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

Remotix 3.1.4 - Access all your computer...
Remotix is a fast and powerful application to easily access multiple Macs (and PCs) from your own Mac. Features Complete Apple Screen Sharing support - including Mac OS X login, clipboard... Read more
DesktopLyrics 2.6.6 - Displays current i...
DesktopLyrics is an application that displays the lyrics of the song currently playing in "iTunes" right on your desktop. The lyrics for the song have to be set in iTunes; DesktopLyrics does nothing... Read more
VOX 2.5.1 - Music player that supports m...
VOX is a beautiful music player that supports many filetypes. The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features and support for all... Read more
NetNewsWire 4.0.0 - RSS and Atom news re...
NetNewsWire is the best way to keep up with the sites and authors you read most regularly. Let NetNewsWire pull down the latest articles, and read them in a distraction-free and Mac-like way. Native... Read more
MacUpdate Desktop 6.0.6 - Search and ins...
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
ForkLift 2.6.5 - Powerful file manager:...
ForkLift is a powerful file manager and ferociously fast FTP client clothed in a clean and versatile UI that offers the combination of absolute simplicity and raw power expected from a well-executed... Read more
Drive Genius 4.1.0 - Powerful system uti...
Drive Genius 4 gives you faster performance from your Mac while also protecting it. The award-winning and improved DrivePulse feature alerts you to hard drive issues before they become major problems... Read more
OnyX 2.9.7 - Maintenance and optimizatio...
OnyX is a multifunctional utility for OS X. It allows you to verify the startup disk and the structure of its System files, to run miscellaneous tasks of system maintenance, to configure the hidden... Read more
DEVONthink Pro 2.8.5 - Knowledge base, i...
DEVONthink Pro is your essential assistant for today's world, where almost everything is digital. From shopping receipts to important research papers, your life often fills your hard drive in the... Read more
Backblaze 4.0.1.878 - Online backup serv...
Backblaze is an online backup service designed from the ground-up for the Mac.With unlimited storage available for $5 per month, as well as a free 15-day trial, peace of mind is within reach with... Read more

Carbo - Handwriting in the Digital Age...
Carbo - Handwriting in the Digital Age 1.0 Device: iOS Universal Category: Productivity Price: $3.99, Version: 1.0 (iTunes) Description: | Read more »
The Apple Watch isn't Great as a Fi...
| Read more »
Show the World What You See With Stre.am...
Live broadcasting is getting popular on mobile devices, which is why you can now get Stre.am, by Infinite Takes. [Read more] | Read more »
PhotoTime's 2.1 Update Adds Apple W...
The latest PhotoTime update is adding even more functionality to the handy photo organizing app. Yep, including Apple Watch support. [Read more] | Read more »
Oh My Glob! Adventure Time Puzzle Quest...
Finn and Jake are taking over D3 Go!'s popular puzzle game series in the upcoming Adventure Time Puzzle Quest. [Read more] | Read more »
Earthcore: Shattered Elements - Tips, Tr...
At first glance, Earthcore: Shattered Elements seems like a rather simple card-battling game. Once you’re introduced to skills that will change quite a bit. Even more so once you start to acquire hero cards. But it’s not so complicated that we... | Read more »
Dungeon999F (Games)
Dungeon999F 1.33 Device: iOS Universal Category: Games Price: $.99, Version: 1.33 (iTunes) Description: "The game you must play at least once in your life!" "The game with potential of million downloads globally!" ...is what the... | Read more »
Mixels Rush - Use Mixes, Maxes and Murps...
Mixels Rush - Use Mixes, Maxes and Murps to Outrun the Nixels 1.0.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.0 (iTunes) Description: Use awesome new Mixels and make crazy combinations to beat the annoying... | Read more »
Battles of the Ancient World II (Games)
Battles of the Ancient World II 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »
Sword Of Xolan (Games)
Sword Of Xolan 1.0.4 Device: iOS Universal Category: Games Price: $.99, Version: 1.0.4 (iTunes) Description: Sword of Xolan is an action platformer game that includes the juice of pixel art style. Xolan is a young and brave man who... | Read more »

Price Scanner via MacPrices.net

Sale! 16GB iPad mini 3 for $349, save $50
B&H Photo has the 16GB iPad mini 3 WiFi 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. Read more
Price drop on 2014 15-inch Retina MacBook Pro...
B&H Photo has dropped prices on 2014 15″ Retina MacBook Pros by $200. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799.99 save $200 - 15″ 2.5GHz... Read more
Will iOS 9 Finally Bring Productivity Friendl...
Ah, the irony. From its original announcement in 2010, Apple has doggedly insisted that the iPad remain “simple,” thus arbitrarily limiting its considerable potential as a content creation and... Read more
13-inch 2.5GHz MacBook Pro (refurbished) avai...
The Apple Store has Apple Certified Refurbished 13″ 2.5GHz MacBook Pros available for $829, or $270 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.... Read more
Mac Pros on sale for up to $260 off MSRP
B&H Photo has Mac Pros on sale for up to $260 off MSRP. Shipping is free, and B&H charges sales tax in NY only: - 3.7GHz 4-core Mac Pro: $2799, $200 off MSRP - 3.5GHz 6-core Mac Pro: $3719.99... Read more
Save up to $600 with Apple refurbished Mac Pr...
The Apple Store has Apple Certified Refurbished Mac Pros available for up to $600 off the cost of new models. An Apple one-year warranty is included with each Mac Pro, and shipping is free. The... Read more
Apple drops price on refurbished 15-inch 2.2G...
The Apple Store has dropped their price on the Apple Certified Refurbished 2014 15″ 2.2GHz Retina MacBook Pro to $1609, $390 off original MSRP. Apple’s one-year warranty is included, and shipping is... Read more
Apple restocks refurbished Mac minis for up t...
The Apple Store has restocked Apple Certified Refurbished 2014 Mac minis, with models available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz... Read more
Cover For iPhone Helps You Create Beautiful P...
Taking photos, editing and sharing them is one of the things most of people do on their iPhones for fun, especially if they’re users of social media. Unfortunately, many iOS photo-editing solutions... Read more
Password Keyboard 1.0 For iOS Handles Logins...
Pfaeffikon, Switzerland based Power APP has introduced Password Keyboard 1.0.0, their new password utility developed for iPhone, iPad and iPod touch devices. Password Keyboard is designed to simplify... Read more

Jobs Board

Project Manager / Business Analyst, WW *Appl...
…a senior project manager / business analyst to work within our Worldwide Apple Fulfillment Operations and the Business Process Re-engineering team. This role will work Read more
Visual Merchandise Manager %u2013 Accessories...
…to be part of an incredible team. Imagine what you could do here. At Apple , great ideas have a way of becoming reality very quickly. Our Accessories assortment makes 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** The ASC is an Apple employee who serves as the Apple business manager and influencer in a hyper-business critical Reseller's store which delivers Read more
Technical Project Manager - *Apple* Pay - A...
**Job Summary** Apple Pay is seeking an experienced technical PM…manage the rollout of features to merchants for the Apple Pay platform in the US Within this role Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.