TweetFollow Us on Twitter

December 92 - BE OUR GUEST

BE OUR GUEST

COMPONENTS AND C++ CLASSES COMPARED

DAVID VAN BRINK

[IMAGE 037-040_Van_Brink_rev1.GIF]

If you're familiar with C++ classes but new to thinking about components, you may find it instructive to know how the two compare. Although each has its own niche in Macintosh software development, components and C++ classes have many features in common.

In general, both components and C++ classes encourage a building-block approach to solving complex problems. But whereas a component is separate from any application that uses it, a class exists only within the application that uses it. Components are intended to add systemwide functionality, while classes are intended to promote a modular approach to developing a program.

We can also compare components and C++ classes in terms of how they're declared and called, their use of data hiding and inheritance, and their implementation. But first, let's briefly review what a class is and what a component is.

SOME BASIC DEFINITIONS
A class, in the programming language C++, is a description of a data structure and the operations (methods) that can be performed on it. An instance of a class is known as an object. Classes are provided in C++ to promote an "object-oriented programming style." By grouping a data type and its methods together, classes enable programmers to take a modular approach to developing a program.

A component, as described in the preceding article ("Techniques for Writing and Debugging Components"), is a single routine that accepts as arguments a selector and a parameter block. The selector specifies which of several (or many) operations to perform, and the parameter block contains the arguments necessary for that operation. Components are "registered" with the Component Manager and can be made available to either the program that registered the component or to any program that's executed, making it possible to add systemwide functionality. For instance, if Joe's Graphics Corporation develops a new image compression technique, it can be sold to users as a component. Users install the component simply by dragging an icon into a folder, and that form of image compression is then automatically available to all programs that make use of graphics.

DECLARING CLASSES AND COMPONENTS
A C++ class is declared in much the same way as a struct, with the addition of routines that operate only on the structure described. Once the class is declared, instances can be declared in exactly the same way as other variables. That is, to create an instance of a class, you either declare a variable of that class or dynamically allocate (and later deallocate) a variable of that class.

A component must be registered with the Component Manager. At that time, its type, subtype, manufacturer, and name are specified. The type, subtype, and manufacturer are long integers; the name is a string. Component instances can only be created dynamically, using specific Component Manager routines. To create an instance of a component that has been registered, a program must first find the component. If the seeking program is the same one that registered the component, it already has the component. If not, it can make Component Manager calls to search for all available components with a given type, subtype, and manufacturer; any part of the description can be a wild card.

Once a component has been found, it must be opened, and this operation produces a reference to the component instance. Operations can be performed on the component instance using this reference.

Table 1 compares how classes and components are declared and how instances of each are created. (Note that for components, the code is idealized.)

CALLING ALL ROUTINES
Calling a routine that operates on a C++ object is slightly different from making a standard routine call: the call more closely resembles a reference to an internal field of a struct. The routine that gets called is identical to any other routine, except that it's declared within the class definition rather than at the same brace level as the main routine.

Calling a component routine is identical to calling any other routine. The first argument is always the component instance, and other arguments may optionally follow. The return type of every component routine is a long integer, and part of the numerical range is reserved for error messages from either the component or the component dispatch mechanism.

The Component Manager lets a program issue calls to a component that it has never "met" before. This form of dynamic linking is crude, because no type checking is performed.

Table 1 compares how classes and components are called.

DATA HIDING
A C++ class can have "private" fields and methods, which are accessible by class methods but not by the caller. The programmer can see these private parts simply by perusing the class declaration. If a change to the implementation of a class requires that the private parts be changed, relinking with the implementation of the class won't be sufficient: all clients must be recompiled, since the positions of public fields might have changed. (One tricky way around this is to include a private field of type char * that's really a pointer to the class's internal state data. The class constructor allocates memory for whatever internal state it likes and coerces a pointer to it to live in that char * field. This technique is useful for object-only software library distribution and also protects proprietary algorithms from curious programmers.)

A component is responsible for allocating memory for its internal state (the component's "globals") when it's opened and releasing that memory when it's closed. There are both component globals and component instance globals. These correspond to static and automatic variables in a C++ class and have similar utility. A component might keep track of how many instances of itself have been opened and restrict that number by failing on the open call.

INHERITANCE
It's often useful to build software on top of existing functionality or, alternatively, to take existing functionality and alter it to perform a more specialized function. Both of these things can be accomplished for C++ classes with inheritance. In the former case, the new class will have methods that don't exist in the base class; in the latter, the new class will have methods with the same name as methods in the base class but that take precedence over the base methods.

Components and the Component Manager support both kinds of inheritance as well, as discussed in the preceding article. All components of a given type must support the same set of calls, although this is enforced only by convention. Components of a particular type and subtype may optionally support other calls as well, and components of a particular type, subtype, and manufacturer may support still more calls. In the case where a component wants to use the services of another component and perhaps override some of its functions with modifications, Component Manager utilities let a component designate another component as its "parent." A simple protocol ensures that the correct variant of a routine gets called. When a component must call itself, it must issue the call to its child component, if any. When a component wants to rely on the existing implementation of the parent component, it must pass the call to its parent.

IMPLEMENTING CLASSES AND COMPONENTS
My discussion of implementation is based on the 68000 platform, since that's the only one I've scrutinized with regard to compiled C++ and Component Manager calls.

The routines that can be used with a C++ class are declared, and optionally implemented, within the class declaration. They behave like normal C routines, as described earlier.

A call to a C++ class that has no parents or descendants is compiled as a direct subroutine call, exactly as is a standard routine call. A call to a C++ class that has parents or descendants is slightly more complicated. A table lookup is used at run time to determine exactly which implementation of a routine gets called for the particular object being operated on. Such a call takes perhaps a dozen assembly instructions.

A component consists of only a single routine. It's passed a selector and a parameter block. The selector is used to decide which operation to actually perform, and the parameter block contains all the arguments passed by the caller.

The component's parameter block is untyped -- the component routine has no way to determine what kinds of arguments were originally passed, and herein lies the danger. Some languages, such as LISP, have untyped arguments; in LISP, however, a routine can determine how many arguments have been passed and what the argument types are. A component interface is more like assembly language -- or C without prototypes! -- in that it can determine nothing about what has been passed to it.

You can't compile a C++ program containing a call to a nonexistent routine; the compiler will balk. (Well, OK, this isn't strictly true: there are dynamically linking systems for C++, and other languages, that let you call a C++ routine that hasn't been linked with the rest of the compiled source code; the routine can be linked to later, at run time. But no facility of this type is currently standard in the Macintosh Operating System or supported under the standard Macintosh development tools.) In the case of components, the compiler can't check for such illegal calls, since the particular components that may be opened are decided at run time. Therefore, the caller must be prepared to handle a "Routine Not Implemented" error if a call is made with an unknown selector.

All calls to components pass through the Component Manager's dispatch mechanism. The dispatcher must locate the component's entry point and globals from the component reference, which is not simply a pointer but a packed record containing an index into a table and some bits used to determine whether the component reference is still valid. If a client makes a call to a component it no longer has open, the Component Manager has a statistical likelihood of catching this call and returning an appropriate error.

The Component Manager has facilities to redispatch the parameter block to one of many routines, and those routines are written to take the arguments as originally passed. The Component Manager was originally written for use on the 68000 series of processor; on computers with that processor, the parameter block doesn't have to be recopied onto the stack for further dispatching. On other processors the parameters might have to be recopied, however.

The Component Manager has been highly optimized and fast dispatching can reduce its overhead still more, but in general its lookup-and-dispatch process still takes several dozen instructions. If the component being called is using the Component Manager's inheritance mechanism, further overhead is incurred by passing control to the parent or child component. Overall, the Component Manager is quite efficient, but still not as efficient as direct routine calls. Table 1 compares how classes and components are implemented.

IN SUM
Components, as supported by the Component Manager, exhibit many of the features of C++ classes. Both encourage a modular approach to solving problems. Both feature inheritance and data hiding. Where they differ is in how they're declared and implemented, how they do (or fail to do) type checking, and how expensive they are to call. Each occupies its own distinct niche in Macintosh programming: classes as a way to ease development of a single program, components as a way to add systemwide functionality and give control and choice to the user.


Table 1A Comparison of Calls: Classes (Actual Code) Versus Components (Idealized Code)

Declaring a Class

class MyClass {
/* Variables and methods for 
    the class */
}

Declaring a Component

myComponent = RegisterComponent(MyEntryRoutine,
        myType, mySubType, myManufacturer, "A Component");

Creating a Class Instance

MyClass x;

Creating a Component Instance

myComponent= FindComponent(myType, mySubType, myManufacturer);
myInstance = OpenComponent(myComponent);

Calling a Class

x.MyMethod(arg1, arg2);

Calling a Component

result = MyMethod(myInstance, arg1, arg2);

Implementing a Class

class MyClass {
    void MyMethod(int arg1, int arg2) {
    /* Some code for MyMethod */
    }
}

Implementing a Component

long MyEntryRoutine(ComponentParams *params, char *globals) {
    switch(params->selector) {
        case kOpen:
        case kClose:
            return noErr;
        . . . /* other required calls here */
        case MyMethod:
        /* Do my method. */
        /* arg1 and arg2 are in params. */ return noErr;
        default:
            return routineNotImplementedErr;
    }
}

DAVID VAN BRINK is a computer programmer. When he's not busy programming computers, he can usually be found writing computer programs. Mostly, he does this in the soothing fluorescent glow of his cubicle at Apple. He's presently writing components (with great fervor) to support musical synthesizers for QuickTime. *

We welcome guest columns from readers who have something interesting or useful to say. Send your column idea or draft to AppleLink DEVELOP or to Caroline Rose at Apple Computer, Inc., 20525 Mariani Avenue, M/S 75-2B, Cupertino, CA 95014.*


Thanks to Casey King and Gary Woodcock for reviewing this column. *

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Kodi 15.0.beta1 - Powerful media center...
Kodi (was XBMC) is an award-winning free and open-source (GPL) software media player and entertainment hub that can be installed on Linux, OS X, Windows, iOS, and Android, featuring a 10-foot user... Read more
DiskCatalogMaker 6.4.12 - Catalog your d...
DiskCatalogMaker is a simple disk management tool which catalogs disks. Simple, light-weight, and fast. Finder-like intuitive look and feel. Super-fast search algorithm. Can compress catalog data... Read more
Macs Fan Control 1.3.0.0 - Monitor and c...
Macs Fan Control allows you to monitor and control almost any aspect of your computer's fans, with support for controlling fan speed, temperature sensors pane, menu-bar icon, and autostart with... Read more
Lyn 1.5.11 - Lightweight image browser a...
Lyn is a lightweight and fast image browser and viewer designed for photographers, graphic artists and Web designers. Featuring an extremely versatile and aesthetically pleasing interface, it... Read more
NeoOffice 2014.11 - Mac-tailored, OpenOf...
NeoOffice is a complete office suite for OS X. With NeoOffice, users can view, edit, and save OpenOffice documents, PDF files, and most Microsoft Word, Excel, and PowerPoint documents. NeoOffice 3.x... Read more
LaunchBar 6.4 - Powerful file/URL/email...
LaunchBar is an award-winning productivity utility that offers an amazingly intuitive and efficient way to search and access any kind of information stored on your computer or on the Web. It provides... Read more
Remotix 3.1.4 - Access all your computer...
Remotix is a fast and powerful application to easily access multiple Macs (and PCs) from your own Mac. Features Complete Apple Screen Sharing support - including Mac OS X login, clipboard... Read more
DesktopLyrics 2.6.6 - Displays current i...
DesktopLyrics is an application that displays the lyrics of the song currently playing in "iTunes" right on your desktop. The lyrics for the song have to be set in iTunes; DesktopLyrics does nothing... Read more
VOX 2.5.1 - Music player that supports m...
VOX is a beautiful music player that supports many filetypes. The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features and support for all... Read more
Microsoft Remote Desktop 8.0.18 - Connec...
With Microsoft Remote Desktop, you can connect to a remote PC and your work resources from almost anywhere. Experience the power of Windows with RemoteFX in a Remote Desktop client designed to help... Read more

Biz Builder Delux (Games)
Biz Builder Delux 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: Ah, there's nothing like the rhythmic bustle of a burgeoning business burg... especially when you're the one building it... | Read more »
Auroch Digital is Bringing Back Games Wo...
| Read more »
Carbo - Handwriting in the Digital Age...
Carbo - Handwriting in the Digital Age 1.0 Device: iOS Universal Category: Productivity Price: $3.99, Version: 1.0 (iTunes) Description: | Read more »
Draggy Dead (Games)
Draggy Dead 1.1 Device: iOS Universal Category: Games Price: $.99, Version: 1.1 (iTunes) Description: Ditch your dead end job and take up a rewarding career in Grave Robbing today!Guide the recently deceased to a fun filled life of... | Read more »
Bad Dinos (Games)
Bad Dinos 1.0.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.0 (iTunes) Description: | Read more »
The Apple Watch isn't Great as a Fi...
| Read more »
Show the World What You See With Stre.am...
Live broadcasting is getting popular on mobile devices, which is why you can now get Stre.am, by Infinite Takes. [Read more] | Read more »
PhotoTime's 2.1 Update Adds Apple W...
The latest PhotoTime update is adding even more functionality to the handy photo organizing app. Yep, including Apple Watch support. [Read more] | Read more »
Oh My Glob! Adventure Time Puzzle Quest...
Finn and Jake are taking over D3 Go!'s popular puzzle game series in the upcoming Adventure Time Puzzle Quest. [Read more] | Read more »
Earthcore: Shattered Elements - Tips, Tr...
At first glance, Earthcore: Shattered Elements seems like a rather simple card-battling game. Once you’re introduced to skills that will change quite a bit. Even more so once you start to acquire hero cards. But it’s not so complicated that we... | Read more »

Price Scanner via MacPrices.net

Memorial Day Weekend Sale: New 27-inch 3.3GHz...
Best Buy has the new 27″ 3.3GHz 5K iMac on sale for $1899.99 this weekend. Choose free shipping or free local store pickup (if available). Sale price for online orders only, in-store prices may vary... Read more
OtterBox Maximizes Portability, Productivity...
From the kitchen recipe book to the boarsroom presentation, the OtterBox Agility Tablet System turns tablets into one of the most versatile pieces of handheld technology available. Available now, the... Read more
Launch of New Car App Gallery and Open Develo...
Automatic, a company on a mission to bring the power of the Internet into every car, has announced the launch of the Automatic App Gallery, an app store for nearly every car or truck on the road... Read more
Memorial Day Weekend Sale: 13-inch 1.6GHz Mac...
Best Buy has the new 13″ 1.6GHz/128GB MacBook Air on sale for $849 on their online store this weekend. Choose free shipping or free local store pickup (if available). Sale price for online orders... Read more
Memorial Day Weekend Sale: 27-inch 3.5GHz 5K...
Best Buy has the 27″ 3.5GHz 5K iMac on sale for $2099.99 this weekend. Choose free shipping or free local store pickup (if available). Sale price for online orders only, in-store prices may vary.... Read more
Sale! 16GB iPad mini 3 for $349, save $50
B&H Photo has the 16GB iPad mini 3 WiFi on sale for $349 including free shipping plus NY sales tax only. Their price is $50 off MSRP, and it’s the lowest price available for this model. Read more
Price drop on 2014 15-inch Retina MacBook Pro...
B&H Photo has dropped prices on 2014 15″ Retina MacBook Pros by $200. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799.99 save $200 - 15″ 2.5GHz... Read more
With a Mission to Make Mobile Free, Scratch W...
Scratch Wireless, claiming to be the world’s first truly free mobile service, has announced the availability of a new Scratch-enabled Android smartphone, the Coolpad Arise. The smartphone is equipped... Read more
First-Ever Titanium Alloy Curved iPhone 6 Scr...
One of the most common problems with mobile phones is damage to the screens. The slightest drop can cause a dreaded spider web of gashes and cracks in the glass panel surface that can cost $hundreds... Read more
Preorder new 12-inch MacBook, $10 off, save o...
Adorama has new 12″ Retina MacBooks available for preorder for $10 off MSRP including free shipping plus NY & NJ sales tax only. For a limited time, Adorama will include a free Apple USB-C to USB... Read more

Jobs Board

*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
*Apple* TV Live Streaming Frameworks Test En...
**Job Summary** Work and contribute towards the engineering of Apple 's state-of-the-art products involving video, audio, and graphics in Interactive Media Group (IMG) at Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Partner Marketing Manager, Merchant- *Apple*...
**Job Summary** The Apple Pay partner marketing team is looking for a marketing manager to develop and drive US marketing programs with our merchant partners. The right Read more
Technical Project Manager - *Apple* Pay - A...
**Job Summary** Apple Pay is seeking an experienced technical PM to…manage the on boarding of new merchants for the Apple Pay platform in the US Within this role you Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.