One reason for controlling access to an object is to defer the full cost of its creation and initialization until we actually need to use it.

Transcription

1 Prxy 1 Intent Prvide a surrgate r placehlder fr anther bject t cntrl access t it. Als Knwn As Surrgate Mtivatin One reasn fr cntrlling access t an bject is t defer the full cst f its creatin and initializatin until we actually need t use it. Creating each expensive bject n demand The image prxy creates the real image nly when the dcument editr asks it t display itself by invking its Draw peratin. Applicability 1. A remte prxy prvides a lcal representative fr an bject in a different address space. 2. A virtual prxy creates expensive bjects n demand. 3. A prtectin prxy cntrls access t the riginal bject.

2 4. A smart reference is a replacement fr a bare pinter that perfrms additinal actins when an bject is accessed. 2 cunting the number f references t the real bject (als called smart pinters). lading a persistent bject int memry when it's first referenced. checking that the real bject is lcked befre it's accessed t ensure that n ther bject can change it. Structure Participants Prxy (ImagePrxy) maintains a reference that lets the prxy access the real subject. Prxy may refer t a Subject if the RealSubject and Subject interfaces are the same. prvides an interface identical t Subject's s that a prxy can by substituted fr the real subject. cntrls access t the real subject and may be respnsible fr creating and deleting it. ther respnsibilities depend n the kind f prxy: remte prxies are respnsible fr encding a request and its arguments and fr sending the encded request t the real subject in a different address space. virtual prxies may cache additinal infrmatin abut the real subject s that they can pstpne accessing it. prtectin prxies check that the caller has the access permissins required t perfrm a request.

3 3 Subject (Graphic) defines the cmmn interface fr RealSubject and Prxy s that a Prxy can be used anywhere a RealSubject is expected. RealSubject (Image) defines the real bject that the prxy represents. Cllabratins Prxy frwards requests t RealSubject when apprpriate, depending n the kind f prxy. Cnsequences 1. A remte prxy can hide the fact that an bject resides in a different address space. 2. A virtual prxy can perfrm ptimizatins such as creating an bject n demand. 3. Bth prtectin prxies and smart references allw additinal husekeeping tasks when an bject is accessed. 4. Cpy-n-write (creatin n demand). We pay the price f cpying the bject nly if it's mdified. T make cpy-n-write wrk The subject must be reference cunted. Cpying the prxy will d nthing mre than increment this reference cunt. Only when the client requests an peratin that mdifies the subject des the prxy actually cpy it. In that case the prxy must als decrement the subject's reference cunt. When the reference cunt ges t zer, the subject gets deleted. Implementatin 1. Overlading the member access peratr "->" in C class Image; 4. extern Image* LadAnImageFile(cnst char*); 5. // external functin class ImagePtr { ImagePtr(cnst char* imagefile); 12. virtual ~ImagePtr(); virtual Image* peratr->(); 15. virtual Image& peratr*(); 16. private: 17. Image* LadImage(); 18. private: 19. Image* _image; 20. cnst char* _imagefile; 21. ; ImagePtr::ImagePtr (cnst char* theimagefile) { 24. _imagefile = theimagefile; 25. _image = 0; 26.

4 Image* ImagePtr::LadImage () { 30. if (_image == 0) { 31. _image = LadAnImageFile(_imageFile); return _image; Image* ImagePtr::peratr-> () { 37. return LadImage(); Image& ImagePtr::peratr* () { 41. return *LadImage(); ImagePtr image = ImagePtr("anImageFileName"); 45. image->draw(pint(50, 100)); // (image.peratr->())->draw(pint(50, 100)) Prxy desn't always have t knw the type f real subject. If a Prxy class can deal with its subject slely thrugh an abstract interface, then there's n need t make a Prxy class fr each RealSubject class. Thus, the prxy can deal with all RealSubject classes unifrmly. But if Prxies are ging t instantiate RealSubjects (such as in a virtual prxy), then they have t knw the cncrete class. 1. Hw t refer t the subject befre it's instantiated. A file name is used fr this purpse in the Mtivatin example. Sample Cde A virtual prxy. class Graphic { virtual ~Graphic(); virtual vid Draw(cnst Pint& at) = 0; virtual vid HandleMuse(Event& event) = 0; virtual cnst Pint& GetExtent() = 0; virtual vid Lad(istream& frm) = 0; virtual vid Save(stream& t) = 0; prtected: Graphic(); ;

5 class Image : public Graphic { Image(cnst char* file); // lads image frm a file virtual ~Image(); 5 virtual vid Draw(cnst Pint& at); virtual vid HandleMuse(Event& event); virtual cnst Pint& GetExtent(); virtual vid Lad(istream& frm); virtual vid Save(stream& t); private: //... ; class ImagePrxy : public Graphic { ImagePrxy(cnst char* imagefile); virtual ~ImagePrxy(); virtual vid Draw(cnst Pint& at); virtual vid HandleMuse(Event& event); virtual cnst Pint& GetExtent(); virtual vid Lad(istream& frm); virtual vid Save(stream& t); prtected: Image* GetImage(); private: Image* _image; Pint _extent; char* _filename; ; ImagePrxy::ImagePrxy (cnst char* filename) { _filename = strdup(filename); _extent = Pint::Zer; // dn't knw extent yet _image = 0; Image* ImagePrxy::GetImage() { if (_image == 0) { _image = new Image(_fileName); return _image; cnst Pint& ImagePrxy::GetExtent () { if (_extent == Pint::Zer) { _extent = GetImage()->GetExtent(); return _extent; vid ImagePrxy::Draw (cnst Pint& at) { GetImage()->Draw(at); vid ImagePrxy::HandleMuse (Event& event) { GetImage()->HandleMuse(event); vid ImagePrxy::Save (stream& t) { t << _extent << _filename;

6 6 vid ImagePrxy::Lad (istream& frm) { frm _extent _filename; class TextDcument { TextDcument(); ; vid Insert(Graphic*); //... TextDcument* text = new TextDcument; //... text-insert(new ImagePrxy("anImageFileName")); Knwn Uses The virtual prxy example is frm the ET++ text building blck classes. Remte prxy is used in distributed bjects such as CORBA and RMI. Related Patterns Adapter: An adapter prvides a different interface t the bject it adapts. In cntrast, a prxy prvides the same interface as its subject. Hwever, a prxy used fr access prtectin might refuse t perfrm an peratin that the subject will perfrm, s its interface may be effectively a subset f the subject's. Decratr: Decratrs have a different purpse. A decratr adds ne r mre respnsibilities t an bject, whereas a prxy cntrls access t an bject. Prxies vary in the degree t which they are implemented like a decratr. A prtectin prxy might be implemented exactly like a decratr. A remte prxy will nt cntain a direct reference t its real subject but nly an indirect reference, such as "hst ID and lcal address n hst." A virtual prxy will start ff with an indirect reference such as a file name but will eventually btain and use a direct reference.