TweetFollow Us on Twitter

August 91 - How to Cope with Nesting Instinct (in C++)

How to Cope with Nesting Instinct (in C++)

Eric M. Berdahl

MacApp 3 will add large amounts of spice to programming recipes. There are new classes, new twists on old classes, and new hoops to jump through.

However, the more interesting items are from the primary language of implementation changing from Pascal to C++. In fact, I propose that MacApp 3's best features are its solutions to the so-called "Nested Procedure" problem.

As a software author, I spend a fair amount of time listening to a short description of some problem a user is having and a longer description of what the user wants to do to solve it. Sound familiar? Sometimes the user is right on the money as to the nature of the "right" solution, but more often it is their description of the problem that is most revealing. I view the Nested Procedure problem in that light. I don't necessarily want nested procedural constructs, but I do have a problem to solve.

One solution is to implement language support for grouping sets of code in some logical order. The Pascal language provides this. However, it's not the only solution, nor is it the only solution for the MacApp programmer-at least not for the ones who use C++.

This article will examine the nested procedure problem, look for solutions, and discuss what this has to do with MacApp. First, I'll describe the problem in more detail. This may seem obvious and repetitive, but the goal is to come up with something that does what nested procedures in Pascal do. In addition, the solution should be link compatible with Object Pascal.

SCOPES?

Let's examine some textbook descriptions of Pascal. Pascal lore provides many ideas from which one may leverage; one of the most useful is the concept of scopes.

Simply put, a scope is the range over which a coding entity-variable, constant, or routine-is meaningful. Languages like assembly generally recognize only one scope, global. That is, once you get down to the assembly level, everything is visible to everyone all the time.

Moving up one step, there is C. C recognizes two basic scopes, global and local. That is, something is either scopeless-global-or it exists within a specific routine.

Languages like Pascal maintain a highly ordered, scoped hierarchy. Pascal's description of scoping is so good, in fact, that to many people, Pascal's implementation is scoping.

ABRACADABRA!

You might ask, at this point, "But when I write Pascal code, I don't really have to worry about scopes, do I?"

Probably not, but you do have to be careful. For example, many of you may remember something like the code in Listing 1 from a Computer Science 101 test. The exam would quiz you on the output of the program; to answer correctly, you would have to recognize that there are overlapping scopes and resolve them appropriately.

In a more practical world, you might see something like Example 2. Example 2 calls a procedure, DoSomethingToIntegers, telling it what action it wants performed, namely DoToInteger. Furthermore, DoToInteger has some magic knowledge of the local variables, oneInt and twoInt. Pascal accomplishes this by giving the nested procedure, DoToInteger, an additional parameter referred to as a "static link" variable. The static link is a pointer to the stack frame of the enclosing routine.

Thus, when a Pascal nested procedure is called, a pointer to the routine's stack frame is passed as a hidden argument. Also, when a routine is passed as a procedure parameter to a Pascal routine, as is done with DoToInteger, a pointer to the routine's stack frame is passed as an additional parameter. In the example, DoSomethingToIntegers actually takes two parameters, the routine to execute, DoToInteger, and a value to pass as the static link parameter to the indicated routine.

OH, I C

A person emulating this concept in a C-based language could write something like Example 3. This construct directly translates Pascal's mechanism into C's. It also looks ugly. For some reason, Pascal's elegance has been lost. Pascal deals elegantly with the static link problem; as a programmer, I don't need to deal with it. I just write my code and let Pascal take care of the overhead.

In Example 3, you could just pass DoSomethingToIntegers a pointer to oneInt, and have CBasedDoToInteger know that staticLink is a pointer to a short, namely oneInt. But that means it won't know about twoInt! Alternatively, you could define a structure that holds oneInt and twoInt, and pass a pointer to the structure for the static link, but that's unattractive. Clearly, it's better to refine and generalize the solution.

DOING IT right IN C++

Simple emulation doesn't seem to solve the nested procedure problem. Nonetheless, Pascal has provided a good starting point, and later we'll return to it. For the moment, however, let's look at another avenue-object technology. Regardless of the language to which it is bound, object concepts provide the programmer with another layer of scoping.

The key to an object-based approach is to recognize that, implicitly, a method is scoped to exist within an instance of an object. I tend to avoid hard-core C++ documentation (except during psychotically masochistic episodes), but in most C++ texts, the descriptions of objects as scopes-and the implications of this for visualizing program constructs-are very good.

For the problem above, a class could be defined, like CDoToInteger in Example 4, to handle the hard work. CDoToInteger looks much closer to what's desired than CBasedDoToInteger in Example 3.

"But where did the mystical staticLink parameter go?" the intrepid reader asks. It's hidden by the magic of object programming, just as it was hidden by the magic of Pascal in the Pascal example. In the object programming case, the scope is determined by the implicit variable this (SELF for you Pascal fans, Current for you Eiffel fans).

Look again at CPlusFoo in Example 4 to see how the CDoToInteger class might be used. I've indicated a routine to invoke-CPlusDoToInteger in the scope of CDoToInteger-and indicated a context in which to execute the routine, aScope. That's it, right?

Not quite. I still have to find some way for aScope to know about oneInt and twoInt within CPlusFoo. One solution might be to set the individual instance variables of aScope to hold the same values as the local variables of CPlusFoo, but that's not what I really want. I want the oneInt and twoInt variables of CDoToInteger to be pointers to oneInt and twoInt in CPlusFoo, but I also need to ensure that those pointers are never nil.

This is the second and last reason that to reasonably use reference variables in C++ MacApp code. (The first involved parameter passing, and was discussed last issue.) So, the CDoToInteger class is finished in Example 5. The careful reader will notice that the declaration of CPlusDoToInteger changed subtly in this last revision. It now carries a Pascal calling sequence, so that it's compatible with Pascal. The difference here is the order in which parameters on the stack are evaluated. The declaration above indicates that the magic this parameter is to be the last parameter pushed on the stack, just like the Pascal magic static link is pushed on last. Ta da.

A PRACTICAL EXAMPLE

So, how might one use such information in a real MacApp application? Consider a common situation, that of iterating over items in a TList:
class CCountSubViews {
private:
long&           numberOfViews;
public:
    CCountSubViews(long& aNumberOfViews) :
    numberOfViews(aNumberOfViews) { }

    pascal void CountSubViews(TView* aView)
   {
    numberOfViews++;
   }
};

pascal void TMyView::CountSubViews()
{
   long             numberOfSubViews;
   CCountSubViews   aCounter(numberOfSubViews);
   fSubViews->Each(CCountSubViews::CountSubViews,
                          &aCounter);
}

You can come up with more relevant examples, I'm sure, but I chose this particular example for a reason. Each is a method that allows a routine to iterate over the items in a TList. This can be used with the magic of Pascal and nested procedures; however, MacApp 3 provides C++ users with another way to iterate.

ITERATORS

Often the best way to solve a problem is to avoid it. It would be nice to avoid writing helper classes, and just bring iteration into the mainstream of code. After all, anyone is used to iterating over integers and other built-in types with simple for loops:
// Find the sum of all integers on the interval [0..4]
short   a;
a = 0;
for (short i = 0; i < 5; i = i + 1)
{
    

a = a + i; }

MacApp 3 provides a nice mechanism for doing this-iterators. A library of iterator objects (sorry, C++ users only) is available for doing a variety of iterative actions. The following is an example of CObjectIterator, the class for iterating over objects in a TList:

CObjectIterator    iter(aTList);
for (TObject* anOb = iter.FirstObject(); iter.More();
    anOb = iter.NextObject())
{
    //  do something with anOb here
}

Within the body of the loop, you can manipulate anOb just as you would any local variable. The reason for this is that anOb is a full-fledged local variable. So, if you happen to know that aTList only contains instances of TFoo, you can easily (and legally) cast anOb to TFoo*.

Iterator classes are provided for most of the basic iterations. For example, the CSubViewIterator class, iterates over each subview of a given TView. You might use it in the following fashion:

CSubViewIterator   iter(aTView);
for (TView* aView = iter.FirstSubView(); iter.More(); aView = iter.NextSubView())
{
    //  do something with aView here
}

NEXT TIME…

Next issue, I'll look at the wild and woolly world of MacApp 3. I'll show what new functionality it provides, what you might want to tap into right away, and what you'll want to study first. Keep those cards and letters coming!

Tech Note #265

For more information on Pascal PROCEDURE parameters, I refer the inquisitive reader to Tech Note #265: Pascal to C - PROCEDURE parameters. n
 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Kodi 17.1. - Powerful media center tool...
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
Bookends 12.8 - Reference management and...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Bookends uses the cloud to sync reference libraries on all the Macs you use.... Read more
Apple iTunes 12.6 - Play Apple Music and...
Apple iTunes lets you organize and stream Apple Music, download and watch video and listen to Podcasts. It can automatically download new music, app, and book purchases across all your devices and... Read more
Default Folder X 5.1.4 - Enhances Open a...
Default Folder X attaches a toolbar to the right side of the Open and Save dialogs in any OS X-native application. The toolbar gives you fast access to various folders and commands. You just click on... Read more
Amazon Chime 4.1.5587 - Amazon-based com...
Amazon Chime is a communications service that transforms online meetings with a secure, easy-to-use application that you can trust. Amazon Chime works seamlessly across your devices so that you can... Read more
CrossOver 16.2 - Run Windows apps on you...
CrossOver can get your Windows productivity applications and PC games up and running on your Mac quickly and easily. CrossOver runs the Windows software that you need on Mac at home, in the office,... Read more
Adobe Creative Cloud 4.0.0.185 - Access...
Adobe Creative Cloud costs $19.99/month for a single app, or $49.99/month for the entire suite. Introducing Adobe Creative Cloud desktop applications, including Adobe Photoshop CC and Illustrator CC... Read more
MegaSeg 6.0.2 - Professional DJ and radi...
MegaSeg is a complete solution for pro audio/video DJ mixing, radio automation, and music scheduling with rock-solid performance and an easy-to-use design. Mix with visual waveforms and Magic... Read more
Bookends 12.8 - Reference management and...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Bookends uses the cloud to sync reference libraries on all the Macs you use.... Read more
Adobe Creative Cloud 4.0.0.185 - Access...
Adobe Creative Cloud costs $19.99/month for a single app, or $49.99/month for the entire suite. Introducing Adobe Creative Cloud desktop applications, including Adobe Photoshop CC and Illustrator CC... Read more

The best deals on the App Store this wee...
Deals, deals, deals. We're all about a good bargain here on 148Apps, and luckily this was another fine week in App Store discounts. There's a big board game sale happening right now, and a few fine indies are still discounted through the weekend.... | Read more »
The best new games we played this week
It's been quite the week, but now that all of that business is out of the way, it's time to hunker down with some of the excellent games that were released over the past few days. There's a fair few to help you relax in your down time or if you're... | Read more »
Orphan Black: The Game (Games)
Orphan Black: The Game 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Dive into a dark and twisted puzzle-adventure that retells the pivotal events of Orphan Black. | Read more »
The Elder Scrolls: Legends is now availa...
| Read more »
Ticket to Earth beginner's guide: H...
Robot Circus launched Ticket to Earth as part of the App Store's indie games event last week. If you're not quite digging the space operatics Mass Effect: Andromeda is serving up, you'll be pleased to know that there's a surprising alternative on... | Read more »
Leap to victory in Nexx Studios new plat...
You’re always a hop, skip, and a jump away from a fiery death in Temple Jump, a new platformer-cum-endless runner from Nexx Studio. It’s out now on both iOS and Android if you’re an adventurer seeking treasure in a crumbling, pixel-laden temple. | Read more »
Failbetter Games details changes coming...
Sunless Sea, Failbetter Games' dark and gloomy sea explorer, sets sail for the iPad tomorrow. Ahead of the game's launch, Failbetter took to Twitter to discuss what will be different in the mobile version of the game. Many of the changes make... | Read more »
Splish, splash! The Pokémon GO Water Fes...
Niantic is back with a new festival for dedicated Pokémon GO collectors. The Water Festival officially kicks off today at 1 P.M. PDT and runs through March 29. Magikarp, Squirtle, Totodile, and their assorted evolved forms will be appearing at... | Read more »
Death Road to Canada (Games)
Death Road to Canada 1.0 Device: iOS Universal Category: Games Price: $7.99, Version: 1.0 (iTunes) Description: Get it now at the low launch price! Price will go up a dollar every major update. Update news at the bottom of this... | Read more »
Bean's Quest Beginner's Guide:...
Bean's Quest is a new take on both the classic platformer and the endless runner, and it's free on the App Store for the time being. Instead of running constantly, you can't stop jumping. That adds a surprising new level of challenge to the game... | Read more »

Price Scanner via MacPrices.net

Updated iPad Price Trackers
Scan our Apple iPad (and iPod touch) Price Trackers for the latest information on sales, bundles, and availability on systems from Apple’s authorized internet/catalog resellers. We update the... Read more
12-inch 32GB Space Gray iPad Pro on sale for...
B&H Photo has 12″ Space Gray 32GB WiFi Apple iPad Pros on sale for $50 off MSRP including free shipping. B&H charges sales tax in NY only: - 12″ Space Gray 32GB WiFi iPad Pro: $749 $50 off... Read more
2.6GHz Mac mini on sale for $559, $140 off MS...
Guitar Center has the 2.6GHz Mac mini (MGEN2LL/A) on sale for $559 including free shipping. Their price is $140 off MSRP, and it’s the lowest price available for this model. Read more
SSD Speeder RAM Disk SSD Life Extender App Fo...
Fehraltorf, Switzerland based B-Eng has announced they are making their SSD Speeder app for macOS publicly available for purchase on their website. SSD Speeder is a RAM disk utility that prevents... Read more
iPhone Scores Highest Overall in Smartphone D...
Customer satisfaction is much higher among smartphone owners who use their device to operate other connected home services such as smart thermostats and smart appliances, according to the J.D. Power... Read more
Swipe CRM Free Photo-Centric CRM Sales DEal C...
Swipe CRM LLC has introduced Swipe CRM: Visual Sales 1.0 for iPad, an app for creating, managing, and sharing visually stunning sales deals. Swipe CRM is targeted to small-and-medium creative... Read more
13-inch 2.0GHz Apple MacBook Pros on sale for...
B&H has the non-Touch Bar 13″ 2.0GHz MacBook Pros in stock today and on sale for $150 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.0GHz MacBook Pro Space Gray (... Read more
15-inch Touch Bar MacBook Pros on sale for up...
B&H Photo has the new 2016 15″ Apple Touch Bar MacBook Pros in stock today and on sale for up to $150 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.7GHz Touch Bar... Read more
Apple’s iPhone 6s Tops Best-Selling Smartphon...
In terms of shipments, the iPhone 6s from Apple bested all competitors for sales in 2016, according to new analysis from IHS Markit, a world leader in critical information, analytics and solutions.... Read more
Logitech Rugged Combo Protective iPad Case an...
Logitech has announced its Logitech Rugged Combo, Logitech Rugged Case, and Logitech Add-on Keyboard for Rugged Case for Apple’s new, more affordable $329 9.7-inch iPad, a complete solution designed... Read more

Jobs Board

*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Fulltime aan de slag als shopmanager in een h...
Ben jij helemaal gek van Apple -producten en vind je het helemaal super om fulltime shopmanager te zijn in een jonge en hippe elektronicazaak? Wil jij werken in Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Fulltime aan de slag als shopmanager in een h...
Ben jij helemaal gek van Apple -producten en vind je het helemaal super om fulltime shopmanager te zijn in een jonge en hippe elektronicazaak? Wil jij werken in Read more
Starte Dein Karriere-Abenteuer in den Hauptst...
…mehrsprachigen Teams betreust Du Kunden von bekannten globale Marken wie Apple , Mercedes, Facebook, Expedia, und vielen anderen! Funktion Du wolltest schon Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.