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How Snow Leopard Cut ObjC Launch Time In Half
MBCook writes "Greg Parker has an excellent technical article on his blog about the changes to the dynamic linker (dyld) for Objective-C that Snow Leopard uses to cut launch time in half and cut about 1/2 MB of memory per application. 'In theory, a shared library could be different every time your program is run. In practice, you get the same version of the shared libraries almost every time you run, and so does every other process on the system. The system takes advantage of this by building the dyld shared cache. The shared cache contains a copy of many system libraries, with most of dyld's linking and loading work done in advance. Every process can then share that shared cache, saving memory and launch time.' He also has a post on the new thread-local garbage collection that Snow Leopard uses for Objective-C."
Or the GAC
I take it since then shared library machine code has had to be patched in memory after it's loaded for a while now, thus preventing easy sharing among processes, and causing the page to need its own space in the swap file.
Sounds like this latest improvement effectively brings things back to the way they were, by effectively writing this patched version back to disk so that it can be mapped read-only as before, and not have to be patched every time the library is loaded into a process. It's odd, because I thought the OS already did this several versions ago when prebinding.
You've had enough of fucking, and would like more Snow Leopard stories? Each to his own, I guess.
Did you even read the article ? Suppose not... this is slashdot after all.
The article states that prebinding (similar to prelink) was used in previous versions of OS X and has been replaced by a much faster shared cache.
And reintroduced dll hell as well?
Well okay then, Apple were the ones who "popularised" it! ("Well I hadn't heard about Superfetch, but I heard about Apple doing it first, therefore, Apple did it first")
Or um ... they "integrated" it better. Yeah, that's it.
Hey, it makes a refreshing change from the daily "Do XYZ On Your Iphone" stories! I'm love the variation today here on Apple- er Slashdot.
Considering that the whole point of Snow Leopard was to refine the internal structure of the operating system and introduce new features for developers, it should come as no surprise that there are far more /. appropriate stories about it than the more eye-candy oriented releases.
dyld - noun. A reminder that regardless of age, you'll always have an adolescent sense of humor.
Maybe I've never realized before how bad slashdot is at beating a dead horse. But, really, this is going overboard with apple stories of the least bit of consequence or relevance (barring this story for once). I'm starting to think slashdot doesn't have pre-arranged deals with apple, and others, but that they get paid if a submission does happen to come through and is approved.
Selah.ca. Pause, and calmly think on that.
no. we've had a variant but lesser hell for years. Which has led to a series of cargo cult like maintenance procedures for Mac OS.
Nothing in the world is more dangerous than sincere ignorance and conscientious stupidity.
It's nothing like Superfetch. Superfetch preloads applications into system memory and this shared cache doesn't do that instead from what I understand it preforms some of the work the linker would do on load in advance.
If all else fails, immortality can always be assured by spectacular error.
Sounds like they've just updated their dynamic (shared) library loader to be able to handle Objective C (aka Cocoa) instead of just plain C, and to be a little smarter about keeping track of what it's already got going on, so it doesn't duplicate things.
As a long-time UNIX and Linux (and other more esoteric OSes) geek, this alone doesn't impress me too much. The idea that they went through the whole OS and worked to get little efficiency/performance gains like this all over the place impresses me a little more.
Village idiot in some extremely smart villages.
Well, the FP is claiming that they're the same thing, and your post seemed to be agreeing with him. But yea, that troll rating probably should have gone to the post you replied to.
I don't know anything about prelink, but Superfetch sounds completely different from dyld. Superfetch keeps frequently launched applications in memory to make them launch faster (much like Winamp Agent does for Winamp). dyld, OTOH, shortens application launch times by not reloading a shared library each time an application is launched. Keeping the shared library loaded in a shared cache also reduces the number of copies of that library you need loaded in memory. It doesn't sound like Superfetch does that.
Both a turbocharger and a cold air intake can improve car performance, but that doesn't make them the same thing.
erm... these don't stick?
Seven Days with Ubuntu Unity
The shared libs are shared. What was not shared before is the linking table, that must be built for accessing that shared code. prelink precalculates that table, and this apple thing does more or less the same.
And maybe kdeinit does something similiar since 2003?
Me thinks you (and many other readers) are mistaking this feature for more traditional static dyld caching.
This enhancement is actually about caching a runtime computation for Objective-C purposes. In practice, as the linked article indicates, this computation is consistent most of the time. In some cases it is not. So to handle the general and most common case, these computations (selector uniquing) are cached and used across different processes.
So the fair question is does Linux cache selector-uniquing?
You phrased things much better than I could fine sir. The link to the wikipedia article that the original poster included read nothing like TFA. Though because of the glaring placement of the post toward the top of the page and how straight forward it is written, I think most mods probably modded it without checking against either of the articles. That is how human nature is :). It's nice that slashdot moderation tends to correct itself over time thanks to insightful replies lower in the tree.
Once you start despising the jerks, you become one.
Moderators, please mod the parent down -- it completely misses the point.
Objective-C selector uniquing caching is NOT the same as Windows Superfetch.
Objective-C uses a two-phase dispatch for method calls. When you see a call in the Objective-C source code that looks like:
the dispatch system:
The problem arises in the method dispatch table when you have multiple methods named "init" -- which is very common. When an application is loaded the dynamic loader ("dyld") needs to separately identify all of the methods named "init" (and any other methods with conflicting names) that apply to different classes. This is done by "tagging" each method in the dispatch table, a process called selector uniquing.
Now, this has to be not only for the application binary itself, but also for any Objective-C classes in shared libraries that are loaded. Almost all apps on Mac OS X load the libobjc.dylib library, which is cached to improve performance. As a part of the caching process, Snow Leopard now does the selector uniquing only once, and then stores the uniqued selectors in the cache. Thus, any application that links against libobjc.dylib (or any other library that is in the cache) only has to unique its own selectors, not those of the library as well. This significantly reduces the amount of overhead for launching an application compared to previous versions of Mac OS X.
This process does not attempt to retain application binary code in memory in the face of page-outs as Superfetch does. Selector uniquing caching speeds application launch times by reducing the amount of computation that has to happen at launch, not by pre-loading the application's binary.
Thread-local garbage collection is NOT the same as Windows Superfetch.
Thread-local garbage collection is a third phase of garbage collection added on top of the Objective-C 2.0 garbage collection system, which speeds up the garbage collection system even further. By concentrating GC to what has occurred in a single thread, the GC system can delay and reduce the cost of a slow global sweep even beyond the generational GC algorithm.
Windows Superfetch is a response to poorly written software.
To quote from the Wikipedia article:
The intent is to improve performance in situations where running an anti-virus scan or back-up utility would result in otherwise recently-used information being paged out to disk, or disposed from in-memory caches, resulting in lengthy delays when a user comes back to their computer after a period of non-use.
In my opinion as an experienced application developer the user should never run into the problem that Superfetch attempts to solve. Anti-malware scans or backups are generally limited by I/O transfer rates, not by CPU. In such situations, using lots of memory to pre-load data makes no sense. It is relatively easy to write a two-buffer, threaded, streaming system for situations that are constrained by disk transfer rates without consuming scads of memory.
In the bigger picture, Superfetch attempts to learn the times of day when apps are used and pre-loads their binaries. This is a nice concept, but I have serious doubts as to how useful it really is. The penalty for guessing wrong is fairly high, and users are more tolerant of consistent small slowdowns than they are of occasional long hangs (see the Mac literature on the spinning beach ball).
Mac OS X is less likely to need such anti-malware scans in the first place as the application binaries are now digitally signed by the developer. Any malware that attempts to insert itself into applications will run into problems. This is not to say that the Mac is immune -- I can think of a number of holes that could be exploited (such as the fact that unsigned binaries w
For reference, normally when a program is launched without prebinding, the program has to look into the symbol table for the shared library and "bind" it (basically, tell the program where it is). Prebinding basically does that in advance and saves the lookup table, but any time the library is changed, the bindings have to be regenerated.
The article says prebinding is actually quite efficient for C/C++ code, but objective-C (used by macOS X and iPhone) is structured more like Smalltalk or Java, and uses selectors, which I believe can't be prebound (for you java programmers, these are equivalent to interfaces - C/C++ does not have this concept and instead allows direct access to the classes using protected or public) to interface into classes and these are instanced once for every application accessing that shared library. According to TFA, by keeping a single cached copy of the selector, they avoid the memory overhead of keeping individual copies. Since the OS itself has over 30000 selectors, you can imagine this cuts overhead by quite a bit, especially with commonly loaded libraries like Cocoa.
For people comparing this to superfetch, it's not really the same thing - superfetch was pre-loading heavily used libraries into memory to avoid the delay in loading them during start time, and this is caching the library lookups onto disk that may or may not be memory resident at any particular time.
I'm not sure whether there are people out there writing Objective-C apps for the Mac without Cocoa, though. I guess there's always someone who won't use the nifty library and shortcuts and all that, because they're hardcore, efficiency nuts, or just masochists...
Or they use only a subset of Cocoa because they plan to port the app to GNU/Linux, *BSD, and Windows using GNUstep, an OpenStep-compatible toolkit that implements only some of Cocoa.
Does Linux need selector uniquing if it doesn't use Objective-C? To me this sounds like an inefficiency in Objective-C that made it less efficient than C++ (the other OO flavour of C) has been improved somewhat.
It's nothing like Superfetch. Superfetch preloads applications into system memory [microsoft.com] and this shared cache doesn't do that instead from what I understand it preforms some of the work the linker would do on load in advance.
The whole dyld sounds a lot like some of the basic features of the .NET runtime...
Or maybe some of the features in this advanced futuristic os:
http://blogs.technet.com/askperf/archive/2008/02/06/ws2008-dynamic-link-library-loader-and-address-space-load-randomization.aspx
mod parent : should be the article itself :D
Comparing this to Superfetch is ignorant beyond belief. Superfetch is part of the paging system on Windows and attempts to trigger page faults before the data is actually needed so that it's already cached when it is needed. This is quite a nice feature and one I am naturally prejudiced to like because my PhD was in this topic.
This is entirely different. Part of it is similar to the existing prebinding / prelinking stuff in Leopard / Linux, which generates the relocation tables in position-independent code. This is nothing like anything in Windows, because Windows doesn't use position-independent code for shared libraries (it uses a horribly ugly hack which performs better in the best case and much worse in the worst case). The article is a bit too light on details to understand exactly why the new version performs so much better.
The other half, however, is very clever. By caching the selector uniquing information, they are saving a lot of time when loading compilation units containing Objective-C code. Even better is the fact that, because these symbols are now not modified, they can be shared between processes without triggering copy-on-write faults. This isn't actually that hard to implement for the GNU runtime; just give the selector symbols mangled names and mark them as having common linkage (it's a bit harder on Darwin because Mach-O is weird), then you can use pointer comparison as a first step in the runtime and avoid the strcmp() call. Combining this with the prelinking support and you get the caching for free, which is very nice. I actually implemented this in Clang while writing this post, so expect to see it on non-Apple platforms soon too.
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Objective-C uses a two-phase dispatch for method calls. When you see a call in the Objective-C source code that looks like:
the dispatch system:
C++ follows the same steps. The big difference is that in Objective-C, the dispatch table is an associative array (C++ unordered_map, Java HashMap, Python dict) from strings to function pointers, not a plain array (C++ vector, Java ArrayList, Python list).
In my opinion as an experienced application developer the user should never run into the problem that Superfetch attempts to solve. Anti-malware scans or backups are generally limited by I/O transfer rates, not by CPU. In such situations, using lots of memory to pre-load data makes no sense. It is relatively easy to write a two-buffer, threaded, streaming system for situations that are constrained by disk transfer rates without consuming scads of memory.
But then you rely on the operating system to provide a method for applications to provide cache hints, and you rely on the antivirus software to provide such hints. SuperFetch tries to infer these even for applications developed prior to widespread knowledge of these hints or ported from systems that lack these hints.
In the bigger picture, Superfetch attempts to learn the times of day when apps are used and pre-loads their binaries. This is a nice concept, but I have serious doubts as to how useful it really is.
Having my applications ready to start at 08:57 when I'm about to grab the mouse at 08:58 improves my productivity. Consider that employees have sued their employers for requiring that employees be present during application startup time but not paid until the application has fully started up.
Mac OS X is less likely to need such anti-malware scans in the first place as the application binaries are now digitally signed by the developer.
But who signs the developer's certificate? And what keeps malware publishers from signing their trojans?
Any malware that attempts to insert itself into applications will run into problems.
Unless an application tries to insert itself as, say, an assistive technology using the accessibility API.
Does Linux need selector uniquing if it doesn't use Objective-C?
No it doesn't. Since the average executable on linux is static code linked to dynamic libraries made up of static code, you get your "selector uniquing" at compile time - you don't get a method selector description, instead you get a pre-calculated and already unique address of the method or function.
To me this sounds like an inefficiency in Objective-C that made it less efficient than C++ (the other OO flavour of C) has been improved somewhat.
It is a tradeoff. You get to worry about the performance of shared library selector uniquing, but you get all the benefits of dynamic language and runtime. In practice such inefficiencies matter most in cases where you are very constrained for resources - e.g. on a phone, as hinted in TFA. I doubt in the context of the rest of the performance and efficiency improvements in Snow Leopard and on a reasonably modern computer, the 1/10 of a second or the few megabytes of memory saved matter all that much.
Perhaps Obj-C has a few nice features but personally I don't see it. If they'd stuck to C or C++ like every other version of Unix then this would never have been an issue in the first place. Plus a lot more people would have been able to cross-code for OS/X without having to learn an obscure OO version of C which never caught on in the wider IT world and is still used on practically no other system.
Also, so what if they're "different languages"? If they were the "same language" there'd be nothing to compare. Do you go around comparing your right eye to your right eye?
Over-the-top Response Guy! Giving "Over-the-Top Responses" since 1970.
Makes sense... but I am not sold on the benefits of a dynamic language... how is this better than dynamically loaded shared libraries? If you really need a dynamic framework there is always XPCOM, which gives you the added benefit of being able to use it from many different languages, and of course the dynamic binding is built into Javascript which makes it ideal for linking together components written in C++.
selectors, which I believe can't be prebound (for you java programmers, these are equivalent to interfaces - C/C++ does not have this concept and instead allows direct access to the classes using protected or public)
I'm sorry, but this and most of the rest of your description is completely wrong. Selectors are nothing like Java interfaces. Interfaces are Java's version of Objective-C Protocols. Selectors are abstract method names (Smalltalk calls them symbols). Each Objective-C class has some data structure mapping these to function pointers. When you send a message (call a method) you look up the function pointer corresponding to the selector in the receiving class. To make this fast, all selector comparisons are done as pointer comparisons. To make this work, the runtime needs to make sure that selectors are unique. This process involves building a large hash table and inserting every selector referenced by every compilation unit into it. By making the linker handle this uniquing, you have several advantages. The first is that the resulting table can be shared more easily between processes, resulting in a memory efficiency gain. The second is that the runtime can first try doing pointer comparison when registering a new selector, and only use the hash if the linker didn't unique the selector.
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Okay, whoever modded the above post as "Troll" is confused. Talking about the moderation system on a thread about some other topic (e.g. Snow Leopard) isn't a "Troll", it's "Off-topic".
"Convictions are more dangerous enemies of truth than lies."
So, Objective-C requires selectors (such as for message names) to be interned, and the old way was to intern all selectors at process startup time when the dynamic linker does its work; but the new way is to cache the interned selectors both on disk (faster startup) and in memory (even faster startup plus saved memory overhead). Is that correct?
Also sounds like the prelink application in Linux.
No, OS X has always done that. Except they call it prebinding.
It is cowardly, and a betrayal of whatever it means to be a Jew, to act as a white man
-James Baldwin
The intent is to improve performance in situations where running an anti-virus scan or back-up utility would result in otherwise recently-used information being paged out to disk, or disposed from in-memory caches, resulting in lengthy delays when a user comes back to their computer after a period of non-use.
In my opinion as an experienced application developer the user should never run into the problem that Superfetch attempts to solve. Anti-malware scans or backups are generally limited by I/O transfer rates, not by CPU. In such situations, using lots of memory to pre-load data makes no sense. It is relatively easy to write a two-buffer, threaded, streaming system for situations that are constrained by disk transfer rates without consuming scads of memory.
I don't think you understood it right: the perf problem is not for the anti-malware programs, but once they have run they have thrown everything out of the cache and subsequent applications have to re-populate it again, thus slowing eveything down. There used to be the same kind of problem under linux after the 'locate' cronjob.
Almost. The old way was to do it at module load time, in the Objective-C runtime library (libobjc), the new way is to do it in the loader (dyld). The loader caches the result of everything it does, including uniquing symbols (two symbols with the same name are resolved to one or other instance), while the runtime library does no caching.
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Apple's dyld comes in handy in both cases.
In theory there is no difference between theory and practice. In practice there is. - Yogi Berra
Thread-local garbage collection is a third phase of garbage collection added on top of the Objective-C 2.0 garbage collection system, which speeds up the garbage collection system even further. By concentrating GC to what has occurred in a single thread, the GC system can delay and reduce the cost of a slow global sweep even beyond the generational GC algorithm.
Do you know how they detect that a pointer has escaped? Is there some sort of write barrier? How does this work with the somewhat unsafe base language?
Sounds like this "solution" is only a benefit because Apple chose to standardize on a language that generates very slow executables by default. Thus, the problem (and solution) are of their own making.
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Dyld's (Dynamic Loader) have existed before Snow Leopard. They are extensively used in all Mac OS X versions since they are at the base of the system. They also appear in BSD and Linux under slightly different names. This just explains how they found out a way to do caching and preloading better than previously. It's like Microsoft finding out a way to automagically load all necessary dll's and correctly find out the dll amongst several different versions of the same dll for a program and preload them before the program even needs them. It does so correctly and consistently every time, they just sped it up now. I don't know if Windows DLL's even do versioning but from what I remember (latest experience was last week in XP) if one program writes over the other programs dll, you're shafted (I'm looking at you Aladdin USB/DRM Keys).
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Why don't they (any OS) just add something onto the generic filesystem caching layer to keep executable bits in RAM as long as the input files stay the same? If it was done that way you could theoretically reuse it for interpreted code as well.
that sound you hear is NoYob's spirit being completely crushed.
bravo, sir.
---
Is this the MPAA? Is this the RIAA? Is this the DMCA? I thought it was the USA!
Sir, I believe you are mistaken. There obviously must have been at least one virus at some point for a Mac otherwise why would Apple include virus and malware protection in Snow Leopard. Only now will none exist since Apple has deemed it so by not allowing the rogue virus to infect their hardware. :P
erm... these don't stick?
Not even sticky files stick when applied to glue languages like Python. Must be glue incompatibility or something.
Ezekiel 23:20
Perhaps Obj-C has a few nice features but personally I don't see it. If they'd stuck to C or C++ like every other version of Unix then this would never have been an issue in the first place.
But you can use either of those perfectly well mixed with ObjC calls.
ObjC is a relatively small set of additions to standard-C, so it really doesn't take that long to pick up the syntax changes if you've encountered C before, while at the same time it allows for some very nice dynamic behavior and things like introspection. The language has evolved to support things like garbage collection (though that specific feature is not available on the iPhone due to performance constraints).
What you are really overlooking though is that Objective-C that Apple uses, has a very rich and diverse set of foundation classes (some inherited from the NeXT days) - just as wide in scope as Java. Any modern language simply has to have a giant toolbox to help get common tasks done, and that's going to be the thing that takes the most time to learn. Happily, Objective-C has a fairly consistent set of tools and conventions, that make learning new parts easier once you have learned a few others.
"There is more worth loving than we have strength to love." - Brian Jay Stanley
The article on Arstechnica about Snow Leopard goes into some detail about the advantages of Obj-C being a dynamic language... primarily due to the new inclusion of Closures aka functions assigned to variables so that you can pass a function to another function with dynamic arguments.
This makes for not necessarily a better performing language but an easier, more efficient and less buggy language.
It's still likely a personal coding preference of course.
A fool throws a stone into a well and a thousand sages can not remove it.
You are reading apple.slashdot.org. It might have occurred to you that at this subdomain you might find stories about Apple? You can filter it out, you know.
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I still don't see what the point is. My solution to this: More RAM and *Autostart*. Really. I start everything I need at boot time. Which is quite rare. So I never felt the need to speed up the first start of any programs.
Any sufficiently advanced intelligence is indistinguishable from stupidity.
Yes, wouldn't it be terrible if Apple actually fixed issues with their OS...
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There is a write barrier. Every pointer assignment is turned (by the compiler) into a call to the GC. You can find the code for this in the clang repository in lib/CodeGen/CGObjCMac.cpp (it's quite easy to read, in spite of being C++). If a pointer is assigned to a global or to an object that is marked as belonging to another thread, then it is treated as having escaped. The same is true if you call CFRetain() on it (which increments the ref count and tells the GC not to free it until the CFRelease() has been called.
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Gretchen, stop trying to make Superfetch happen. It's not going to happen.
Green Monkey
I'm not so sure about that. Typically, code in shared libraries is re-entrant and code pages are loaded once, then mapped into the address space of the process using them. I don't know of any modern OS that wastefully makes a copy of the library's code for each process.
I don't know about other platforms, but I wouldn't be surprised if they do something to share the memory as well.
Dude, I think you're hearing things that people aren't saying. The article describes how apple has improved their dynamic loading. The author doesn't claim they invented it. Apple doesn't claim they invented it. Nobody said anything about amazing new technologies Apple created.
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They probably just rolled in the 2.6.31 kernel.
Using OS X/Mac since 10.2.8, I haven't seen another abused tool like "update_prebinding" even while it is a very risky process in pre 10.5 systems since it deals with actual binary headers.
Also thanks to uninformed IT blogs etc, people always considered prebinding a thing which will go away in next release. Like, Apple is really stupid to do such thing. They basically misunderstood the added flexibility to prebinding scheme where tools without (or broken) prebinding will continue to run.
Anyway, want to see how much Apple users are abused by some shareware developers? Just watch for a applescript to basically issue this command and asks for money or donation. I don't have 10.6 but on 10.5, in its FIRST line of man page (man update_prebinding), Apple states "normally, there shouldn't be any need to issue this command manually" or something equivalent to that. On pre 10.5, like 10.4.11, it can get catastrophic if you keep doing it, as explained on article http://unsanity.org/archives/mac_os_x/shock_and_awe.php
Unless you lived a power loss in middle of a OS X/Quicktime update or kernel crash, there shouldn't be ANY reason to manually update prebinding. In fact, it can lead to a horrible cache fragmentation which may slow things down. Don't fix a working thing.
Oh boy, its my good buddy Alexander Peter Kowalski
Why do you always have to chime in with some bizarrely formatted story completely tangential to the topic at hand?
I'm surprised you're not whining about the HOSTS file today. Boo-hoo, I can't put 0 in the HOSTS file. STFU.
Go back and write some more useless Delphi shit you crazy fuck.
Applesoft BASIC on the Apple II mapped keyword tokens directly to a jump table of ROM entry points.
Other than the fact that it was not object-based, didn't reference system or external libraries, and all of the token offsets were fixed and predetermined...
It's exactly the same. :)
Any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so.
Comparing JavaScript to Objective-C is a big leap. They're completely different.
Yes, they're both "dynamic". But:
* JavaScript is not a "proper" object oriented language, Objective-C is.
* JavaScript is a lot more dynamic than Objective-C, it's compiled at run time. Objective-C just does some decisions at runtime.
* Objective-C is orders of magnitude faster than JavaScript (I don't actually know how much faster, but having programmed in both I'm confident a few test would reveal tasks that take 2 seconds in javascript would take less than a millisecond in Objective-C)
* Objective-C is compiled by a *C* compiler (more often than not, a C++ compiler). This means that any C/C++ code you find on the 'net, or any C/C++ library you find on the net, can be accessed seamlessly within your Objective-C code.
* Because you can mix C/C++ code into your Objective C code, you can use the language which bests suites your task, for example a function could start with 3 lines of Objective-C code, then have 20 lines of C++, and then 4 more lines of Objective-C.
As for not being convinced of dynamic languages... it really does depend on the task at hand. But it is generally accepted (definitely by me) that software written in a dynamic language tends to be less buggy (buffer over-runs and memory leaks are virtually impossible in many dynamic languages for example) and faster to write. Where you draw the line between bugs/faster to write and performance/flexibility depends on the individual project.
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I was referring to XPCOM, which allows many languages (including C++) to have the same kind of dynamic binding as objective-c. It is used by firefox to implement plugins for example. Of course in C++ it looks messy as the management of XPCOM objects is exposed. In Javascript this management is hidden. There is nothing wrong with implementing the computationally heavy parts of an application in C++, and then using Javascript as the toplevel. Besides, Javascript is a proper object oriented language. Javascript is both object oriented (using the object prototype model) and supports closures (something neither C, C++, and objective C cannot manage). See: http://javascript.crockford.com/javascript.html
I just checked and a 1-year code signing cert from Comodo is $179.95, with discounts for multi-year certs. Other vendors also seem to have pretty reasonable prices.
That's at least on the order of $100 per platform. The certificate for Windows is $179.95 per year, and the certificate for a secure web site from which to distribute copies of the software is another $99 per year. It gets even more expensive to target more than one platform: the certificate for XNA is $99 per year, the certificate for iPod Touch is $99 per year, and by the time one has ported an application to all the platforms that his audience uses, he'd be out of his hobby money.
Anyone who has the time to put together a serious app (even for freeware) can afford that amount.
Say I develop a video game and want to distribute it to the public. How would I recover the nearly $480 per year across three platforms without selling either copies or advertisements? It's almost enough to push someone toward a less expensive hobby.
selectors, which I believe can't be prebound (for you java programmers, these are equivalent to interfaces - C/C++ does not have this concept and instead allows direct access to the classes using protected or public)
I'm sorry, but this and most of the rest of your description is completely wrong. Selectors are nothing like Java interfaces. Interfaces are Java's version of Objective-C Protocols. Selectors are abstract method names (Smalltalk calls them symbols). Each Objective-C class has some data structure mapping these to function pointers.
Although I will agree with you that GPP is somewhat misinformed I take issue with your statement that selectors are nothing like Java Interfaces.
It is true that the class structure of Objective-C (one root NSObject class, at least in common practice) and the class structure of Java (one root Object class) are virtually identical. And it is true that an Objective-C protocol has feature parity with a Java Interface and when you think of formal interfaces in Java the equivalent to that in Objective-C is a protocol.
So Java has anObj instanceof SomeClass which will indicate that anObj is an instance of SomeClass or an instance of some other class that derives from SomeClass. The Objective-C equivalent to this is [anObj isKindOfClass:[SomeClass class]]. And Java has anObj instanceof SomeInterface where the equivalent in Objective-C is [anObj conformsToProtocol:@protocol(SomeInterface)].
But then Objective-C also has this nifty thing [anObj respondsToSelector:@selector(doSomething:)] which does exactly what it says and allows you to see if the object will respond to the doSomething selector that takes one argument. Java has no analogue to this. I mean, you can sort of fake it using reflection to find methods but it isn't quite the same thing.
The bottom line is that when a unique selector can be looked up like this each selector functions almost as if it were its own Java-style interface. There is clearly a parallel between Java if(anObj instanceof DoSomethingInterface) ((DoSomethingInterface)anObj).doSometing(1); and Objective-C if([anObj respondsToSelector:@selector(doSomething:)]) [(id)anObj doSomething: 1];
From a coding standpoint where I would think to use a respondsToSelector: in Objective-C I wind up making an interface containing exactly 1 method in Java. Sometimes it's the right choice to add the extra lines and make an interface (and if so, then you should add all the extra lines and make a protocol in Objective-C). But often times I find the required formality of Java to be a distraction.
For thoroughness, would you mind defining 'module load time' in the Objective-C context for me? I am familiar with the language but much less familiar with its runtime support components.
When you compile any C-family language, you get an object code file. This contains symbols which will be resolved by either the static or dynamic linker and various sections. One of these sections contains an array of pointers to functions which are called by the loader when that object file is loaded. These are not used in C. In C++, they are used for static constructors. In Objective-C, there is one that issues a call to __objc_exec() with a pointer to the struct objc_module created for that compilation unit as the argument. This registers all of the selectors, protocols, classes, and categories declared in that compilation unit with the module.
When you statically link object code files together, these sections will be concatenated. When you then load the resulting object code file (either an executable or a binary), the loader will iterate through the combined section and call each function in turn. This is how you get code executing before main() in a program.
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Thanks. The part about Objective-C using that mechanism to register, on a per-compilation-unit basis, selectors, classes, and the like is news to me. :)
With Objective-C, a method may not even exist at compile time. It is impossible to define formal interfaces in many cases. respondsToSelector allows you to ensure that any arbitrary object walks, swims, and quacks like a duck without strict class definitions.