Objective-C has no namespaces; it's much like C, everything is within one global namespace. Common practice is to prefix classes with initials, e.g. if you are working at IBM, you could prefix them with "IBM"; if you work for Microsoft, you could use "MS"; and so on. Sometimes the initials refer to the project, e.g. Adium prefixes classes with "AI" (as there is no company behind it of that you could take the initials). Apple prefixes classes with NS and says this prefix is reserved for Apple only.
So far so well. But appending 2 to 4 letters to a class name in front is a very, very limited namespace. E.g. MS or AI could have an entirely different meanings (AI could be Artificial Intelligence for example) and some other developer might decide to use them and create an equally named class. Bang, namespace collision.
Okay, if this is a collision between one of your own classes and one of an external framework you are using, you can easily change the naming of your class, no big deal. But what if you use two external frameworks, both frameworks that you don't have the source to and that you can't change? Your application links with both of them and you get name conflicts. How would you go about solving these? What is the best way to work around them in such a way that you can still use both classes?
In C you can work around these by not linking directly to the library, instead you load the library at runtime, using dlopen(), then find the symbol you are looking for using dlsym() and assign it to a global symbol (that you can name any way you like) and then access it through this global symbol. E.g. if you have a conflict because some C library has a function named open(), you could define a variable named myOpen and have it point to the open() function of the library, thus when you want to use the system open(), you just use open() and when you want to use the other one, you access it via the myOpen identifier.
Is something similar possible in Objective-C and if not, is there any other clever, tricky solution you can use resolve namespace conflicts? Any ideas?
Update:
Just to clarify this: answers that suggest how to avoid namespace collisions in advance or how to create a better namespace are certainly welcome; however, I will not accept them as the answer since they don't solve my problem. I have two libraries and their class names collide. I can't change them; I don't have the source of either one. The collision is already there and tips on how it could have been avoided in advance won't help anymore. I can forward them to the developers of these frameworks and hope they choose a better namespace in the future, but for the time being I'm searching a solution to work with the frameworks right now within a single application. Any solutions to make this possible?
Prefixing your classes with a unique prefix is fundamentally the only option but there are several ways to make this less onerous and ugly. There is a long discussion of options here. My favorite is the #compatibility_alias Objective-C compiler directive (described here). You can use #compatibility_alias to "rename" a class, allowing you to name your class using FQDN or some such prefix:
#interface COM_WHATEVER_ClassName : NSObject
#end
#compatibility_alias ClassName COM_WHATEVER_ClassName
// now ClassName is an alias for COM_WHATEVER_ClassName
#implementation ClassName //OK
//blah
#end
ClassName *myClass; //OK
As part of a complete strategy, you could prefix all your classes with a unique prefix such as the FQDN and then create a header with all the #compatibility_alias (I would imagine you could auto-generate said header).
The downside of prefixing like this is that you have to enter the true class name (e.g. COM_WHATEVER_ClassName above) in anything that needs the class name from a string besides the compiler. Notably, #compatibility_alias is a compiler directive, not a runtime function so NSClassFromString(ClassName) will fail (return nil)--you'll have to use NSClassFromString(COM_WHATERVER_ClassName). You can use ibtool via build phase to modify class names in an Interface Builder nib/xib so that you don't have to write the full COM_WHATEVER_... in Interface Builder.
Final caveat: because this is a compiler directive (and an obscure one at that), it may not be portable across compilers. In particular, I don't know if it works with the Clang frontend from the LLVM project, though it should work with LLVM-GCC (LLVM using the GCC frontend).
If you do not need to use classes from both frameworks at the same time, and you are targeting platforms which support NSBundle unloading (OS X 10.4 or later, no GNUStep support), and performance really isn't an issue for you, I believe that you could load one framework every time you need to use a class from it, and then unload it and load the other one when you need to use the other framework.
My initial idea was to use NSBundle to load one of the frameworks, then copy or rename the classes inside that framework, and then load the other framework. There are two problems with this. First, I couldn't find a function to copy the data pointed to rename or copy a class, and any other classes in that first framework which reference the renamed class would now reference the class from the other framework.
You wouldn't need to copy or rename a class if there were a way to copy the data pointed to by an IMP. You could create a new class and then copy over ivars, methods, properties and categories. Much more work, but it is possible. However, you would still have a problem with the other classes in the framework referencing the wrong class.
EDIT: The fundamental difference between the C and Objective-C runtimes is, as I understand it, when libraries are loaded, the functions in those libraries contain pointers to any symbols they reference, whereas in Objective-C, they contain string representations of the names of thsoe symbols. Thus, in your example, you can use dlsym to get the symbol's address in memory and attach it to another symbol. The other code in the library still works because you're not changing the address of the original symbol. Objective-C uses a lookup table to map class names to addresses, and it's a 1-1 mapping, so you can't have two classes with the same name. Thus, to load both classes, one of them must have their name changed. However, when other classes need to access one of the classes with that name, they will ask the lookup table for its address, and the lookup table will never return the address of the renamed class given the original class's name.
Several people have already shared some tricky and clever code that might help solve the problem. Some of the suggestions may work, but all of them are less than ideal, and some of them are downright nasty to implement. (Sometimes ugly hacks are unavoidable, but I try to avoid them whenever I can.) From a practical standpoint, here are my suggestions.
In any case, inform the developers of both frameworks of the conflict, and make it clear that their failure to avoid and/or deal with it is causing you real business problems, which could translate into lost business revenue if unresolved. Emphasize that while resolving existing conflicts on a per-class basis is a less intrusive fix, changing their prefix entirely (or using one if they're not currently, and shame on them!) is the best way to ensure that they won't see the same problem again.
If the naming conflicts are limited to a reasonably small set of classes, see if you can work around just those classes, especially if one of the conflicting classes isn't being used by your code, directly or indirectly. If so, see whether the vendor will provide a custom version of the framework that doesn't include the conflicting classes. If not, be frank about the fact that their inflexibility is reducing your ROI from using their framework. Don't feel bad about being pushy within reason — the customer is always right. ;-)
If one framework is more "dispensable", you might consider replacing it with another framework (or combination of code), either third-party or homebrew. (The latter is the undesirable worst-case, since it will certainly incur additional business costs, both for development and maintenance.) If you do, inform the vendor of that framework exactly why you decided to not use their framework.
If both frameworks are deemed equally indispensable to your application, explore ways to factor out usage of one of them to one or more separate processes, perhaps communicating via DO as Louis Gerbarg suggested. Depending on the degree of communication, this may not be as bad as you might expect. Several programs (including QuickTime, I believe) use this approach to provide more granular security provided by using Seatbelt sandbox profiles in Leopard, such that only a specific subset of your code is permitted to perform critical or sensitive operations. Performance will be a tradeoff, but may be your only option
I'm guessing that licensing fees, terms, and durations may prevent instant action on any of these points. Hopefully you'll be able to resolve the conflict as soon as possible. Good luck!
This is gross, but you could use distributed objects in order to keep one of the classes only in a subordinate programs address and RPC to it. That will get messy if you are passing a ton of stuff back and forth (and may not be possible if both class are directly manipulating views, etc).
There are other potential solutions, but a lot of them depend on the exact situation. In particular, are you using the modern or legacy runtimes, are you fat or single architecture, 32 or 64 bit, what OS releases are you targeting, are you dynamically linking, statically linking, or do you have a choice, and is it potentially okay to do something that might require maintenance for new software updates.
If you are really desperate, what you could do is:
Not link against one of the libraries directly
Implement an alternate version of the objc runtime routines that changes the name at load time (checkout the objc4 project, what exactly you need to do depends on a number of the questions I asked above, but it should be possible no matter what the answers are).
Use something like mach_override to inject your new implementation
Load the new library using normal methods, it will go through the patched linker routine and get its className changed
The above is going to be pretty labor intensive, and if you need to implement it against multiple archs and different runtime versions it will be very unpleasant, but it can definitely be made to work.
Have you considered using the runtime functions (/usr/include/objc/runtime.h) to clone one of the conflicting classes to a non-colliding class, and then loading the colliding class framework? (this would require the colliding frameworks to be loaded at different times to work.)
You can inspect the classes ivars, methods (with names and implementation addresses) and names with the runtime, and create your own as well dynamically to have the same ivar layout, methods names/implementation addresses, and only differ by name (to avoid the collision)
Desperate situations call for desperate measures. Have you considered hacking the object code (or library file) of one of the libraries, changing the colliding symbol to an alternative name - of the same length but a different spelling (but, recommendation, the same length of name)? Inherently nasty.
It isn't clear if your code is directly calling the two functions with the same name but different implementations or whether the conflict is indirect (nor is it clear whether it makes any difference). However, there's at least an outside chance that renaming would work. It might be an idea, too, to minimize the difference in the spellings, so that if the symbols are in a sorted order in a table, the renaming doesn't move things out of order. Things like binary search get upset if the array they're searching isn't in sorted order as expected.
#compatibility_alias will be able to solve class namespace conflicts, e.g.
#compatibility_alias NewAliasClass OriginalClass;
However, this will not resolve any of the enums, typedefs, or protocol namespace collisions. Furthermore, it does not play well with #class forward decls of the original class. Since most frameworks will come with these non-class things like typedefs, you would likely not be able to fix the namespacing problem with just compatibility_alias.
I looked at a similar problem to yours, but I had access to source and was building the frameworks.
The best solution I found for this was using #compatibility_alias conditionally with #defines to support the enums/typedefs/protocols/etc. You can do this conditionally on the compile unit for the header in question to minimize risk of expanding stuff in the other colliding framework.
It seems that the issue is that you can't reference headers files from both systems in the same translation unit (source file). If you create objective-c wrappers around the libraries (making them more usable in the process), and only #include the headers for each library in the implementation of the wrapper classes, that would effectively separate name collisions.
I don't have enough experience with this in objective-c (just getting started), but I believe that is what I would do in C.
Prefixing the files is the simplest solution I am aware of.
Cocoadev has a namespace page which is a community effort to avoid namespace collisions.
Feel free to add your own to this list, I believe that is what it is for.
http://www.cocoadev.com/index.pl?ChooseYourOwnPrefix
If you have a collision, I would suggest you think hard about how you might refactor one of the frameworks out of your application. Having a collision suggests that the two are doing similar things as it is, and you likely could get around using an extra framework simply by refactoring your application. Not only would this solve your namespace problem, but it would make your code more robust, easier to maintain, and more efficient.
Over a more technical solution, if I were in your position this would be my choice.
If the collision is only at the static link level then you can choose which library is used to resolve symbols:
cc foo.o -ldog bar.o -lcat
If foo.o and bar.o both reference the symbol rat then libdog will resolve foo.o's rat and libcat will resolve bar.o's rat.
Just a thought.. not tested or proven and could be way of the mark but in have you considered writing an adapter for the class's you use from the simpler of the frameworks.. or at least their interfaces?
If you were to write a wrapper around the simpler of the frameworks (or the one who's interfaces you access the least) would it not be possible to compile that wrapper into a library. Given the library is precompiled and only its headers need be distributed, You'd be effectively hiding the underlying framework and would be free to combine it with the second framework with clashing.
I appreciate of course that there are likely to be times when you need to use class's from both frameworks at the same time however, you could provide factories for further class adapters of that framework. On the back of that point I guess you'd need a bit of refactoring to extract out the interfaces you are using from both frameworks which should provide a nice starting point for you to build your wrapper.
You could build upon the library as you and when you need further functionality from the wrapped library, and simply recompile when you it changes.
Again, in no way proven but felt like adding a perspective. hope it helps :)
If you have two frameworks that have the same function name, you could try dynamically loading the frameworks. It'll be inelegant, but possible. How to do it with Objective-C classes, I don't know. I'm guessing the NSBundle class will have methods that'll load a specific class.
With swift, isn't it possible to dump functions and variables into a file instead of implementing a singleton? Is there a good reason (like memory management or something) that I am missing, other than style concerns?
It is very much possible to do so. Module level private variables could be used as private variables for singleton class.
For me it is rather a choice dictated by what makes more sense as a solution to the problem. The language has given us tools each with its particular usage, so use them the way they are meant to be used.
If your solution requires an object with data, state and related functionality which could have only one instance then it is better to use a singleton. This will make more sense when you or someone else will try to understand code at later time.
If your solution is just bunch of loosely related functions without data or state then it is better to use functions in a file.
With swift, isn't it possible to dump functions and variables into a file instead of implementing a singleton?
Yes, it is definitely possible. However, the same is possible in Objective-C, with the same pluses and minuses.
Is there a good reason (like memory management or something) that I am missing, other than style concerns?
Two considerations are relevant here:
You can make your singleton polymorphic, expose an interface, define several implementations, and pick the specific one at runtime
You can "reset" your singleton by discarding the instance and creating a new one.
If you need concurrency, having a singleton makes it a little easier to implement.
When I first began working on a long iOS project, my first, I had need for a small class that was readily accessible throughout the project. At first I passed it around as a property and it became way too much of a headache.
So, even though I've seen posts here advising against it, I created that class in the AppDelegate and access it as needed through a pointer to the AppDelegate. Then, I would revisit it later.
So, it is now later, I'm approaching release of the project, and I want to deal with this issue. It works just fine as it is but if it is lousy practice I want to fix it before release.
I don't understand why it is lousy practice. Sometimes you just need a class that is readily available and these seems like as good a way as any to get it. But there could be some downside I'm not understanding.
Assuming I need this class instance (it is very lightweight but heavily used) to be accessible throughout the project (probably about 50 VCs, total), what would be a good alternative to just referencing it via the App Delegate?
TIA for comments. I hope it doesn't start a war.
There's no hard-fast rule for for this and hundreds of opinions to be had. Here's my take on it.
I don't understand why it is lousy practice. Sometimes you just need a class that is readily available and these seems like as good a way as any to get it. But there could be some downside I'm not understanding.
You're right to feel this way and yes sometimes you need a class readily available with your data. It's lousy because it is not how AppDelegate should be used. In an ideal world it shouldn't contain lots of unrelated state-data and should serve a singular purpose: delegating system calls to your app. The trick is where do you put your model so that everyone can have access to it?
Assuming I need this class instance (it is very lightweight but heavily used) to be accessible throughout the project (probably about 50 VCs, total), what would be a good alternative to just referencing it via the App Delegate?
Put your state in a class using the singleton pattern. It will ensure that only one copy of it is ever created and all your classes should be able to access it. This is a well accepted pattern in the iOS SDK (NSUserDefaults, UIApplication, etc..). Just beware of this class growing too large or doing too many things. Try your best to keep it simple and focused and the Object-Oriented Police will leave you alone.
Other resources and opinions worth considering:
http://www.cocoawithlove.com/2008/11/singletons-appdelegates-and-top-level.html
Is it a bad practice to use your AppDelegate as your Singleton?
Is it good practice to use AppDelegate for data manipulation and Handling?
I've defined some message types that I'd like to use in my protorpc service on app engine, but the service in throwing an error because it is finding members that are methods, instead of messages.Field instances. Is there a way to get around this? Because defining methods on messages is really useful. In my example, I have a credentials message, and I've defined a method to check if the credentials are correct, on the message. I know I can just define the methods as functions, but it just seams less logical.
One of the goals of ProtoRPC is to remain as faithful to the implementation of Protocol buffers as possible. Writing a protocol buffer using .proto notation does not allow for method definitions. So if one could define methods you would end up with classes that could not correctly be represented as a .proto file. As an aside, the hope is that it makes the developer enforce a clear distinction between their message classes and client/server functionality.
We also went through a lot of effort to make sure that if we needed to make future changes (such as adding support for the protocol buffer mechanism) it easier to do so without breaking compatibility with older versions. Because it was intended for being built in to the App Engine runtime the library is updated automatically when a new version of App Engine is released, not when the developer is ready to upgrade. Restricting what can be added to messages now allows greater flexibility for the library to grow in the future (in truth, at the expense of the user of the library).
Although using associative references seems to be the most widely used method to fake category variables, I can't help from thinking of it as a hack. I'm probably wrong. Thus this question. So, I know Apple uses categories for many of the framework classes, and they did so way before 3.1 (when associative references were added). Which makes me wonder, were they doing it in a different way? Are associative references used this way recommended by Apple? (any docs for sustaining this?) From the design point of view, adding storage to a category is something to avoid?
Associated Objects are not a hack, as they are provided to us as standard functions in the "modern" runtime.
You are right, Associated Objects were not implemented before the "Modern" runtime, however Apple had no use for them. If they wished to add variables to a class, it was done with a combination of an explicit setter/getter/internal iVar (you'll notice this especially with old Cocoa classes like NSSplitView) in the main class (because why would you need to use associated objects if you owned the framework?) Apple's use of categories was the old way of grouping similar functions together, something like a language level #pragma mark -.