TweetFollow Us on Twitter

Using Assert()

Volume Number: 13 (1997)
Issue Number: 12
Column Tag: Beginner's Corner

Using Assert()

by Peter N. Lewis, Perth, Australia

Understanding assertions and how and when to use them

Introduction

Many programmers believe it is impossible to write bug free code. They just assume bugs are a part of life and that beta testers and QA departments (or even end users!) will find and report the bugs which will then (hopefully) be tracked down and resolved. I'm not sure I believe it is possible to write bug free code, but one thing I certainly believe is that it will not happen without a conscious effort on the part of the programmer.

Once you have decided that writing bug free code is a worthwhile goal, the first and most important tool at your disposal is the "Assertion". An assertion is simply a piece of code that validates part of the state of your program, and alerts you if something is wrong. This article describes assertions, why you want to use them, what they are, when and where to use them, and how you implement them. Throughout this article I will use examples in Pascal or C, but the concepts apply to almost any language.

Why You Should Use Assertions

Bugs come in many shapes and sizes, but there is a general rule of thumb that the earlier you detect a bug the less time it takes to fix it:

  • If you detect it as you are typing, it takes basically no time.
  • If you detect it during syntax check, compile, or link, it takes only a few seconds.
  • If you detect it as soon as the program launches, it takes only a minute.
  • If you detect it in your own testing, you probably will not waste much time, especially if an assertion fires to tell you exactly what went wrong.
  • If you send it to your beta testers/QA department, you are wasting days (and other people's time).
  • If you ship it and your end users find the bugs, the results can be arbitrarily bad. (Imagine your company going out of business because of bad reviews of your buggy product!)

Assertions make it easier to detect bugs earlier. By automatically detecting bugs you can find them before the bug has a chance to cascade and destroy the evidence. The more liberally you use assertions, the more quickly you will find bugs. And because assertions can be "compiled out" of your code (I'll show you how to do this in C and Pascal later), assertions only slow down your beta versions so you are free to use lots of them.

What is an Assertion?

At its most basic form, an assertion is simply a procedure that takes a boolean parameter and reports (to the programmer) if the boolean is false, for example:

procedure Assert( must: boolean );
    begin
    if not must then begin
      DebugStr( 'Assertion failed;sc' );
    end;
  end;

You can report the error using any method you like (DebugStr, Alert, writeln/printf, etc), and can include any extra information you want (such as the source file and line or a text message explaining what happened). I generally use DebugStr since it will work even in interrupt level code, but it does require you install MacsBug or some other low level debugger. Also, since the Metrowerks Debugger can catch DebugStr and leave you pointing at exactly the place where the assertion failed (once you step out of the Assert function), there is no need to include an explanatory message.

It is important to note that assertions are not a form of error checking. Assertions exist to detect programatical errors, they are not useful for detecting real life error conditions like disk or network errors -- errors such as these must be detected, handled and reported by error checking code that remains in the shipping version and that reports to the user in a helpful manner. Assertions are to help you as the programmer, your users should never see them if you do your job properly.

When and Where to Use Assertions

The short answer is to use assertions everywhere. Anywhere you are using facts about your programs state that are not obvious from the proceeding lines of code, you should consider using an assertion to confirm that the "facts" really are true. The most important places are:

  • At the start of each procedure (check that the parameters are acceptable).
  • At the start of each loop (check that the loop invariants hold true).
  • At the end of each loop (check that the loop has done its job).
  • At the end of each procedure (check that the procedure has done its job).
  • Before using any pointer (check that the pointer is not nil).
  • Before using any structure (check that the structure is valid).

For example, say we want to write a routine that accepts two string pointers, source and dest, where the source is suppose to be an 8 character string of lowercase letters, and its job is to uppercase the string and store it in dest. With assertions added, we might write it like this:

procedure Uppercase( source: StringPtr; dest: StringPtr );
  const
    required_length = 8;
  var
    i: integer;
begin
  Assert( (source <> nil) & (length(source^) = required_length) );
  Assert( dest <> nil );    // Ideally we would like to test that dest^
                          // is long enough
  Assert( source <> dest );  // Ideally we would like to test that they 
                          // do not overlap
  dest^[0] := chr(required_length);
  for i := 1 to required_length do begin
    Assert( source^[i] in ['a'..'z'] );
    dest^[i] := UpCase( source^[i] );
  end;
  Assert( EqualString( source^, dest^, false, true ) );
end;

So, we start off checking the preconditions (that source is not nil and is the right length and that dest is not nil). We should really also check that source is made up entirely of lowercase letters, but we defer that to the loop where it is easier to check. And then at the end we check that we have done the job - it is a pretty loose test, (checking only that dest is case insensitively equal to source), but it at least checks that we have done something like what we said we would do.

We also assert that source must not be the same as dest -- it would be easy enough to ensure that the code worked properly in this case, but I don't feel like checking the code for that case so instead I save myself some work and simply disallow it - if at some future time a programmer tries to use this routine with that case they will immediately get an assertion, they can then either fix their code to use two strings, or update Uppercase to ensure this code works properly in the case where source and dest are the same.

This is another use for assertions, they are a form of self documenting code. If I simply added a comment to the documentation that source and dest are not allowed to overlap, a programmer might not notice and might accidentally use the procedure in this manner. Worse, the code might work sometimes but not always. It is much better to enforce the restriction in the code so that a any future user of this routine immediately learns of their mistake.

Compiler Generated Assertions and Warnings

It is worth noting that the compiler is capable of generating some assertions of its own, and you should take advantage of these whenever possible. Take the time to go through the compiler settings and ensure all possible warnings and checks are enabled. For example, the compiler may have range checking or nil checking options. It may also be able to detect things like unused variables, variables used before they are initialised and functions that do not return results. These warnings and errors can save you a lot of time so turn them on!

Duplicate your code and check your data structures

In the example above, we checked at the end of the routine that dest was case insensitively equal to source. We can actually go further and check that dest is exactly what it should be by duplicating the routine, something like this:

var
    i: integer;
{$ifc do_debug}
    test_string: Str255;
{$endc}
begin
  ...
{$ifc do_debug}
  test_string := source^;
  UpperString( test_string, false );
  Assert( dest^ = test_string );
{$endc}
end;

Now when this routine executes we don't have to wonder if it is doing the right thing and hope that we spot the problem if it isn't. We know it works correctly every time because if it ever fails it will immediately notify us of the problem.

Note how the debugging variable test_string is compiled out if do_debug is false. This is for two reasons, first it avoids the unused variable warning when you build the non-debugging version, but more importantly it makes it clear that test_string is for debugging purposes only and ensures it is not accidentally used in the "real" code.

Many programs have a single important job that they perform. For example, in a drawing program it might be rendering to the screen, in a spreadsheet it might be the recalculation engine, in a game it might be updating the game state. These all involve taking some input state and mapping it to a new state. You can use assertions to validate your code in two important ways:

  • By checking that the state is valid before and after the update.
  • By duplicating the engine and running both and ensuring they get the same results.

Checking the state is generally pretty easy, you just go through each variable in each structure and ensure that it is within acceptable ranges. For each interaction, you ensure the variables are compatible. For example in a drawing package, you might assert that each object is within range, that each rectangle has four points, that each colour or pattern is valid, that each group is made up of objects that are inside the group, and so forth.

Duplicating the code engine can be a fair amount of work, but it can also be very valuable. Often these engines must be very efficient so they end up being highly optimised. At the start of the project, you might write a very simple engine as a proof of concept - rather than throw this engine away, keep it and execute it in parallel with the new optimised engine you write and then check that the results are identical. For example, in a drawing package, you might do something like this:

procedure UpdateOffscreenWorld( offscreen: GWorldPtr );
  var
    rgn: RgnHandle;
{$ifc do_debug}
    debug_world: GWorldPtr;
{$endc}
begin
  FindChangeRegion( rgn );
  RedrawOnlyChanges( offscreen, rgn );
  DisposeRgn( rgn );
{$ifc do_debug}
  MakeNewOffscreenWorld( debug_world );
  DrawEverything( debug_world );
  Assert( IdenticalBits( offscreen, debug_world ) );
{$endc}
end;

How to Implement Assertions

As described above, the basic assertion is simply a procedure that takes a boolean and reports if the boolean is false. However, since computing the assertion condition may be computationally expensive and since it does not in any way affect the execution of the program, it is desirable to have them automatically removed from your code before you ship the final version. To do this, we use compiler macros (#define in C, {$definec} in Pascal) like this:

#ifndef do_debug
#define do_debug 1
#endif

#if !do_debug
#define Assert(b)
#else
#define Assert(b) AssertCode(b)
#endif

#if do_debug
void AssertCode( Boolean b );
#endif

Or

{$ifc not defined do_debug}
{$setc do_debug := 1}
{$endc}

{$ifc not do_debug}
{$definec Assert(b)}
{$elsec}
{$definec Assert(b) AssertCode(b)}
{$endc}

{$ifc do_debug}
  procedure AssertCode (b: boolean);
{$endc}

First, we default do_debug to true. Then we define the macro, mapping Assert( condition ) to either nothing at all or to a call to AssertCode -- the actual procedure is renamed to AssertCode so that it's definition is not mangled by the Assert macro. For final builds you can use a prefix file to set do_debug to false and then recompile all your source.

There are two things you have to be careful with. First, since the macro mechanism effectively removes the Assert lines from your program, you must never use a function with a side effect in your assertion. For example, you might be tempted to do something like this:

Assert( NewGWorld( ... ) == noErr );

However, when you compile that with do_debug set to false, the line will disappear and the GWorld will not be created. Instead you should write:

err = NewGWorld( ... );
Assert( err == noErr );

For this reason, and just for general safety, it is import that you set do_debug to false for at least your last few beta builds (after you have resolved all bugs that cause assertions to fire of course!) so that you can get some serious testing with a build that is almost identical to the final build.

Extending Assertions

Assertions can be as simple or as complicated as you choose to make them. I have described a very simple implementation, but you can expand on the concept in several ways.

You could use a more interesting reporting mechanism than simple DebugStrs such as Alerts or sending the reports out a serial or TCP connection. You could build on what you want to assert, such as asserting that a pointer or file reference or TCP stream is valid. You could also add information to the assertion such as the file name or line number or a message describing the cause of the assertion.

Always keep in mind that you want to use assertions frequently in all your code so there may be some constraints on what you can do in the Assert routine if you are writing any low level code like interrupt routines or drivers, and you should avoid making the act of including an assertion overly tedious (that is why I generally don't include an explanatory message in my assertions).

You can also look around for other assertion libraries -- CodeWarrior's MSL and PowerPlant both include support for assertions.

Conclusion

The single most important question to ask yourself whenever you find a bug in your code is "How could I have prevented this bug?" or at the very least "How could I have found this bug earlier?" Assertions are one way of finding bugs very early. Steve Maguire's excellent book, Writing Solid Code, describes assertions and many other ways of finding or preventing bugs (including stepping through any new code you write, writing good interfaces, choosing safe/debugable implementations).

These techniques really do work. They will save you time and frustration, and they will dramatically increase the level of confidence you have in your code. Where you would previously have said "this routine probably does more or less what I expect" you can say with confidence that it does exactly what it is suppose to do, and if it ever fails, you'll hear about it immediately. So if you are going to write code (especially if it will end up running on my Mac!) go and read Writing Solid Code, get an attitude adjustment, and start writing bug free code!

References

  • Writing Solid Code by Steve Maguire. This book is the definitive reference in my opinion. I believe all programmers should read this book, it does a great job at explaining this topic and at motivating the reader to strive to write bug-free code.
  • Effective C++ by Scott Meyers. C++ has many ways to introduce bugs in to your code that are very difficult to debug. Effective C++ describes many of these and how to avoid them.

Peter N. Lewis is a successful shareware author. He founded Stairways Software Pty Ltd in 1995 and specializes in Macintosh TCP/IP products but has been known to diversify into other areas.

 
AAPL
$118.85
Apple Inc.
-0.15
MSFT
$48.10
Microsoft Corpora
+0.35
GOOG
$541.36
Google Inc.
+0.99

MacTech Search:
Community Search:

Software Updates via MacUpdate

Skype 7.2.0.412 - Voice-over-internet ph...
Skype allows you to talk to friends, family and co-workers across the Internet without the inconvenience of long distance telephone charges. Using peer-to-peer data transmission technology, Skype... Read more
HoudahSpot 3.9.6 - Advanced file search...
HoudahSpot is a powerful file search tool built upon MacOS X Spotlight. Spotlight unleashed Create detailed queries to locate the exact file you need Narrow down searches. Zero in on files Save... Read more
RapidWeaver 6.0.3 - 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
iPhoto Library Manager 4.1.10 - Manage m...
iPhoto Library Manager lets you organize your photos into multiple iPhoto libraries. Separate your high school and college photos from your latest summer vacation pictures. Or keep some photo... Read more
iExplorer 3.5.1.9 - View and transfer al...
iExplorer is an iPhone browser for Mac lets you view the files on your iOS device. By using a drag and drop interface, you can quickly copy files and folders between your Mac and your iPhone or... Read more
MacUpdate Desktop 6.0.3 - Discover and i...
MacUpdate Desktop 6 brings seamless 1-click installs and version updates to your Mac. With a free MacUpdate account and MacUpdate Desktop 6, Mac users can now install almost any Mac app on macupdate.... Read more
SteerMouse 4.2.2 - Powerful third-party...
SteerMouse is an advanced driver for USB and Bluetooth mice. It also supports Apple Mighty Mouse very well. SteerMouse can assign various functions to buttons that Apple's software does not allow,... Read more
iMazing 1.1 - Complete iOS device manage...
iMazing (was DiskAid) is the ultimate iOS device manager with capabilities far beyond what iTunes offers. With iMazing and your iOS device (iPhone, iPad, or iPod), you can: Copy music to and from... Read more
PopChar X 7.0 - Floating window shows av...
PopChar X helps you get the most out of your font collection. With its crystal-clear interface, PopChar X provides a frustration-free way to access any font's special characters. Expanded... Read more
OneNote 15.4 - Free digital notebook fro...
OneNote is your very own digital notebook. With OneNote, you can capture that flash of genius, that moment of inspiration, or that list of errands that's too important to forget. Whether you're at... Read more

Latest Forum Discussions

See All

7 tips to get ahead of the competition i...
7 tips to get ahead of the competition in Dynasty of Dungeons Posted by Simon Reed on November 28th, 2014 [ permalink ] Playcrab has launched their action-packed new dungeon crawler, Dynasty of Dungeons, today. | Read more »
Monster Strike Review
Monster Strike Review By Campbell Bird on November 28th, 2014 Our Rating: :: BILLIARD STRATEGYUniversal App - Designed for iPhone and iPad Collect monsters and battle by flinging them across the battlefield in this strangely... | Read more »
Proun+ Review
Proun+ Review By Jennifer Allen on November 28th, 2014 Our Rating: :: TWITCHY RACINGUniversal App - Designed for iPhone and iPad Twitchy racing aplenty in Proun+, an enjoyably tricky title.   | Read more »
Lucha Amigos (Games)
Lucha Amigos 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Forget Ninja Turtles, and meet Wrestlers Turtles! Crazier, Spicier and…Bouncier! Sling carapaces of 7 Luchadores to knock all... | Read more »
Record of Agarest War Zero (Games)
Record of Agarest War Zero 1.0 Device: iOS Universal Category: Games Price: $7.99, Version: 1.0 (iTunes) Description: HyperDevbox Holiday Turkey Black Friday Special Pricing! To celebrate the opening of the holiday season HyperDevbox... | Read more »
Raby (Games)
Raby 1.0.3 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.3 (iTunes) Description: ***WARNING - Raby runs on: iPhone 5, iPhone 5C, iPhone 5S, iPhone 6, iPhone 6 Plus, iPad Mini Retina, iPad Mini 3, iPad 4, iPad Air,... | Read more »
Oddworld: Stranger's Wrath (Games)
Oddworld: Stranger's Wrath 1.0 Device: iOS Universal Category: Games Price: $5.99, Version: 1.0 (iTunes) Description: ** PLEASE NOTE: Oddworld Stranger's Wrath requires at least an iPhone 4S, iPad 2, iPad Mini or iPod Touch 5th gen... | Read more »
Bounce On Back (Games)
Bounce On Back 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »
Dwelp (Games)
Dwelp 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: === 50% off for a limited time, to celebrate release === Dwelp is an elegant little puzzler with a brand new game mechanic. To complete a... | Read more »
Make Way for Fat Chicken, from the Maker...
Make Way for Fat Chicken, from the Makers of Scrap Squad Posted by Jessica Fisher on November 26th, 2014 [ permalink ] Relevant Games has announced they will be releasing their reverse tower defense game, | Read more »

Price Scanner via MacPrices.net

Black Friday: 27-inch 5K iMac for $2299, save...
 B&H Photo continues to offer Black Friday sale prices on the 27″ 3.5GHz 5K iMac, in stock today and on sale for $2299 including free shipping plus NY sales tax only. Their price is $200 off MSRP... Read more
Karalux Announces 24K Gold-Plated iPhone 6
Karalux, a Vietnam-based jewellery firm, has launched a unique 24 karat gold-plated iPhone 6 version with gold-cast monolithic dragon on its back panel. The real 24 karat gold plated enclosure doesn’... Read more
Black Friday: 13-inch 2.6GHz Retina MacBook P...
 B&H Photo has lowered their price for the 13″ 2.6GHz/128GB Retina MacBook Pro to $1159 for Black Friday. That’s $140 off MSRP, and it’s the lowest price for this model (except for Apple’s $1099... Read more
View all the Black Friday sales on our Mac Pr...
We’ve updated our Mac Price Trackers with the latest information on prices, bundles, and availability on systems from Apple’s authorized internet/catalog resellers. View Black Friday sale prices at a... Read more
Black Friday: 11-inch MacBook Air for $779, s...
 Best Buy has lowered their price for the 2014 11″ 1.4GHz/128GB MacBook Air to $779.99 for Black Friday. That’s $120 off MSRP. Choose free shipping or free local store pickup (if available). Sale... Read more
Apple Store Black Friday sale for 2014: $100...
BLACK FRIDAY The Apple Store has posted their Black Friday deals for 2014. Receive a $100 PRODUCT(RED) branded iTunes gift card with the purchase of select Macs, $50 with iPads, and $25 with iPods,... Read more
Black Friday: 15% off iTunes Gift Cards
Staples is offering 15% off $50 and $100 iTunes Gift Cards on their online store as part of their Black Friday sale. Click here for more information. Shipping is free. Best Buy is offering $100... Read more
BEVL Releases Dock Tailored for iPhone 6 and...
Seattle based BEVL has released their first product: an iPhone dock that is divergent in build quality, rock-solid function and visual simplicity to complement the iPhone. BEVL is now accepting... Read more
Black Friday: $150 off 13-inch Retina MacBook...
 Best Buy has 13-inch 2.6GHz Retina MacBook Pros on sale for $150 off MSRP on their online store as part of their Black Friday sale. Choose free shipping or free local store pickup (if available).... Read more
Black Friday: $300 off 15-inch Retina MacBook...
 B&H Photo has the new 2014 15″ Retina MacBook Pros on sale for $300 off MSRP as part of their Black Friday sale. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.2GHz Retina... Read more

Jobs Board

*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, 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 (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.