Design Patterns: Elements of Reusable Object-Oriented Software - page 20

Proxy

Intent

Provide a surrogate or placeholder for another object to control access to it.

Also Known As

Surrogate

Motivation

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. Consider a document editor that can embed graphical objects in a document. Some graphical objects, like large raster images, can be expensive to create. But opening a document should be fast, so we should avoid creating all the expensive objects at once when the document is opened. This isn't necessary anyway, because not all of these objects will be visible in the document at the same time.

These constraints would suggest creating each expensive object on demand, which in this case occurs when an image becomes visible. But what do we put in the document in place of the image? And how can we hide the fact that the image is created on demand so that we don't complicate the editor's implementation? This optimization shouldn't impact the rendering and formatting code, for example.

The solution is to use another object, an image proxy, that acts as a stand-in for the real image. The proxy acts just like the image and takes care of instantiating it when it's required.

The image proxy creates the real image only when the document editor asks it to display itself by invoking its Draw operation. The proxy forwards subsequent requests directly to the image. It must therefore keep a reference to the image after creating it.

Let's assume that images are stored in separate files. In this case we can use the file name as the reference to the real object. The proxy also stores its extent, that is, its width and height. The extent lets the proxy respond to requests for its size from the formatter without actually instantiating the image.

The following class diagram illustrates this example in more detail.

The document editor accesses embedded images through the interface defined by the abstract Graphic class. ImageProxy is a class for images that are created on demand. ImageProxy maintains the file name as a reference to the image on disk. The file name is passed as an argument to the ImageProxy constructor.

ImageProxy also stores the bounding box of the image and a reference to the real Image instance. This reference won't be valid until the proxy instantiates the real image. The Draw operation makes sure the image is instantiated before forwarding it the request. GetExtent forwards the request to the image only if it's instantiated; otherwise ImageProxy returns the extent it stores.

Applicability

Proxy is applicable whenever there is a need for a more versatile or sophisticated reference to an object than a simple pointer. Here are several common situations in which the Proxy pattern is applicable:

  1. A remote proxy provides a local representative for an object in a different address space. NEXTSTEP [Add94] uses the class NXProxy for this purpose. Coplien [Cop92] calls this kind of proxy an "Ambassador."
  2. A virtual proxy creates expensive objects on demand. The ImageProxy described in the Motivation is an example of such a proxy.
  3. A protection proxy controls access to the original object. Protection proxies are useful when objects should have different access rights. For example, KernelProxies in the Choices operating system [CIRM93] provide protected access to operating system objects.
  4. A smart reference is a replacement for a bare pointer that performs additional actions when an object is accessed. Typical uses include
    • counting the number of references to the real object so that it can be freed automatically when there are no more references (also called smart pointers [Ede92]).
    • loading a persistent object into memory when it's first referenced.
    • checking that the real object is locked before it's accessed to ensure that no other object can change it.

Structure

Here's a possible object diagram of a proxy structure at run-time:

Participants

  • Proxy (ImageProxy)
    • maintains a reference that lets the proxy access the real subject. Proxy may refer to a Subject if the RealSubject and Subject interfaces are the same.
    • provides an interface identical to Subject's so that a proxy can by substituted for the real subject.
    • controls access to the real subject and may be responsible for creating and deleting it.
    • other responsibilities depend on the kind of proxy:
      • remote proxies are responsible for encoding a request and its arguments and for sending the encoded request to the real subject in a different address space.
      • virtual proxies may cache additional information about the real subject so that they can postpone accessing it. For example, the ImageProxy from the Motivation caches the real image's extent.
      • protection proxies check that the caller has the access permissions required to perform a request.
  • Subject (Graphic)
    • defines the common interface for RealSubject and Proxy so that a Proxy can be used anywhere a RealSubject is expected.
  • RealSubject (Image)
    • defines the real object that the proxy represents.

Collaborations

  • Proxy forwards requests to RealSubject when appropriate, depending on the kind of proxy.

Consequences

The Proxy pattern introduces a level of indirection when accessing an object. The additional indirection has many uses, depending on the kind of proxy:

  1. A remote proxy can hide the fact that an object resides in a different address space.
  2. A virtual proxy can perform optimizations such as creating an object on demand.
  3. Both protection proxies and smart references allow additional housekeeping tasks when an object is accessed.

There's another optimization that the Proxy pattern can hide from the client. It's called copy-on-write, and it's related to creation on demand. Copying a large and complicated object can be an expensive operation. If the copy is never modified, then there's no need to incur this cost. By using a proxy to postpone the copying process, we ensure that we pay the price of copying the object only if it's modified.

To make copy-on-write work, the subject must be reference counted. Copying the proxy will do nothing more than increment this reference count. Only when the client requests an operation that modifies the subject does the proxy actually copy it. In that case the proxy must also decrement the subject's reference count. When the reference count goes to zero, the subject gets deleted.

Copy-on-write can reduce the cost of copying heavyweight subjects significantly.

Implementation

The Proxy pattern can exploit the following language features:

  1. Overloading the member access operator in C++. C++ supports overloading operator->, the member access operator. Overloading this operator lets you perform additional work whenever an object is dereferenced. This can be helpful for implementing some kinds of proxy; the proxy behaves just like a pointer.

    The following example illustrates how to use this technique to implement a virtual proxy called ImagePtr.

    class Image;
    extern Image* LoadAnImageFile(const char*);
        // external function
    
    class ImagePtr {
    public:
        ImagePtr(const char* imageFile);
        virtual ~ImagePtr();
    
        virtual Image* operator->();
        virtual Image& operator*();
    private:
        Image* LoadImage();
    private:
        Image* _image;
        const char* _imageFile;
    };
    
    ImagePtr::ImagePtr (const char* theImageFile) {
        _imageFile = theImageFile;
        _image = 0;
    }
    
    Image* ImagePtr::LoadImage () {
        if (_image == 0) {
            _image = LoadAnImageFile(_imageFile);
        }
        return _image;
    }
     

    The overloaded -> and * operators use LoadImage to return _image to callers (loading it if necessary).

    Image* ImagePtr::operator-> () {
        return LoadImage();
    }
    
    Image& ImagePtr::operator* () {
        return *LoadImage();
    }
     

    This approach lets you call Image operations through ImagePtr objects without going to the trouble of making the operations part of the ImagePtr interface:

    ImagePtr image = ImagePtr("anImageFileName");
    image->Draw(Point(50, 100));  
        // (image.operator->())->Draw(Point(50, 100))
     

    Notice how the image proxy acts like a pointer, but it's not declared to be a pointer to an Image. That means you can't use it exactly like a real pointer to an Image. Hence clients must treat Image and ImagePtr objects differently in this approach.

    Overloading the member access operator isn't a good solution for every kind of proxy. Some proxies need to know precisely which operation is called, and overloading the member access operator doesn't work in those cases.

    Consider the virtual proxy example in the Motivation. The image should be loaded at a specific time—namely when the Draw operation is called—and not whenever the image is referenced. Overloading the access operator doesn't allow this distinction. In that case we must manually implement each proxy operation that forwards the request to the subject.

    These operations are usually very similar to each other, as the Sample Code demonstrates. Typically all operations verify that the request is legal, that the original object exists, etc., before forwarding the request to the subject. It's tedious to write this code again and again. So it's common to use a preprocessor to generate it automatically.

  2. Using doesNotUnderstand in Smalltalk. Smalltalk provides a hook that you can use to support automatic forwarding of requests. Smalltalk calls doesNotUnderstand: aMessage when a client sends a message to a receiver that has no corresponding method. The Proxy class can redefine doesNotUnderstand so that the message is forwarded to its subject.

    To ensure that a request is forwarded to the subject and not just absorbed by the proxy silently, you can define a Proxy class that doesn't understand any messages. Smalltalk lets you do this by defining Proxy as a class with no superclass.6

    The main disadvantage of doesNotUnderstand: is that most Smalltalk systems have a few special messages that are handled directly by the virtual machine, and these do not cause the usual method look-up. The only one that's usually implemented in Object (and so can affect proxies) is the identity operation ==.

    If you're going to use doesNotUnderstand: to implement Proxy, then you must design around this problem. You can't expect identity on proxies to mean identity on their real subjects. An added disadvantage is that doesNotUnderstand: was developed for error handling, not for building proxies, and so it's generally not very fast.

  3. Proxy doesn't always have to know the type of real subject. If a Proxy class can deal with its subject solely through an abstract interface, then there's no need to make a Proxy class for each RealSubject class; the proxy can deal with all RealSubject classes uniformly. But if Proxies are going to instantiate RealSubjects (such as in a virtual proxy), then they have to know the concrete class.

Another implementation issue involves how to refer to the subject before it's instantiated. Some proxies have to refer to their subject whether it's on disk or in memory. That means they must use some form of address space-independent object identifiers. We used a file name for this purpose in the Motivation.

Sample Code

The following code implements two kinds of proxy: the virtual proxy described in the Motivation section, and a proxy implemented with doesNotUnderstand:.7

  1. A virtual proxy. The Graphic class defines the interface for graphical objects:
    class Graphic {
    public:
        virtual ~Graphic();
    
        virtual void Draw(const Point& at) = 0;
        virtual void HandleMouse(Event& event) = 0;
    
        virtual const Point& GetExtent() = 0;
    
        virtual void Load(istream& from) = 0;
        virtual void Save(ostream& to) = 0;
    protected:
        Graphic();
    };
     

    The Image class implements the Graphic interface to display image files. Image overrides HandleMouse to let users resize the image interactively.

    class Image : public Graphic {
    public:
        Image(const char* file);  // loads image from a file
        virtual ~Image();
    
        virtual void Draw(const Point& at);
        virtual void HandleMouse(Event& event);
    
        virtual const Point& GetExtent();
    
        virtual void Load(istream& from);
        virtual void Save(ostream& to);
    private:
        // ...
    };
     

    ImageProxy has the same interface as Image:

    class ImageProxy : public Graphic {
    public:
        ImageProxy(const char* imageFile);
        virtual ~ImageProxy();
    
        virtual void Draw(const Point& at);
        virtual void HandleMouse(Event& event);
    
        virtual const Point& GetExtent();
    
        virtual void Load(istream& from);
        virtual void Save(ostream& to);
    protected:
        Image* GetImage();
    private:
        Image* _image;
        Point _extent;
        char* _fileName;
    };
     

    The constructor saves a local copy of the name of the file that stores the image, and it initializes _extent and _image:

    ImageProxy::ImageProxy (const char* fileName)  {
        _fileName = strdup(fileName);
        _extent = Point::Zero;  // don't know extent yet
        _image = 0;
    }
    
    Image* ImageProxy::GetImage() {
        if (_image == 0) {
            _image = new Image(_fileName);
        }
        return _image;
    }
     

    The implementation of GetExtent returns the cached extent if possible; otherwise the image is loaded from the file. Draw loads the image, and HandleMouse forwards the event to the real image.

    const Point& ImageProxy::GetExtent () {
        if (_extent == Point::Zero) {
            _extent = GetImage()->GetExtent();
        }
        return _extent;
    }
    
    void ImageProxy::Draw (const Point& at) {
        GetImage()->Draw(at);
    }
    
    void ImageProxy::HandleMouse (Event& event) {
        GetImage()->HandleMouse(event);
    }
     

    The Save operation saves the cached image extent and the image file name to a stream. Load retrieves this information and initializes the corresponding members.

    void ImageProxy::Save (ostream& to) {
        to << _extent << _fileName;
    }
    
    void ImageProxy::Load (istream& from) {
        from>> _extent>> _fileName;
    }
     

    Finally, suppose we have a class TextDocument that can contain Graphic objects:

    class TextDocument {
    public:
        TextDocument();
    
        void Insert(Graphic*);
        // ...
    };
     

    We can insert an ImageProxy into a text document like this:

    TextDocument* text = new TextDocument;
    // ...
    text->Insert(new ImageProxy("anImageFileName"));
     
  2. Proxies that use doesNotUnderstand. You can make generic proxies in Smalltalk by defining classes whose superclass is nil8 and defining the doesNotUnderstand: method to handle messages.

    The following method assumes the proxy has a realSubject method that returns its real subject. In the case of ImageProxy, this method would check to see if the the Image had been created, create it if necessary, and finally return it. It uses perform:withArguments: to perform the message being trapped on the real subject.

    doesNotUnderstand: aMessage
        ^ self realSubject
            perform: aMessage selector
            withArguments: aMessage arguments
     

    The argument to doesNotUnderstand: is an instance of Message that represents the message not understood by the proxy. So the proxy responds to all messages by making sure that the real subject exists before forwarding the message to it.

    One of the advantages of doesNotUnderstand: is it can perform arbitrary processing. For example, we could produce a protection proxy by specifying a set legalMessages of messages to accept and then giving the proxy the following method:

    doesNotUnderstand: aMessage
        ^ (legalMessages includes: aMessage selector)
            ifTrue: [self realSubject
                perform: aMessage selector
                withArguments: aMessage arguments]
            ifFalse: [self error: 'Illegal operator']
     

    This method checks to see that a message is legal before forwarding it to the real subject. If it isn't legal, then it will send error: to the proxy, which will result in an infinite loop of errors unless the proxy defines error:. Consequently, the definition of error: should be copied from class Object along with any methods it uses.

Known Uses

The virtual proxy example in the Motivation section is from the ET++ text building block classes.

NEXTSTEP [Add94] uses proxies (instances of class NXProxy) as local representatives for objects that may be distributed. A server creates proxies for remote objects when clients request them. On receiving a message, the proxy encodes it along with its arguments and then forwards the encoded message to the remote subject. Similarly, the subject encodes any return results and sends them back to the NXProxy object.

McCullough [McC87] discusses using proxies in Smalltalk to access remote objects. Pascoe [Pas86] describes how to provide side-effects on method calls and access control with "Encapsulators."

Related Patterns

Adapter : An adapter provides a different interface to the object it adapts. In contrast, a proxy provides the same interface as its subject. However, a proxy used for access protection might refuse to perform an operation that the subject will perform, so its interface may be effectively a subset of the subject's.

Decorator : Although decorators can have similar implementations as proxies, decorators have a different purpose. A decorator adds one or more responsibilities to an object, whereas a proxy controls access to an object.

Proxies vary in the degree to which they are implemented like a decorator. A protection proxy might be implemented exactly like a decorator. On the other hand, a remote proxy will not contain a direct reference to its real subject but only an indirect reference, such as "host ID and local address on host." A virtual proxy will start off with an indirect reference such as a file name but will eventually obtain and use a direct reference.


6The implementation of distributed objects in NEXTSTEP [Add94] (specifically, the class NXProxy) uses this technique. The implementation redefines forward, the equivalent hook in NEXTSTEP.

7Iterator describes another kind of proxy on .

8Almost all classes ultimately have Object as their superclass. Hence this is the same as saying "defining a class that doesn't have Object as its superclass."