TweetFollow Us on Twitter

January 93 - Persistent Objects and Object-Oriented Databases for C++

Persistent Objects and Object-Oriented Databases for C++

Dirk Bartels and Jonathan Robie

Reprinted with permission from C++ Report September 1992. Copyright© 1992 SIGS Publications, 588 Broadway, NY, NY; 212/274-0640; 212/274-0646 (fax).

Many programmers first learn about object-oriented programming by buying a C++ compiler and a GUI class library. These libraries are generally well structured, support true object-oriented design and offer a simple, powerful programming interface. This is often enough to convince programmers to use true object-oriented designs. Unfortunately, many of these programmers find themselves tearing their designs apart when they integrate their programs with conventional database systems, which provide almost no support for objects or for expressing the relationships among objects.

Conventional databases are good at managing large amounts of data, sharing data among programs, and fast value-based queries. They are not very good at modeling the relationships among data, however, since everything must be represented as series of a two-dimensional tables.

Object-oriented database systems (OODBS) are a relatively new tool for software developers. Unlike relational and table-oriented systems, they provide full support for the object-oriented programming model used in languages like C++ and Smalltalk. This model is intuitive, good at modeling relationships, and very suitable for large software projects.

An object-oriented database combines the semantics of an object-oriented programming language with the data management and query facilities of a conventional database system. This makes it easy to manage large amounts of data and to model the relationships among the data. If an object-oriented database is integrated with an object-oriented language, it should support the semantics of that language; relationships established in the program should automatically be represented in the database when objects are stored.

This article focuses on the advantages of object-oriented databases over conventional table-oriented and relational databases and the integration of an OODBS into C++. For small applications these advantages mean that your program will be less complex and easier to understand. For large or complex applications these advantages may mean the difference between success and failure.

LIMITS OF CONVENTIONAL DATABASE SYSTEMS

Database systems are designed for managing large amounts of data, and they provide many important features that object-oriented programming languages do not: permanent storage, fast queries, sharing of objects among programs, device independent formats, and sophisticated error handling for database operations.

Relational database systems (RDBS) and table-oriented systems based on B-Tree or Indexed Sequential Access Method (ISAM) are the standard systems currently used in most software development. Each requires that all data be portrayed as a series of two-dimensional tables. The relational model declares the structures, operations, and design principles to be used with these tables.

These systems are quite appropriate for some applications and were a real breakthrough in their time, but software developers are rapidly learning that life is not a series of two-dimensional tables. The growing complexity of modern programs and the increasing use of dynamic data models have pushed traditional databases to their limit. The limited data models they support can result in significant software development costs since they do not allow program designs that closely match the problem domain. They are not even worth considering for some application areas like computer-aided design (CAD), computer-aided engineering (CAE), multimedia, and office automation.

Limited Data Types

Modern software systems often contain data types that are not easily modeled using such predeclared types. For example, a CAD program might have an array of shapes, or a desktop publishing program might model a page as a series of frames which may contain bitmaps, paragraphs, or vector drawings. We have already seen that object-oriented programs allow us to declare new data types as needed.

Conventional databases have a fixed set of data types. The better systems include both simple data types like INTEGER, FLOAT, or CHAR and complex data types like DATE, TIME, or CURRENCY. New data types cannot be added by the user. If your database does not have the data type you need then you are stuck. Aggregate data types like arrays are rarely available. The only way to group data is to put it in a table.

Limited Modeling of Data Relationships

In conventional database systems, each item is represented as a row in a table. Tables may be accessed sequentially or by searching for values. The only way to express relationships among items is by setting values in the rows. In each table one or more columns is chosen as the primary key; this key must be unique for each row in the table. For instance, the primary keys for a student, a teacher, and a class might each be represented as identification numbers.

The relational model is weak when showing many-to-one relationships, which generally require the introduction of a new table. In our example, the only way to show which students are taking a class is to create an "enrollment" table which has a row for each student and contains the student identification number and class identification number in each row.

Since relational databases have no concept of hierarchy, it is difficult to model the ISA relationship. Suppose we have a "people" table, a "students" table, and a "teachers" table. Every student is also a person, and some of his fields are in each table. To update all of a student's information you must find the rows of each table whose identification numbers match. Every level in the hierarchy requires a new table, and every program using the database must update every relevant table appropriately. The hierarchy is not explicitly represented in the database; you simply have to know why the various tables are there.

No Way of Grouping Code With Data

We have already seen that object-oriented programming languages allow related code and data to be combined to form objects. There is no way to do this in a conventional database system. If you know the name of a table you may use it, and the system will not prevent you from changing the wrong table. As long as you have the right password everything in the database is globally accessible to all of your code.

Limited Manipulation of Data

Database languages are often very poor at manipulating data. SQL, for instance, does not allow you to perform computations on your data as input to a query, nor does it allow you to perform computations on the result of a query. A computer language designer would say that SQL is not computationally complete even though it is relationally complete; a normal human being might say that SQL is great for searching but lousy for anything else. Because of this, most serious applications are written in conventional programming languages using some kind of SQL-based interface to the database.

Poor Integration

Since the database and the host programming language use two different models and different data types the programmer must either perform all operations directly in the database or constantly convert between the two systems. The first method does not let the programmer use many features of the host language; the second means a great deal of overhead and frustration since the relationships among data must be constantly converted to support both programming models. Such a program has two distinct designs, one for the program itself and one for the database.

Summary

To store data in a conventional database, it must be dissected into a series of two-dimensional tables. Only predeclared data types are supported. Object-oriented programming languages have a rich set of features for creating data types and representing the relationships among data that are not supported in such databases. In the rest of this article, we discuss features that an object-oriented database must support. To illustrate these features, we examine POET, a commercial object-oriented database system with which we are connected.

POET
Persistence

The original implementation of Smalltalk had a simple method for storing objects: the program's entire memory image could be dumped to disk and restored when running the program later. This scheme has some real advantages. It is very simple to implement, requires almost no effort from the programmer, and fully implements all aspects of the programming language (after all, the program sticks everything in memory somewhere!). It also has some real disadvantages. The number of objects that can be stored depends on the amount of main memory available, the programming context must be stored and retrieved as a whole, objects may not be shared among programs or retrieved on another kind of computer, and there is no way to implement intelligent error recovery.
 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Sandvox 2.10.2 - Easily build eye-catchi...
Sandvox is for Mac users who want to create a professional looking website quickly and easily. With Sandvox, you don't need to be a Web genius to build a stylish, feature-rich, standards-compliant... Read more
LibreOffice 5.0.1.2 - Free, open-source...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
f.lux 36.1 - Adjusts the color of your d...
f.lux makes the color of your computer's display adapt to the time of day, warm at night and like sunlight during the day. Ever notice how people texting at night have that eerie blue glow? Or wake... Read more
VirtualBox 5.0.2 - x86 virtualization so...
VirtualBox is a family of powerful x86 virtualization products for enterprise as well as home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers... Read more
File Juicer 4.43 - Extract images, video...
File Juicer is a drag-and-drop can opener and data archaeologist. Its specialty is to find and extract images, video, audio, or text from files which are hard to open in other ways. In computer... Read more
Apple MainStage 3.2 - Live performance t...
MainStage 3 makes it easy to bring to the stage all the same instruments and effects that you love in your recording. Everything from the Sound Library and Smart Controls you're familiar with from... Read more
formZ 8.5 - Solid and surface modeler. (...
formZ pro is a powerful 3D design application featuring a variety of modeling personalities and tools with an easy to use interface to express and communicate your imagination. It is based on... Read more
PopChar 7.2 - Floating window shows avai...
We're also selling a 5-license family pack for only $25.99! PopChar helps you get the most out of your font collection. With its crystal-clear interface, PopChar provides a frustration-free way to... Read more
FileMaker Pro 14.0.2 - Powerful, easy-to...
FileMaker Pro is powerful, easy-to-use software used to create custom solutions for your business that run on iPad, iPhone, Windows, Mac and the web. Use FileMaker Pro to manage and share information... Read more
djay Pro 1.2 - Transform your Mac into a...
djay Pro provides a complete toolkit for performing DJs. Its unique modern interface is built around a sophisticated integration with iTunes and Spotify, giving you instant access to millions of... Read more

ReBoard: Revolutionary Keyboard (Utilit...
ReBoard: Revolutionary Keyboard 1.0 Device: iOS Universal Category: Utilities Price: $1.99, Version: 1.0 (iTunes) Description: Do everything within the keyboard without switching apps! If you are in WhatsApp, how do you schedule a... | Read more »
Tiny Empire (Games)
Tiny Empire 1.1.3 Device: iOS Universal Category: Games Price: $2.99, Version: 1.1.3 (iTunes) Description: Launch cannonballs and blow tiny orcs into thousands of pieces in this intuitive fantasy-themed puzzle shooter! Embark on an... | Read more »
Astropad Mini (Productivity)
Astropad Mini 1.0 Device: iOS iPhone Category: Productivity Price: $4.99, Version: 1.0 (iTunes) Description: *** 50% off introductory price! ​*** Get the high-end experience of a Wacom tablet at a fraction of the price with Astropad... | Read more »
Emo Chorus (Music)
Emo Chorus 1.0.0 Device: iOS Universal Category: Music Price: $1.99, Version: 1.0.0 (iTunes) Description: Realistic Choir simulator ranging from simple Chorus emulation to full ensemble Choir with 128 members. ### introductory offer... | Read more »
Forest Spirit (Games)
Forest Spirit 1.0.5 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.5 (iTunes) Description: | Read more »
Ski Safari 2 (Games)
Ski Safari 2 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: The world's most fantastical, fun, family-friendly skiing game is back and better than ever! Play as Sven's sister Evana, share... | Read more »
Lara Croft GO (Games)
Lara Croft GO 1.0.47768 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.47768 (iTunes) Description: Lara Croft GO is a turn based puzzle-adventure set in a long-forgotten world. Explore the ruins of an ancient... | Read more »
Whispering Willows (Games)
Whispering Willows 1.23 Device: iOS Universal Category: Games Price: $4.99, Version: 1.23 (iTunes) Description: **LAUNCH SALE 50% OFF** - Whispering Willows is on sale for 50% off ($4.99) until September 9th. | Read more »
Calvino Noir (Games)
Calvino Noir 1.1 Device: iOS iPhone Category: Games Price: $3.99, Version: 1.1 (iTunes) Description: The film noir stealth game. Calvino Noir is the exploratory, sneaking adventure through the 1930s European criminal underworld.... | Read more »
Angel Sword (Games)
Angel Sword 1.0 Device: iOS Universal Category: Games Price: $6.99, Version: 1.0 (iTunes) Description: Prepare to adventure in the most epic full scale multiplayer 3D RPG for mobile! Experience amazing detailed graphics in full HD.... | Read more »

Price Scanner via MacPrices.net

12-inch 1.1GHz Gold MacBook on sale for $1149...
B&H Photo has the 12″ 1.1GHz Gold Retina MacBook on sale for $1149.99 including free shipping plus NY sales tax only. Their price is $150 off MSRP, and it’s the lowest price available for this... Read more
27-inch 3.3GHz 5K iMac on sale for $1849, sav...
Best Buy has the 27″ 3.3GHz 5K iMac on sale for $1849.99. Their price is $150 off MSRP, and it’s the lowest price available for this model. Choose free shipping or free local store pickup (if... Read more
Worldwide Tablet Shipments Expected to Declin...
Does Apple badly need a touchscreen convertible/hybrid laptop MacBook? Yes, judging from a new market forecast from the International Data Corporation (IDC) Worldwide Quarterly Tablet Tracker, which... Read more
Continued PC Shipment Shrinkage Expected Thro...
Worldwide PC shipments are expected to fall by -8.7 percent in 2015 and not stabilize until 2017, according to the latest International Data Corporation (IDC) Worldwide Quarterly PC Tracker data. The... Read more
Imminent iPhone 6s Announcement Leads To 103%...
NextWorth Solutions, with its online and in-store electronics trade-in programs including http://NextWorth.com, reports that it has experienced a 103 percent surge in quoted trade-in values over the... Read more
Weekend Deal: 13-inch Retina MacBook Pros for...
Save up to $100 on the purchase of a new 2015 13″ Retina MacBook Pro at the following resellers this weekend. Shipping is free with each model: 2.7GHz/128GB MSRP $1299 2.7GHz/... Read more
13-inch 2.5GHz MacBook Pro on sale for $999,...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $999.99 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Read more
Apple refurbished iPad Air 2s available for u...
The Apple Store has Apple Certified Refurbished iPad Air 2s available for up to $140 off the price of new models. Apple’s one-year warranty is included with each model, and shipping is free: - 128GB... Read more
The ‘Book Mystique Still Magic After All Thes...
This column has been called The ‘Book Mystique for what has been a 15 year run so far, and I have no interest in changing the name. The reference is to what I’ve observed is a the near magical... Read more
15-inch Retina MacBook Pros on sale for up to...
B&H Photo has 2015 15″ Retina MacBook Pros on sale for up to $184 off MSRP including free shipping plus NY sales tax only: - 15″ 2.2GHz Retina MacBook Pro: $1815 $184 off - 15″ 2.5GHz Retina... Read more

Jobs Board

*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
Software QA Engineer, *Apple* Pay Security...
Changing the world is all in a day039s work at Apple . If you love innovation, here039s your chance to make a career of it. You039ll work hard. But the job comes with 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
*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* 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
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.