I have a protocol that has a method returning NSArray*.
In the implementation I had made the return type of that method to be NSView*
I see this is happening only in case of Objective C class pointers and not in other cases like returning void vs returning int.
I would expect a complier warning at the minimum but the compilation happens just fine.
#protocol prot <NSObject>
-(NSArray*)array;
#end
#interface impl : NSObject<prot>
#end
#implementation impl
//Should return NSArray. Returns NSView instead
- (NSView *)array
{
return nil;
}
#end
First things first:
impl should be Implementation since class names are written in upper camel case and abbreviations are bad(TM). Moreover, Class is a class pointer, NSView* and NSArray* are instance pointers.
To your Q, even I'm a bit tired of this discussion (dynamic vs. static typing, early vs. late binding):
A: Why should the compiler warn? Both are instance pointers and maybe the messages sent to the object are supported by both. The compiler does not care about binding, it is done at runtime.
B: But this is very unsafe!
A: Did you ever ship code with such an error?
B: No. But it is unsafe by theory.
A: Yes, that's true for alle theories that ship code without running it at least one time.
B: But you have to admit, that this is more unsafe than type checking at compile time.
A: Yes, theoretically that's true.
B: So why do you support it?
A: Because there are many situations in which dynamic typing has advantages. I. e. it is very easy to write generic code without having templates. (Even sometimes they are called generics, they are still silly templates.) It is very easy to give around responsibility, what needs contra-conceptual extensions in other languages (signals & slots in C++, delegates in C#, …) It is very easy to create stand-in objects for lowering memory pressure. It is very easy to write an ORIM. Shall I continue?
B: Yes
A: Is is that flexible that you can write a whole AOP framework within that language. It is that flexible that you can write a prototype based framework within that language.
However, sometimes it is easy to detect for the compiler that something makes no sense at all. And sometimes the compiler warns about that. But in many cases the compiler is not more intelligent than the developer.
Agreed that it should generate a warning, but it doesn't. Part of the issue is that all ObjC objects are id at runtime, which is why you're seeing different behavior for int (which isn't id). But that's not really an excuse. It's a limitation of the compiler. There are numerous places where it doesn't do a good job of distinguishing between ObjC object types. ObjC objects are duck-typed, so as long as they respond to the right messages "they work."
Sometimes this is a benefit; for example, NSArray is actually a class cluster, and there are several (private) types that pretend to be NSArray by just implementing the same interface. That's something that is easy in ObjC, but hard in Swift. Still no excuse, since it would be easy to get that benefit without this frustrating lack of a compiler warning, but it gets back to how ObjC thinks about class types.
This limitation is fixed in Swift, and another benefit of moving over, but that doesn't really help you, I know.
Related
While working on on open source project, I came across the following C function declaration and implementation:
// FSNData.h
NSString *stringForMimeType(MimeType type);
#interface FSNData : NSObject
// All the expected objective-c property and instance method declarations
#end
// FSNData.m
#import "FSNData.h"
// where 'type' is an enum
// this does work as expected
NSString *stringForMimeType(MimeType type) {
switch (type) {
case MimeType_image_jpeg: return #"image/jpeg";
case MimeType_image_png: return #"image/png";
default:
NSLog(#"ERROR: FSNData: unknown MimeType: %d", type);
// do not return "application/octet-stream"; instead, let the recipient guess
// http://en.wikipedia.org/wiki/Internet_media_type
return nil;
}
}
#implementation
// all properties and methods defined in FSData.h implemented as expected
#end
This example could easily be re-written as a class level method with out any problem. As it is, using stringFormMimeType() sill requires importing the FSNData header file anyway.
Looking at the Apple docs, it states only:
Because Objective-C rests on a foundation of ANSI C, you can freely
intermix straight C code with Objective-C code. Moreover, your code
can call functions defined in non-Cocoa programmatic interfaces, such
as the BSD library interfaces in /usr/include.
There is no mention of when C functions should favour Objective-C methods.
The only benefit I can see at this point, is that calling the above function, as opposed to a class method, some Objective-C runtime call(s) would be skipped. In a typical use case of FSNData, this would not give a noticeable boost in performance to the user (probably even to developers)*.
What benefit exists (other than coding style) for favouring a C function over a class method?
*FSNData is used as part of the FSNetworking library, so I doubt there would be thousands upon thousands of network operations being performed during any application's life cycle.
In short, C (or C++) implementations are very useful:
For Abstraction
For Reusability
When making medium and large scale programs
In performance critical paths
For 'Interior' implementations
What benefit exists (other than coding style) for favouring a C function over a class method?
ObjC messaging introduces indirect function calls. These are firewalls for optimizers.
C functions can easily restrict access, whereas 'private' ObjC implementations may be looked up using the ObjC runtime, or accidentally overridden.
C functions may be removed from your executable if not referenced, or they may be made private. If you write reusable code (and you should), this can have a huge impact on your binary sizes and load times -- C functions which are not referenced/used may be removed, but ObjC types and methods will be preserved (including everything they reference). This is why your app's binary size may grow significantly when you use only small part of an ObjC static library -- every objc class in the library is preserved. If that library were C or C++, then you could get by with very small growth because you need only what is referenced. What is or is not referenced is easier to prove with C and C++.
C functions can be inlined, either during compilation or during Link Time Optimization phases.
The compiler and optimizers are able to do much optimization with C functions (e.g. inter-procedural optimizations), but very little with ObjC methods because they are always indirect.
To avoid ObjC message dispatch overhead (as you mentioned)
Potential for additional reference counting operations and autorelease pool activity when interacting with ObjC objects.
Of course you won't always hurt paying for things you don't need or use -- and remember that ObjC class methods have some benefits over C functions, too. So, just look at C or C++ implementations as another tool in your toolbox. I find them very useful as complexity and project sizes increase, and they can be used to make your programs much faster. Just do what you are least likely to regret in 2015 ;)
You already touched on the marginal performance difference of avoiding an objc_msgSend call. Objective-C class methods are also subject to overriding in subclasses, so implementing a method in C will prevent it from being overridden in a subclass. Relatedly, because of that runtime inheritance/polymorphism, an Objective-C method can never be inlined, whereas a C function can potentially be inlined by the compiler for added performance.
When it comes to avoiding objc_msgSend, a wise man once told me, "If the overhead of objc_msgSend is too great for you, Objective-C is probably the wrong tool for the job."
Assume that:
New Protocol is declared
Method in this protocol is marked #required
Class conforms to Protocol
Class does not implement the method mentioned in Protocol
At compile time, information about this method is known: i.e. that it is required and that this class and any other classes this class may may extend do not implement it.
Why in this case the compiler issues a warning and not an error?
Errors are only issued when the compiler cannot continue because something went terribly wrong.
When calling a method in Objective-C, the method lookup is done during runtime and not during compilation, which C++ does. In Objective-C a "message" is simply sent to the object, something like obj.executeCommand("Hey, can you execute function <name> for me?"). In C++ the object will be called directly, in a way like obj.<name>(). In the case of Objective-C the executeCommand() method is called, which exists. In C++'s case the function is called but it does not exist. These are methods that are linked on the compiler level, which means they both become memory addresses rather than names. executeCommand becomes 0x12345678 but it still uses the same message ("execute function <name>").
This is probably very confusing, but it's related to the way methods are implemented in different languages.
If you feel strongly about it, why not turn on -Werror?
I don't know the real answer but here is a use case that would go against it.
What if you implemented all of the protocol methods in a category???
Main interface declaration adopts the protocol however the protocol method implementation is in a category.
This is valid code but will show compile error if compiler was that strict !!
Objective-C is a dynamic language. The idea of what an implementation is, is different to a static language.
For the most part, it's in code that most of us implement inside the #implementation ... #end block.
But what if a method is not found? Then an object has a chance deal with it dynamically.
Imagine you have an interface for a sound effect player:
#protocol FX
- (void)playBeep;
- (void)playSiren;
- (void)playHonk;
#end
An implementation could have the files Beep.mp3, Siren.mp3, Honk.mp3 to play, but instead of implementing each of the methods, it could override -forwardInvocation: and parse the selector string, something like this pseudocode:
NSString *selName = NSStringFromSelector([invocation selector]);
if ([selName startsWith:#"play"]) {
NSString filename = fileNameFromSelector(selName);
[self playSoundFileNamed:filename];
}
This may seem contrived, but once you start using the dynamic features of the language, you will start finding more and more places where it makes sense. And by sense I mean, does this effort help in the long run?
In the above case, just add a -sound* method name to the interface, and drop in a appropriately named sound file. It just works.
Another example from personal experiments: how to deal with Core Data entities in a more natural way. I want to do this:
NSArray *people = [Person findAllWithNameLike:#"B%"];
instead of mucking about with predicates, fetch requests etc.
But I don't want to define every permutation of method in code.
How about if I wanted to build an XML builder? I would look at a dynamic approach. It has served Groovy Builders well (look at Groovy/Grails for examples).
One last example: I have a traits system where I can define behaviours in the form of groups of methods and have my objects assimilate this behaviour. So, while the compiler doesn't see an implementation for the interface my object conforms to, the implementation is injected into it from a trait class, using the Objective-C runtime. Why would I do this? I find many delegate methods are boiler plate, but at the same time, a single base class for each situation is not flexible enough. Instead of cut and paste from code samples, my 'samples' compile and run :) and any changes are reflected across all projects using the trait.
To really understand why all this is available to you, it is worth playing around with a Smalltalk environment (search Pharo or Squeak). This is where Objective-C has its roots.
And finally, to stop these warnings:
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wprotocol"
#implementation ... #end
#pragma clang diagnostic pop
Because there are times when there are bogus "required" methods in a poorly designed protocol.
They should have been optional but someone insisted they are "required".
Thusly making this a run time issue rather than a compile bug is very very wise.
I read everywhere that Objective-C has true dynamic binding, where as C++ has only Late binding. Unfortunately none of the books go on to explain it clearly or discuss the underlying implementation. For e.g C++ uses virtual table. How about Objective-C?
http://www.gnu.org/software/gnustep/resources/ObjCFun.html has a pretty good description.
Basically what dynamic binding means is that at the time that the method call is actually made, the decision is made about what method to invoke. And the method can, if you wish, be dynamically chosen at that point.
Edit: Here is a lot more detail to the best of my understanding. I do not promise that it is entirely correct, but it should be mostly right. Every object in Objective C is a struct whose first member, named isa, is a pointer to a class. Each class is itself an object that is traditionally laid out as:
struct objc_class {
Class isa;
Class super_class;
const char *name;
long version;
long info;
long instance_size;
struct objc_ivar_list *ivars;
struct objc_method_list **methodLists;
struct objc_cache *cache;
struct objc_protocol_list *protocols;
};
At runtime, here is pseudo-code for what happens on a method lookup:
Follow isa to find the class
if implementation = class.lookup_method(method):
call implementation
else if get_implementation = class.lookup_method(forwardInvocation):
implementation = get_implementation(method)
if implementation:
call implementation
else:
raise runtime error
else:
raise runtime error
And how does that lookup_method work?
def lookup_method (class, method):
if method in class.objc_cache:
return implementation from objc_cache
else if method in class.objc_method_list:
cache implementation from objc_method_list
return implementation
else if implementation = class.super_class.lookup_method(method):
cache implementation
return implementation
else:
return null
In response to the obvious question, yes this is much slower than C++'s virtual tables. According to benchmarks, about 1/3 of the speed. Every Objective C text immediately follows that up with the fact that in the real world, method lookup speed is almost never a bottleneck.
This is much more flexible than C's method lookups. For instance you can use forwardInvocation to cause unrecognized methods to go to an object that you have in a variable. This kind of delegation can be done without knowing what the type of that object will be at run time, or what methods it will support. You can also add methods to classes - even at runtime if you wish - without having access to the source code. You also have rich runtime introspection on classes and methods.
The obvious flip side, that any C++ programmer will be jumping up and down about, is that you've thrown away any hope of compile time type checking.
Does that explain the differences and give you sufficient detail to understand what is going on?
Both dynamic binding and late binding are the same,in fact. In we have static binding ,or early binding , which checks the issues which happen at compile time(errors regarding thevariables,expressions etc) and these information are stored in a v-table(virtual method table). What late binding does is that it just binds the methods with those in the v-table.
What is better and why ?
What is better in such situations as the class init method and usual variables in a code ?
What is better and why ?
Explicit typing information is always better unless you just can't use it for some reason (see below).
It allows the compiler to much more stringently validate the code and will catch many errors at compile time that would otherwise cause your app to crash at runtime.
A long, long, time ago, everything in the APIs used id. This proved to be a complete pain in the butt; fragile and led to many crashes that would have been caught with specific types.
So, it was changed. (This was ~1994).
What is better in such situations as
the class init method and usual
variables in a code ?
For init, you have no choice but to use the generic (id) return type. Objective-C does not support either co-variant or contra-variant declarations, nor is there a mechanism for generalizing the declaration of init while also providing support for specific type checking.
Same goes for retain, objectAtIndex:, addObject: and many other methods that take or return one of many kinds of objects (or take 'em as arguments).
And, no, there is absolutely no performance difference whatsoever between id and, say, NSView*.
can you give an example when explicit
typing will cause a problem please?
If you wrote:
- (MyClass *) init;
And in a subclass:
- (MySubclass *) init;
You'd get compiler warnings out the wazoo most likely or you'd have to typecast out the wazoo.
On recent versions of clang (in Lion) you should actually not return id, and instead return instancetype. This is a keyword that is used in return types to specify that the type it returns is an instance of the class receiving the message. It is now the preferred return type for init methods on OS X Lion.
Explicit typing provides build-time protection, informing you of likely problems if you do things such as casting or performing operations on something that probably won't work.
Explicit typing also helps prevent non-obvious transfer of mistyped objects, something traveling through a path in your code you hadn't considered that turns out to be of an unexpected type. This kind of bug often doesn't become clear until the program has been tested a lot, more commonly after its release.
It's also helpful for future programmers (including your future self) trying to work with your code, making to more likely that they'll be able to tell at glance what an object is supposed to be. It makes code more "self-documenting" as a result.
Some things cannot have a meaningful type because no type actually applies. Other times you need to use id because you need to be able to accept any type of object. Cocoa Touch uses it, for example, when referring to the sender of a message because anything could have sent it; specifying an explicit type simply wouldn't work.
The vast majority of the time, though, an explicit type is to your advantage.
Use a type as specific as you can but not more so. Consider how you are using any particular variable, argument, or return type and set its type appropriately.
For example a UITableView's dataSource property is declared as id<UITableViewDataSource> because the table view only cares that its data source is some object which conforms to the UITableViewDataSource protocol. This allows the data source to be flexible enough for use with any specific class which implements the protocol but still allows the compiler to warn you if you attempt to assign an object which does not implement that protocol as the data source.
If you are too specific then your code becomes inflexible, accepting only specific implementations which are not strictly necessary (ie demanding a NSMutableString when you could really work with any NSString).
If you are too vague (typing everything as id for example) then you lose the ability to identify when you are sending unrecognized selectors to a particular instance and the compiler cannot identify any number of invalid statements.
For init methods follow the advice in The Objective-C Programming Language
The return type of an initializer method should be id.
The reason for this is that id gives an indication that the class is purposefully not considered—that the class is unspecified and subject to change, depending on context of invocation. For example, NSString provides a method initWithFormat:. When sent to an instance of NSMutableString (a subclass of NSString), however, the message returns an instance of NSMutableString, not NSString. (See also, though, the singleton example given in “Combining Allocation and Initialization.”)
I don't think there is a performance difference between both.
You can let id return type for init because you can cast the result of your init.
For exemple :
Toto *foo = [[Toto alloc] init];
id foo2 = [[Toto alloc] init];
Both work but you'll have to cast foo2 variable like that (Toto *)foo in order to access property or methods of your instance without creating a compiler warning. Even if it works fine...
I think some developers let id because they just pass there variable trough instance and don't use it. That kind of use allow to not import the .h
Regards,
KL94
I've seen a number of strategies for declaring semi-private methods in Objective-C, but there does not seem to be a way to make a truly private method. I accept that. But, why is this so? Every explanation I've essentially says, "you can't do it, but here's a close approximation."
There are a number of keywords applied to ivars (members) that control their scope, e.g. #private, #public, #protected. Why can't this be done for methods as well? It seems like something the runtime should be able to support. Is there an underlying philosophy I'm missing? Is this deliberate?
The answer is... well... simple. Simplicity and consistency, in fact.
Objective-C is purely dynamic at the moment of method dispatch. In particular, every method dispatch goes through the exact same dynamic method resolution point as every other method dispatch. At runtime, every method implementation has the exact same exposure and all of the APIs provided by the Objective-C runtime that work with methods and selectors work equally the same across all methods.
As many have answered (both here and in other questions), compile-time private methods are supported; if a class doesn't declare a method in its publicly available interface, then that method might as well not exist as far as your code is concerned. In other words, you can achieve all of the various combinations of visibility desired at compilation time by organizing your project appropriately.
There is little benefit to duplicating the same functionality into the runtime. It would add a tremendous amount of complexity and overhead. And even with all of that complexity, it still wouldn't prevent all but the most casual developer from executing your supposedly "private" methods.
EDIT: One of the assumptions I've
noticed is that private messages would
have to go through the runtime
resulting in a potentially large
overhead. Is this absolutely true?
Yes, it is. There's no reason to suppose that the implementor of a class would not want to use all of the Objective-C feature set in the implementation, and that means that dynamic dispatch must happen. However, there is no particular reason why private methods couldn't be dispatched by a special variant of objc_msgSend(), since the compiler would know that they were private; i.e. this could be achieved by adding a private-only method table to the Class structure.
There would be no way for a private
method to short-circuit this check or
skip the runtime?
It couldn't skip the runtime, but the runtime wouldn't necessarily have to do any checking for private methods.
That said, there's no reason that a third-party couldn't deliberately call objc_msgSendPrivate() on an object, outside of the implementation of that object, and some things (KVO, for example) would have to do that. In effect, it would just be a convention and little better in practice than prefixing private methods’ selectors or not mentioning them in the interface header.
To do so, though, would undermine the pure dynamic nature of the language. No longer would every method dispatch go through an identical dispatch mechanism. Instead, you would be left in a situation where most methods behave one way and a small handful are just different.
This extends beyond the runtime as there are many mechanisms in Cocoa built on top of the consistent dynamism of Objective-C. For example, both Key Value Coding and Key Value Observation would either have to be very heavily modified to support private methods — most likely by creating an exploitable loophole — or private methods would be incompatible.
The runtime could support it but the cost would be enormous. Every selector that is sent would need to be checked for whether it is private or public for that class, or each class would need to manage two separate dispatch tables. This isn't the same for instance variables because this level of protection is done at compile time.
Also, the runtime would need to verify that the sender of a private message is of the same class as the receiver. You could also bypass private methods; if the class used instanceMethodForSelector:, it could give the returned IMP to any other class for them to invoke the private method directly.
Private methods could not bypass the message dispatch. Consider the following scenario:
A class AllPublic has a public instance method doSomething
Another class HasPrivate has a private instance method also called doSomething
You create an array containing any number of instances of both AllPublic and HasPrivate
You have the following loop:
for (id anObject in myArray)
[anObject doSomething];
If you ran that loop from within AllPublic, the runtime would have to stop you sending doSomething on the HasPrivate instances, however this loop would be usable if it was inside the HasPrivate class.
The answers posted thus far do a good job of answering the question from a philosophical perspective, so I'm going to posit a more pragmatic reason: what would be gained by changing the semantics of the language? It's simple enough to effectively "hide" private methods. By way of example, imagine you have a class declared in a header file, like so:
#interface MyObject : NSObject {}
- (void) doSomething;
#end
If you have a need for "private" methods, you can also put this in the implementation file:
#interface MyObject (Private)
- (void) doSomeHelperThing;
#end
#implementation MyObject
- (void) doSomething
{
// Do some stuff
[self doSomeHelperThing];
// Do some other stuff;
}
- (void) doSomeHelperThing
{
// Do some helper stuff
}
#end
Sure, it's not quite the same as C++/Java private methods, but it's effectively close enough, so why alter the semantics of the language, as well as the compiler, runtime, etc., to add a feature that's already emulated in an acceptable way? As noted in other answers, the message-passing semantics -- and their reliance on runtime reflection -- would make handling "private" messages non-trivial.
The easiest solution is just to declare some static C functions in your Objective-C classes. These only have file scope as per the C rules for the static keyword and because of that they can only be used by methods in that class.
No fuss at all.
Yes, it can be done without affecting the runtime by utilizing a technique already employed by the compiler(s) for handling C++: name-mangling.
It hasn't been done because it hasn't been established that it would solve some considerable difficulty in the coding problem space that other techniques (e.g., prefixing or underscoring) are able to circumvent sufficiently. IOW, you need more pain to overcome ingrained habits.
You could contribute patches to clang or gcc that add private methods to the syntax and generated mangled names that it alone recognized during compilation (and promptly forgot). Then others in the Objective-C community would be able to determine whether it was actually worthwhile or not. It's likely to be faster that way than trying to convince the developers.
Essentially, it has to do with Objective-C's message-passing form of method calls. Any message can be sent to any object, and the object chooses how to respond to the message. Normally it will respond by executing the method named after the message, but it could respond in a number of other ways too. This doesn't make private methods completely impossible — Ruby does it with a similar message-passing system — but it does make them somewhat awkward.
Even Ruby's implementation of private methods is a bit confusing to people because of the strangeness (you can send the object any message you like, except for the ones on this list!). Essentially, Ruby makes it work by forbidding private methods to be called with an explicit receiver. In Objective-C it would require even more work since Objective-C doesn't have that option.
It's an issue with the runtime environment of Objective-C. While C/C++ compiles down into unreadable machine code, Objective-C still maintains some human-readable attributes like method names as strings. This gives Objective-C the ability to perform reflective features.
EDIT: Being a reflective language without strict private methods makes Objective-C more "pythonic" in that you trust other people that use your code rather than restrict what methods they can call. Using naming conventions like double underscores is meant to hide your code from a casual client coder, but won't stop coders needing to do more serious work.
There are two answers depending on the interpretation of the question.
The first is by hiding the method implementation from the interface. This is used, typically with a category with no name (e.g. #interface Foo()). This permits the object to send those messages but not others - though one might still override accidentally (or otherwise).
The second answer, on the assumption that this is about performance and inlining, is made possible but as a local C function instead. If you wanted a ‘private foo(NSString *arg)‘ method, you would do void MyClass_foo(MyClass *self, NSString *arg) and call it as a C function like MyClass_foo(self,arg). The syntax is different, but it acts with the sane kind of performance characteristics of C++'s private methods.
Although this answers the question, I should point out that the no-name category is by far the more common Objective-C way of doing this.
Objective-C doesn't support private methods because it doesn't need them.
In C++, every method must be visible in the declaration of the class. You can't have methods that someone including the header file cannot see. So if you want methods that code outside your implementation shouldn't use, you have no choice, the compiler must give you some tool so you can tell it that the method must not be used, that is the "private" keyword.
In Objective-C, you can have methods that are not in the header file. So you achieve the same purpose very easily by not adding the method to the header file. There's no need for private methods. Objective-C also has the advantage that you don't need to recompile every user of a class because you changed private methods.
For instance variables, that you used to have to declare in the header file (not anymore), #private, #public and #protected are available.
A missing answer here is: because private methods are a bad idea from an evolvability point of view. It might seem a good idea to make a method private when writing it, but it is a form of early binding. The context might change, and a later user might want to use a different implementation. A bit provocative: "Agile developers don't use private methods"
In a way, just like Smalltalk, Objective-C is for grown-up programmers. We value knowing what the original developer assumed the interface should be, and take the responsibility to deal with the consequences if we need to change implementation. So yes, it is philosophy, not implementation.