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
 
AAPL
$101.79
Apple Inc.
+0.21
MSFT
$46.68
Microsoft Corpora
+0.16
GOOG
$589.27
Google Inc.
+4.50

MacTech Search:
Community Search:

Software Updates via MacUpdate

Duplicate Annihilator 5.0 - Find and del...
Duplicate Annihilator takes on the time-consuming task of comparing the images in your iPhoto library using effective algorithms to make sure that no duplicate escapes. Duplicate Annihilator detects... Read more
jAlbum Pro 12.2 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code!... Read more
jAlbum 12.2 - Create custom photo galler...
With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly, with pro results Simply drag and drop photos into groups, choose a design... Read more
Quicken 2015 2.0.4 - Complete personal f...
Quicken 2015 helps you manage all your personal finances in one place, so you can see where you're spending and where you can save. Quicken automatically categorizes your financial transactions,... Read more
iMazing 1.0 - Complete iOS device manage...
iMazing (formerly 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... Read more
Xcode 6.0.1 - Integrated development env...
Apple Xcode is Apple Computer's integrated development environment (IDE) for OS X. The full Xcode package is free to ADC members and includes all the tools you need to create, debug, and optimize... Read more
Apple Safari 7.1 - Apple's Web brow...
Apple Safari in OS X Mavericks brings you all-new ways to find and enjoy the best of the web. It works with iCloud to give you a seamless browsing experience across all your devices. It looks out for... Read more
Delivery Status 6.1.2 - Check delivery s...
Delivery Status displays delivery status of packages for a variety of shipment services. Can't wait for your packages to arrive? Don't waste your time checking the site constantly, just open this all... Read more
Mavericks Cache Cleaner 8.0.9 - Clear ca...
Mavericks Cache Cleaner is an award-winning general purpose tool for OS X. MCC makes system maintenance simple with an easy point-and-click interface to many OS X functions. Novice and expert users... Read more
OneNote 15.2.2 - Free digital notebook f...
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

Project Life (Photography)
Project Life 1.0 Device: iOS Universal Category: Photography Price: $2.99, Version: 1.0 (iTunes) Description: Imagine scrapbooking without scissors or adhesive or tools … or without having to print photos! Never before has... | Read more »
Skater (Games)
Skater 1.0 Device: iOS iPhone Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: All of Skateboarding In The Palm Of Your Hand Designed by skaters for skaters, we teamed up with 17 of the most prominent brands in... | Read more »
Huerons (Games)
Huerons 1.1 Device: iOS Universal Category: Games Price: $.99, Version: 1.1 (iTunes) Description: EXCLUSIVE LAUNCH PRICE! Huerons is 50% off until September 20th! Huerons are tiny colored circles. Merge them by clicking on an empty... | Read more »
Down Among the Dead Men (Games)
Down Among the Dead Men 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: Avast! Take to the high seas in a fully interactive piratical tale of broadsides and buccaneers. From author Dave... | Read more »
Sling Adds Chromecast Support Through Sl...
Sling Adds Chromecast Support Through Slingplaye​r Mobile Apps Posted by Jessica Fisher on September 18th, 2014 [ permalink ] | Read more »
How to Completely Delete Your iPhone’s C...
The iPhone 6 is out tomorrow, and plenty of people are excited about it. So much so that they’re planning to – or already have – traded in their old iPhone to go towards it. The thing about trading in hardware is it’s very important to make sure... | Read more »
Dragon Quest I Review
Dragon Quest I Review By Andrew Fisher on September 18th, 2014 Our Rating: :: THINE QUEST AWAITETHUniversal App - Designed for iPhone and iPad Its historical significance aside, Dragon Quest 1 is a fun, campy, difficult, thoroughly... | Read more »
It Came From Canada: Overkill 3
Overkill 3 is like every trope of big modern gaming rolled into one. It’s a sequel to an action-packed military shooter. It’s flashy and scripted and flaunts its sophisticated graphics. And it’s a mobile game with a heavy emphasis on in-app... | Read more »
New Modes and Leader Boards in Update fo...
New Modes and Leader Boards in Update for Rules! Posted by Jessica Fisher on September 18th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
TwistedRun Review
TwistedRun Review By Rob Thomas on September 18th, 2014 Our Rating: :: DON'T TWIST YOUR ANKLE!Universal App - Designed for iPhone and iPad TwistedRun is kind of like running up a giant curly fry into the sky. Or maybe that was just... | Read more »

Price Scanner via MacPrices.net

iOS 8 Adoption Rate Slower than iOS 7, 6, Hit...
Apple began pushing out iOS 8 updates to eligible devices around 1pm ET on September 17, 2014. However, unlike with iOS 7, which boasted a wide variety of differences from its predecessor iOS 6, in... Read more
Save up to $300 on the price of a new Mac wit...
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
13-inch 2.8GHz Retina MacBook Pro available f...
B&H Photo has the new 2014 13″ 2.8GHz Retina MacBook Pro on sale for $1699.99 including free shipping plus NY sales tax only. They’ll also include free copies of Parallels Desktop and LoJack for... Read more
16GB iPad Air on sale for $449, save $50
Walmart has the 16GB iPad Air WiFi on sale for $449 on their online store for a limited time. Choose free home shipping or free local store pickup. Their price represents a $50 savings over standard... Read more
13-inch 256GB MacBook Air on sale for $1099,...
B&H Photo has the 2014 13″ 1.4GHz 256GB MacBook Air on sale for $1099.99. Shipping is free, and B&H charges NY sales tax only. Their price is $100 off MSRP. Read more
Toshiba Introduces TransMemory ID High-Speed...
Toshiba’s Digital Products Division (DPD), a division of Toshiba America Information Systems, Inc., today introduced the TransMemory ID USB 3.0 Flash Drive, a simpler storage solution for people who... Read more
New iPads and OS X Yosemite Release Coming Oc...
The DailyDot’s Micah Singleton reports that Apple is planning to hold its next product announcement event on Oct. 21, at which it will unveil the iPad Air 2 and iPad mini 3 and release a final build... Read more
Logitech Bluetooth Multi-Device Cross-Platfor...
Logitech has an enviable track record of making some of the best computer keyboards and mice. At least in my estimation, the best freestanding keyboards I’ve ever used have been Logitech units,... Read more
Roundup of Apple refurbished iPad Airs and iP...
Apple is offering Certified Refurbished iPad Airs for up to $140 off MSRP. Apple’s one-year warranty is included with each model, and shipping is free. Stock tends to come and go with some of these... Read more
Sprint offers 16GB iPad mini for $199.99 with...
Sprint is offering 1st generation 16GB iPad minis for $199.99 with a 2-year service agreement. Standard MSRP for this iPad is $429. Their price is the lowest available for this model. Read more

Jobs Board

Project Manager, *Apple* Financial Services...
**Job Summary** Apple Financial Services (AFS) offers consumers, businesses and educational institutions ways to finance Apple purchases. We work with national and 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* 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* 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* 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
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.