals of an object that don’t change it’s interface can (and do) break code that uses the object.
      • Consider adding private member variables.
  • 9. Versioned Libraries
    • A quick look in /usr/lib or %WINDIR% will likely reveal a number of “identical” libraries with different versions.
      • libFoo.so.1
      • libFoo.so.2
    • With enough diligence the library developer can insulate applications from change buy explicitly versioning the library.
      • I think we all agree this is sub-optimal solution.
  • 10. Interface v. Implementation
    • C++ supports separation of interface and implementation at the syntax level – not at the binary level.
      • So changes of the implementation are “seen” by clients.
    • We could hide the actual implementing class behind an opaque pointer in the interface exposed to the client and delegate interface calls through this pointer to the “real” object.
      • Easy for simple, cumbersome for complex interfaces
  • 11. Abstract Classes as Interfaces
    • With three assumptions, we can use abstract classes to solve these problems:
      • C-style structs are represented identically across (C++) compilers.
      • All compilers can be forced to use common call conventions.
      • All compilers on a platform use equivalent virtual call implementations.
  • 12. vtbls and vptrs
    • Assumption 3 is critical, and turns out to be not unfounded, as nearly all C++ compilers use vptrs and vtbls.
    • For each class the compiler generates a (static) array of func pointers to it’s members (it’s vtbl).
    • Each instance of each class has a (hidden) member that points to the vtbl (it’s vprt).
  • 13. Example
    • class ISearchableString {
    • public:
    • virtual int Length(void) const = 0;
    • virtual int Find(const char *s) = 0;
    • };
    IsearchableString vptr vtbl Length (null) Find (null)
  • 14. Example (cont.)
    • class SString : public ISearchableString { public:
      • SearchableString(const char *s);
      • ~SearchableString(void); int Length(void) const; int Find(const char *s); };
    SString vptr vtbl SString::Length SString::Find
  • 15. Instantiating an Abstract Class
    • Clearly the client can’t instantiate an ISearchableString – it’s pure abstract, nor do we want them instantiating a SString – that breaks (binary) encapsulation.
    • So we need a factory method – and we can force it (using extern “C”) to be accessible to all clients.
  • 16. Virtual Destructors
    • Unfortunately, there’s a problem – our class lacks a virtual d’tor – so calls to delete will use the (default) d’tor on the ISearchableString class.
    • We can’t add a virtual d’tor to the abstract class because different compilers put dtor’s in different places in the vtbl. (blech)
    • So we add a virtual “Delete” method to the abstract class.
  • 17. What is COM
    • A “substrate” for building re-usable components.
    • Language neutral
      • it’s easier to use in C++, but can be used from any language that can generate/grok vtbl’s and vptrs.
      • Interfaces are defined in COM IDL (IDL+COM extensions for inheritance and polymorphism)
    • OS Neutral
      •  commercial Unix implementations, and MS supports COM on Mac System (OS X?)
      • Using only on the COM spec, we (OMKT) rolled our own.
  • 18. Interfaces
    • Interfaces are uniquely identified by UUID’s (often called GUID’s – the terms are equivalent) called their IID (interface ID).
    • Implementers of an interface are uniquely identified by a UUID called their CLSID (class ID).
    • All COM objects implement the IUnknown interface.
  • 19. IUnknown
    • Provides three methods:
      • HRESULT QueryInterface(IID iid, void **ppv)
      • ULONG AddRef(void);
      • ULONG Release(void);
    • AddRef and Release are for resource management (reference counting). We’ll mostly ignore them.
  • 20. QueryInterface
    • QueryInterface is essentially a run-time cast – it allows you to ask a component if it implements a specific interface.
      • If it does, it returns a pointer to that interface pointer in ppv.
      • Think of it as a compiler/language neutral dynamic_cast operation.
  • 21. HRESULT
    • This is language neutral – so no exceptions. HRESULTS are a packed bit field return value used all over COM.
      • Honestly it’s one of the ugliest parts of COM.
    • The most used return value is defined as S_OK (success, ok), the other is E_FAIL (error, failure) but there are others.
    • There are macros SUCCEEDED() and FAILED() that take an HRESULT and report success or failure.
  • 22. Instantiating Objects
    • So as developers, we have interfaces (defined in IDL) for the components available in a library/on the system.
    • How do we actually obtain an instance of an object we want to use?
      • In COM this is termed Activation – there are three basic types, and each involves the SCM (service control manager).
  • 23. Activation and the SCM
    • The SCM manages the mapping between IIDs, CLSIDs, and implementations.
    • You can ask the SCM for a particular CLSID and it will instantiate an instance and return it’s interface pointer.
      • CoGetClassObject()
    • There’s an additional layer of indirection through ProgIDs – strings of the form libraryname.classname.version that map to CLSIDs.
  • 24. Activation (cont.)
    • Sometimes you want “an implementation of the following interface that meets some set of constraints”
      • enter category IDs (CATIDs)
    • You can define a set of categories, and each COM class can advertise the categories it implements.
  • 25. COM as CORBA-light
    • COM provides a very efficient in-process component model.
      • Once past the initial COCreateInstance and QueryInterface calls, each method call is simply a call-by-func-pointer call, essentially free.
    • Instantiating a component doesn’t require any out of process, or shared memory operations – it’s all DLL (or Shared Object) magic.
  • 26. There’s Much, Much More
    • COM is specific about many topics that C++ (and other languages) are not. It specifies:
      • Execution environment options (so-called Apartments)
      • Inter-process Marshalling
      • Remote Object Activation mechanism and protocols
      • Threading models
  • 27. The Definitive References
    • [1] The Component Object Model Specification
      • Microsoft and Digital Equipment Corp, 1992-1995
      • www.microsoft.com/com/resources/comdocs.asp
    • [2] Essential COM
      • Don Box, Addison Wesley
      • ISBN 0-201-63446-5