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

Apple iOS 8.1.3 - The latest version of...
The latest version of iOS can be downloaded through iTunes. Apple iOS 8 comes with big updates to apps you use every day, like Messages and Photos. A whole new way to share content with your family.... Read more
SpamSieve 2.9.19 - Robust spam filter fo...
SpamSieve is a robust spam filter for major email clients that uses powerful Bayesian spam filtering. SpamSieve understands what your spam looks like in order to block it all, but also learns what... Read more
RapidWeaver 6.0.5 - Create template-base...
RapidWeaver is a next-generation Web design application to help you easily create professional-looking Web sites in minutes. No knowledge of complex code is required, RapidWeaver will take care of... Read more
Duet 1.2.2 - Use your iPad as an externa...
Duet is the first app that allows you to use your iDevice as an extra display for your Mac using the Lightning or 30-pin cable. Note: This app requires a $14.99 iOS companion app. Version 1.2.2:... Read more
Alfred 2.6 - Quick launcher for apps and...
Alfred is an award-winning productivity application for OS X. Alfred saves you time when you search for files online or on your Mac. Be more productive with hotkeys, keywords, and file actions at... Read more
1Password 5.1 - Powerful password manage...
1Password is a password manager that uniquely brings you both security and convenience. It is the only program that provides anti-phishing protection and goes beyond password management by adding Web... Read more
GarageSale 6.9.2 - Create outstanding eB...
GarageSale is a slick, full-featured client application for the eBay online auction system. Create and manage your auctions with ease. With GarageSale, you can create, edit, track, and manage... Read more
calibre 2.17 - Complete e-library manage...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
OmniGraffle Pro 6.1.2 - Create diagrams,...
OmniGraffle Pro helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use... Read more
OmniGraffle 6.1.2 - Create diagrams, flo...
OmniGraffle helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use Graffle to... Read more

New WOTA: U-Boat Compass ‘Wets’ Your App...
New WOTA: U-Boat Compass ‘Wets’ Your Appetite for WOTA: Wolves of the Atlantic. Posted by Jessica Fisher on January 27th, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Warship Solitaire Review
Warship Solitaire Review By Nadia Oxford on January 27th, 2015 Our Rating: :: A WAR OF NUMBERSUniversal App - Designed for iPhone and iPad Warship Solitaire might lack frills and looks, but it offers some thought provoking... | Read more »
Keep Track of Your Hectic Work Schedule...
Keep Track of Your Hectic Work Schedule With Shifts Posted by Jessica Fisher on January 27th, 2015 [ permalink ] iPhone App - Designed for the iPhone, compatible with the iPad | Read more »
All Star Quarterback Review
All Star Quarterback Review By Campbell Bird on January 27th, 2015 Our Rating: :: PLAY YOUR PARTUniversal App - Designed for iPhone and iPad Train your way to the Super Bowl in this free-to-play sports management game.   | Read more »
Sentinel 4: Dark Star Adds New Campaign...
Sentinel 4: Dark Star Adds New Campaign and More Posted by Ellis Spice on January 27th, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Wheel & Deal Review
Wheel & Deal Review By Tre Lawrence on January 27th, 2015 Our Rating: :: SIMPLE FUNUniversal App - Designed for iPhone and iPad A simple arcade shooter that has a retro feel.   | Read more »
Join the NAVY in a Big New Update for WW...
Join the NAVY in a Big New Update for WW2: Sandbox. Strategy & Tactics Posted by Jessica Fisher on January 27th, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Earthworm Alchemy Review
Earthworm Alchemy Review By Campbell Bird on January 27th, 2015 Our Rating: :: FEED THE WORMUniversal App - Designed for iPhone and iPad Feed this funny looking worm and dodge bombs in this charming, free-to-play arcade game.   | Read more »
Graphic Novel App netwars – The Butterfl...
Graphic Novel App netwars – The Butterfly Attack has Received Some New Episodes Posted by Jessica Fisher on January 27th, 2015 [ permalink ] | Read more »
DataVault Password Manager Review
DataVault Password Manager Review By Jennifer Allen on January 27th, 2015 Our Rating: :: SECURE DATA STORAGEUniversal App - Designed for iPhone and iPad Store your passwords and other sensitive information safely, thanks to... | Read more »

Price Scanner via MacPrices.net

Save up to $340 on iMacs with Apple Certified...
The Apple Store has Apple Certified Refurbished iMacs available for up to $340 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free. These are the best prices on... Read more
AppleCare Protection Plans on sale for up to...
B&H Photo has 3-Year AppleCare Warranties on sale for up to $105 off MSRP including free shipping plus NY sales tax only: - Mac Laptops 15″ and Above: $244 $105 off MSRP - Mac Laptops 13″ and... Read more
Apple refurbished Time Capsules available sta...
The Apple Store has certified refurbished Time Capsules available for up to $60 off MSRP. Apple’s one-year warranty is included with each Time Capsule, and shipping is free: - 2TB Time Capsule: $255... Read more
3TB AirPort Time Capsule on sale for $349, $5...
B&H Photo has 3TB AirPort Time Capsules 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 (except... Read more
16GB iPad Air 2 on sale for $446, save $53
Walmart has 16GB WiFi iPad Air 2s on sale for $53 off MSRP on their online store. Choose free shipping or free local store pickup (if available). Sale prices available for online orders only, in-... Read more
Save up to $300 on a new Mac, $30 on an iPad,...
Purchase a new Mac or iPad at The Apple Store for Education and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free,... Read more
MasterCard Brings Contactless Payment, Apple...
MasterCard, the official card of the Waste Management Phoenix Open, and the PGA TOUR are bringing the latest payment technology to the spectator experience with the introduction of contactless... Read more
Petitioning Dropbox For Mac OS X 10.4 and 10....
Last week Dropbox announced to its users that app support for Mac OS X 10.4 and 10.5 will end May 18 — disappointing news for those of us who are still getting useful service out of older PPC Macs... Read more
Stop Street Harassment, Bullying, and Assault...
The STOP-ATTACK (http://www.stop-attack.com) app will leverage smartphone technology to make the world a safer place. Whether it’s bullying, street harassment or something even more sinister, the app... Read more
Stir Kinetic Desk M1 Standing Or Sitting Desk...
The age of the standing desk is upon us, and according to medical research, it’s arriving none too soon. The World Health Organization (WHO), reports that 60 to 85 percent of people worldwide lead... 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* Solutions Consultant - Retail Sales...
As an ambassador of the Apple brand, the ASC is accountable for driving sales performance by: Connecting with customers. Discovering customers' needs and values. Showing Read more
*Apple* Solutions Consultant- Retail Sales -...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Resellers store. The ASCs role is to grow Apple product Read more
Detailer *Apple* Ford Body Shop / Collision...
Apple Automotive is one of the fastest growing dealer…and it shows. Consider making the switch to the Apple Automotive Group today! At Apple Automotive , Read more
*Apple* Acura/Subaru Service Technicians - A...
Apple Automotive is one of the fastest growing dealer…and it shows. Consider making the switch to the Apple Automotive Group today! At Apple Automotive , Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.