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COM: A Brief Introduction

Dan Berger dberger@cs.ucr.edu

mailto:dberger@cs.ucr.edumailto:dberger@cs.ucr.edu

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A Brief History of COM  $%e Brain C%ild of Ant%ony &illiams +

outlined in t'o !internal# M, aers: Object Arc%itecture: Dealing 'it% t%e

-nno'n or $ye ,afety in a Dynamically/0tensible Class !1233# On In%eritance: &%at It Means and Ho' $o

-se it !1224#

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History !Cont.# Origins of COM 'ere O5/ !Object 5ining

and /mbedding# 1 t%at s%ied 'it%&indo's 6.1 !1227#

 $%e *rst ublic version of COM s%iedin O5/ 7.4 !1226#. DCOM !Distributed COM# 'as released

in 1228 in ans'er to CO(BA. !&e9ll

ignore it.# COM" 'as released along 'it% &indo's7444 and 'as rimarily concerned 'it%M$,. $%e DCOM monier 'as droed.

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Objects vs. Comonents Object Oriented ;rogramming <

;olymor%ism " !,ome# 5ate Binding "!,ome# /ncasulation " In%eritance

Comonent Oriented ;rogramming <;olymor%ism " !(eally# 5ate Binding "!(eal= /nforced# /ncasulation "Interface In%eritance " Binary (euse>

C%arlie ?indel COM uy> Microsoft Cor. 22

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COM as !C""#"" !Adated rom7E#

If you get> t%is= it9s all do'n %ill from%ere.

In C""= in articular= t%e linage model

maes binary distribution and reusediFcult.

Consider: Gou9ve 'ritten a class t%at9sart of a C"" class library.

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C%allenges of Distribution Imagine 'e distribute t%e source for t%e

class !as is common in C"" class libraries#. If eac% alication t%at uses it statically lins it=

it gets dulicated !'aste# and is imossible to

udate*0 in t%e *eld 'it%out redistributing ane' alication.

If it9s acaged as a s%ared libraryobject= t%elac of binary standardiation moves t%e

roblem to one of interoeration.  $%e D55 model !but not t%e .so model# can actually

deal 'it% t%is lac of standardiation= but it9s notretty.

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C%allenges of /ncasulation Assume 'e side)ste t%e comilerliner

trouble. $%e coast still isn9t clear. $%eC"" standard also lacs any standard

de*nition for binary encasulation. ,o c%anges to t%e internals of an object

t%at don’t change it’s interface can!and do# brea code t%at uses t%e

object. Consider adding rivate member variables.

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Interface v. Imlementation C"" suorts searation of interface and

imlementation at t%e synta0 level + notat t%e binary level. ,o c%anges of t%e imlementation are seen>

by clients. &e could %ide t%e actual imlementing

class be%ind an oaJue ointer in t%einterface e0osed to t%e client and

delegate interface calls t%roug% t%isointer to t%e real> object. /asy for simle= cumbersome for comle0

interfaces

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Abstract Classes as

Interfaces &it% t%ree assumtions= 'e canuse abstract classes to solve t%eseroblems:

1. C)style structs are reresentedidentically across !C""# comilers.

7. All comilers can be forced to usecommon call conventions.

6. All comilers on a latform useequivalent virtual callimlementations.

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vtbls and vtrs Assumtion 6 is critical= and turns

out to be not unfounded= as nearlyall C"" comilers use vtrs and

vtbls. or eac% class t%e comiler

generates a !static# array of funcointers to it9s members !it9s vtbl#.

/ac% instance of eac% class %as a!%idden# member t%at oints to t%evtbl !it9s vrt#.

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/0amle

class I,earc%able,tring N

  ublic:

  virtual int 5engt%!void# const < 4  virtual int ind!const c%ar Ps# < 4

Q

vtr

vtbl

5engt% !null#

ind !null#

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/0amle !cont.#

class ,,tring : ublicI,earc%able,tring N  ublic:

  ,earc%able,tring!const c%ar Ps#

  R,earc%able,tring!void#  int 5engt%!void# const

  int ind!const c%ar Ps#Q

,,tring

vtr

vtbl

,,tring::5engt%

,,tring::ind

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Instantiating an Abstract

Class Clearly t%e client can9t instantiatean I,earc%able,tring + it9s ureabstract= nor do 'e 'ant t%em

instantiating a ,,tring + t%atbreas !binary# encasulation.

,o 'e need a factory met%od + and

'e can force it !using e0tern C>#to be accessible to all clients.

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irtual Destructors -nfortunately= t%ere9s a roblem + our

class lacs a virtual d9tor + so calls todelete 'ill use t%e !default# d9tor on t%e

I,earc%able,tring class. &e can9t add a virtual d9tor to t%e

abstract class because dierentcomilers ut dtor9s in dierent laces

in t%e vtbl. !blec%# ,o 'e add a virtual Delete> met%od to

t%e abstract class.

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&%at is COM A substrate> for building re)usable

comonents.

5anguage neutral it9s easier to use in C""= but can be used from any

language t%at can generategro vtbl9s and vtrs. Interfaces are de*ned in COM ID5 !ID5"COM

e0tensions for in%eritance and olymor%ism#

O, Leutral

∃ commercial -ni0 imlementations= and M,suorts COM on Mac ,ystem !O, ST#

-sing only on t%e COM sec= 'e !OM?$# rolled ouro'n.

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Interfaces Interfaces are uniJuely identi*ed by

--ID9s !often called -ID9s + t%e termsare eJuivalent# called t%eir IID 

!interface ID#. Imlementers of an interface are

uniJuely identi*ed by a --ID calledt%eir CLSID !class ID#.

All COM objects imlement t%eIUnknown interface.

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I-nno'n ;rovides t%ree met%ods:

H(/,-5$ UueryInterface!IID iid= void PPv# -5OL Add(ef!void#

-5OL (elease!void# Add(ef and (elease are for resource

management !reference counting#.&e9ll mostly ignore t%em.

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UueryInterface UueryInterface is essentially a run)time

cast + it allo's you to as a comonentif it imlements a seci*c interface.

If it does= it returns a ointer to t%atinterface ointer in v.  $%in of it as a comilerlanguage neutral

dynamicVcast oeration.

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H(/,-5$  $%is is language neutral + so no

e0cetions. H(/,-5$, are a aced bit*eld return value used all over COM.

Honestly it9s one of t%e ugliest arts of COM.  $%e most used return value is de*ned as

,VO? !success= o#= t%e ot%er is /VAI5!error= failure# but t%ere are ot%ers.

 $%ere are macros ,-CC//D/D!# andAI5/D!# t%at tae an H(/,-5$ andreort success or failure.

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Instantiating Objects ,o as develoers= 'e %ave interfaces

!de*ned in ID5# for t%e comonentsavailable in a libraryon t%e system.

Ho' do 'e actually obtain aninstance of an object 'e 'ant to useT In COM t%is is termed Activation + t%ere

are t%ree basic tyes= and eac% involvest%e ,CM !service control manager#.

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Activation !cont.# ,ometimes you 'ant an

imlementation of t%e follo'inginterface t%at meets some set of

constraints> enter category IDs !CA$IDs#

 Gou can de*ne a set of categories=

and eac% COM class can advertiset%e categories it imlements.

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COM as CO(BA)lig%t COM rovides a very eFcient in)rocess

comonent model. Once ast t%e initial COCreateInstance and

UueryInterface calls= eac% met%od call issimly a call)by)func)ointer call= essentiallyfree.

Instantiating a comonent doesn9t

reJuire any out of rocess= or s%aredmemory oerations + it9s all D55 !or,%ared Object# magic.

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 $%e De*nitive (eferences

1E $%e Comonent Object Model,eci*cation Microsoft and Digital /Juiment Cor= 1227)

122W '''.microsoft.comcomresourcescomdocs

.as

7E /ssential COM

Don Bo0= Addison &esley I,BL 4)741)86XX8)W

http://www.microsoft.com/com/resources/comdocs.asphttp://www.microsoft.com/com/resources/comdocs.asphttp://www.microsoft.com/com/resources/comdocs.asphttp://www.microsoft.com/com/resources/comdocs.asp