In Objc string, array and dictionary are all reference types, while in Swift they are all value types.
I want to figure out what's the reason behind the scenes, for my understanding, no matter it is a reference type or value type, the objects live in the heap in both Objc and Swift.
Was the change for making coding easier? i.e. if it is reference type then the pointer to the object might not be nil, so need to check both pointer and the object not nil for accessing the object. While if it is value type then only need to check the object itself?
But in terms of memory allocation, value types and reference types are same, right? both allocated same size of memory?
thanks
Arrays, dictionaries etc. in Objective-C are often mutable. That means when I pass an array to another method, and then that array is modified behind the back of the other method, surprising (to put it gently) behaviour will happen.
By making arrays, dictionaries etc. value types, this surprising behaviour is avoided. When you receive a Swift array, you know that nobody is going to modify it behind your back. Objects that can be modified behind your back are a major source for problems.
In reality, the Swift compiler tries to avoid unnecessary copying whenever possible. So even if it says that an array is officially copied, it doesn't mean that it is really copied.
The Swift team is very active on the official developer forums. So, I'm assuming that since you didn't ask there, you're more curious about the community's broader "sense" of what the change means, as opposed to the technical implementation details. If you want to understand exactly "why", just go ask them :)
The explanation that makes the most sense to me is that Objects should be responsible for reacting to, and updating the state of your application. Values should be the state of your application. In other words, an Array or a String or a Dictionary (and other value types) should never be responsible for responding to user input or network input or error conditions, etc. The Objects handle that and store the resulting data into those values.
One cool feature in Swift, which makes a complex Value Type (like a Dictionary or a custom type like Person, as opposed to a simple Float) more viable, is that the value types can encapsulate rules and logic because they can have functions. If I write a value type Person as a struct, then the Person struct can have a function for updating a name due to marriage, etc. That's solely concerned with the data, and not with /managing/ the state. The Objects will still decide WHEN and WHY to updating a Person's name, but the business logic of how to go about doing so safely/test-ably can be included in the Value Type itself. Hence giving you a nice way to increase isolation and reduce complexity.
In addition to the previous answers, there are also multi-threading issues to consider with sharing a Reference-Based collection type that we don't have to worry as much with sharing an instance of a type that is Value-Based and has Copy-On-Write behavior. Multi-core is becoming more and more proliferant even on iOS devices, so it has become more of an issue for the Swift language developers to consider.
I do not know, whether this is the real idea behind it, but have a historical view on it:
At the beginning, an array copy behaved by reference, when you changed an item in it. It behaved by value, when you changed the length of the array. They did it for performance reasons (less array copy). But of course this was, eh, how can I express that politly, eh, difficult with Swift at all, eh, let's call it a "do not care about a good structure if you can win some performance, you probably never need" approach. Some called that copy-on-write, what is not much more intelligent, because COW is transparent, while that behavior was not transparent. Typical Swift wording: Use a buzzword, use it the way, it fits to Swift, don't care about correctness.
Later on arrays got a complete by copy behavior, what is less confusing. (You remember, Swift was for readability. Obviously in Swift's concept, readability means "less characters to read", but does not mean "better understandable". Typical Swift wording: Use a buzzword, use it the way, it fits to Swift, don't care about correctness. Did I already mention that?)
So, I guess it is still performance plus understandable behavior probably leading to less performance. (You will better know when a copy is needed in your code and you can still do that and you get a 0-operation from Cocoa, if the source array is immutable.) Of course, they could say: "Okay, by value was a mistake, we changed that." But they will never say.
However, now arrays in Swift behave consistently. A big progress in Swift! Maybe you can call it a programming language one sunny day.
With all the useful things you get from NSArray and NSMutableArray, why would you ever use a "C Style" array with Objective-C objects?
NSString *array[] = {#"dog", #"cat", #"boy"};
For short, fixed arrays, the availability of a nice compact initialization syntax (as you've demonstrated) can be nice. In certain cases, a C style array may also offer a performance benefit compared to using NSArray. Another thing that comes to mind is that NSArray doesn't offer any built in support for multidimensional arrays, while multidimensional C arrays are easy.
And of course, there's the fact that you can only store objects in NSArray, not C-primitive types, but you asked specifically about using C arrays with Objective-C objects.
All useful things come at a price.
Ever try making an NSArray of ints? You can't. You MUST use NSNumber.
Why? Because NSArray doesn't know what to do with things it doesn't know how to memory-manage... and the only things that conform to it's memory-management requirements are things based on NSObject.
It's just one example, but there are plenty of others.
Bottom line is, there is rarely ever one single "best" answer to anything. NSArray is no exception.
This question is just out of curiosity but, how is NSSet implemented? What data structure is behind it and what are the access times for adding and removing elements? If I had to guess, I'd say it was some sort of hashtable/dictionary data structure, but in that case why differentiate between NSSet and NSMutableSet?
Well, as Bavarious pointed out in a comment, Apple's actual CoreFoundation source is open and available for your perusal too. NSSet is implemented on top of CFSet, whose code is generated (as is that of CFDictionary) from a hash table template, using CFBasicHash to do the work.
The difference between mutablility and immutability seems to be the matter of a flag in the structure (line 91 of CFBasicHash.h), and from my reading so far just affects calls to functions such as CFBasicHashAddValue; there's a simple check for the mutability. It seems likely, however, that Cobbal is right about the copy/retain behavior between the two (I just haven't read that far yet).
PREVIOUSLY:
I find it interesting and educational occasionally to peruse the GNUstep sources when I'm wondering about implementation details. They are, of course, not at all guaranteed to be implemented the way that Apple did it, but they can be helpful in some cases. Their version of Foundation: http://gnu.ethz.ch/debian/gnustep/gnustep-base-1.20.0/Headers/Foundation/ (I hope that's the most recent version. If not, someone please correct me.)
To answer the second half of your question: one benefit of having a non-mutable version is that it allows for a very fast copy method that simply calls retain.
I find this link to be an interesting answer to your question. Apple's data structures (NSArray, NSSet, NSDictionary, etc.) are not implemented in a straightforward and "standard way." In most cases, they perform in the same way any other set would perform, but overall, they optimize automatically for the best performance. So, in truth, it's rather difficult to say. While Apple provides documentation on the efficiency of arrays in CFArray.h (equivalent for NSArrays), it offers no such documentation on the efficiency of sets, though you're free to poke around /System/Library/Frameworks/CoreFoundation.framework/Headers/ to look through other data structure implementations.
In addition, there has to be a distinction between a set and its mutable counterpart, just as there is a distinction between NSString and NSMutableString, NSArray and NSMutableArray, and NSDictionary and NSMutableDictionary (among others). For data structures and strings (and few other classes), Apple offers 'readonly' versions of classes to retain generality, along with standard 'mutable' counterparts for manipulation. It's simply Apple's standard practice.
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We now have the "dot" notation for properties. I've seen various back and forths about the merits of dot notation vs. message notation. To keep the responses untainted I'm not going to respond either way in the question.
What is your thought about dot notation vs. message notation for property accessing?
Please try to keep it focused on Objective-C - my one bias I'll put forth is that Objective-C is Objective-C, so your preference that it be like Java or JavaScript aren't valid.
Valid commentary is to do with technical issues (operation ordering, cast precedence, performance, etc), clarity (structure vs. object nature, both pro and con!), succinctness, etc.
Note, I'm of the school of rigorous quality and readability in code having worked on huge projects where code convention and quality is paramount (the write once read a thousand times paradigm).
Do not use dot for behavior. Use dot to access or set attribute like stuff, typically attributes declared as properties.
x = foo.name; // good
foo.age = 42; // good
y = x.retain; // bad
k.release; // compiler should warn, but some don't. Oops.
v.lockFocusIfCanDraw; /// ooh... no. bad bad bad
For folks new to Objective-C, I would recommend not using the dot for anything but stuff declared as #property. Once you have a feel for the language, do what feels right.
For example, I find the following perfectly natural:
k = anArray.count;
for (NSView *v in myView.subviews) { ... };
You can expect that the clang static analyzer will grow the ability to allow you to check that the dot is being used only for certain patterns or not for certain other patterns.
Let me start off by saying that I started programming in Visual/Real Basic, then moved on to Java, so I'm fairly used to dot syntax. However, when I finally moved to Objective-C and got used to brackets, then saw the introduction of Objective-C 2.0 and its dot syntax, I realized that I really don't like it. (for other languages it's fine, because that's how they roll).
I have three main beefs with dot syntax in Objective-C:
Beef #1: It makes it unclear why you might be getting errors. For example, if I have the line:
something.frame.origin.x = 42;
Then I'll get a compiler error, because something is an object, and you can't use structs of an object as the lvalue of an expression. However, if I have:
something.frame.origin.x = 42;
Then this compiles just fine, because something is a struct itself that has an NSRect member, and I can use it as an lvalue.
If I were adopting this code, I would need to spend some time trying to figure out what something is. Is it a struct? Is it an object? However, when we use the bracket syntax, it's much clearer:
[something setFrame:newFrame];
In this case, there is absolutely no ambiguity if something is an object or not. The introduction of ambiguity is my beef #1.
Beef #2: In C, dot syntax is used to access members of structs, not call methods. Programmers can override the setFoo: and foo methods of an objects, yet still access them via something.foo. In my mind, when I see expressions using dot syntax, I'm expecting them to be a simple assignation into an ivar. This is not always the case. Consider a controller object that mediates an array and a tableview. If I call myController.contentArray = newArray;, I would expect it to be replacing the old array with the new array. However, the original programmer might have overridden setContentArray: to not only set the array, but also reload the tableview. From the line, there's no indication of that behavior. If I were to see [myController setContentArray:newArray];, then I would think "Aha, a method. I need to go see the definition of this method just to make sure I know what it's doing."
So I think my summary of Beef #2 is that you can override the meaning of dot syntax with custom code.
Beef #3: I think it looks bad. As an Objective-C programmer, I'm totally used to bracket syntax, so to be reading along and see lines and lines of beautiful brackets and then to be suddenly broken with foo.name = newName; foo.size = newSize; etc is a bit distracting to me. I realize that some things require dot syntax (C structs), but that's the only time I use them.
Of course, if you're writing code for yourself, then use whatever you're comfortable with. But if you're writing code that you're planning on open sourcing, or you're writing something you don't expect to maintain forever, then I would strong encourage using bracket syntax. This is, of course, just my opinion.
Blog post against dot syntax: https://bignerdranch.com/blog/dot-notation-syntax/
Rebuttal to above post: http://eschatologist.net/blog/?p=226 (with original article in favor of dot syntax: http://eschatologist.net/blog/?p=160)
I'm a new Cocoa/Objective-C developer, and my take on it is this:
I stick to the messaging notation, even though I started with Obj-C 2.0, and even though the dot notation is more familiar feeling (Java is my first language.) My reason for this is pretty simple: I still don't understand exactly why they added the dot notation to the language. To me it seems like an unnecessary, "impure" addition. Although if anyone can explain how it benefits the language, I'd be happy to hear it.
However, I consider this a stylistic choice, and I don't think there is a right or wrong way, as long as it's consistent and readable, just as with any other stylistic choice (like putting your opening curly brace on the same line as the method header or the next line).
Objective-C dot notation is a syntactic sugar that is translated to normal message passing, so under the hood changes nothing and makes no difference at runtime. Dot notation it is absolutely not faster than message passing.
After this needed little preamble here's pros and cons seen by me :
Dot notation pros and cons
pros
readability : dot notation is easier to read than nested brackets massages passing
It simplifies interaction with Attributes and Properties: using dot notation for properties and message notation for methods you can achieve separation of state and behavior at the synthax level
It is possible to use compound assignment operator (1).
using the #property and dot notation the compiler do a lot of work for you, it can generate code for good Memory Management when getting and setting the property; this is why dot notation is suggested by Apple itself official guides.
cons
Dot notation is allowed only for access to a declared #property
Since Objective-C is a layer above standard C(language extension), the dot notation doesn’t really make clear if the accessed entity is a an object or a struct. Often, it looks like you are accessing properties of a struct.
calling a method with the dot notation you lose named parameters readability advantages
when mixed message notation and dot notation seems like you are coding in two different languages
Code Examples :
(1)Compound operator usage code example :
//Use of compound operator on a property of an object
anObject.var += 1;
//This is not possible with standard message notation
[anObject setVar:[anObject var] + 1];
Using the style of a language, consistent with the language itself, is the best advice here. However, this isn't a case of writing functional code in an OO system (or vice versa) and the dot notation is part of the syntax in Objective-C 2.0.
Any system can be misused. The existence of the preprocessor in all C based languages is enough to do really quite weird things; just look at the Obfuscated C Contest if you need to see exactly how weird it can get. Does that mean the preprocessor is automatically bad and that you should never use it?
Using the dot syntax for accessing properties, which have been defined as such in the interface, is open to abuse. The existence of abuse in potentia shouldn't necessarily be the argument against it.
Property access may have side-effects. This is orthogonal to the syntax used to acquire that property. CoreData, delegation, dynamic properties (first+last=full) will all necessarily do some work under the covers. But that would be confusing 'instance variables' with 'properties' of an object. There's no reason why properties should necessarily need to be stored as-is, especially if they can be computed (e.g. length of a String, for example). So whether you use foo.fullName or [foo fullName] there's still going to be dynamic evaluation.
Lastly, the behaviour of the property (when used as an lvalue) is defined by the object itself, like whether a copy is taken or whether it is retained. This makes it easier to change the behaviour later - in the property definition itself - rather than having to re-implement methods. That adds to the flexibility of the approach, with the resulting likelihood of less (implementation) errors occurring. There's still the possibility of choosing the wrong method (i.e. copy instead of retain) but that's an architectural rather than implementation issue.
Ultimately, it boils down to the 'does it look like a struct' question. This is probably the main differentiator in the debates so far; if you have a struct, it works differently than if you have an object. But that's always been true; you can't send a struct a message, and you need to know if it's stack-based or reference/malloc based. There are already mental models which differ in terms of usage ([[CGRect alloc] init] or struct CGRect?). They've never been unified in terms of behaviour; you need to know what you're dealing with in each case. Adding property denotation for objects is very unlikely to confuse any programmer who knows what their data types are; and if they don't, they've got bigger problems.
As for consistency; (Objective-)C is inconsistent within itself. = is used both for assignment and equality, based on lexical position in the source code. * is used for pointers and multiplication. BOOLs are chars, not bytes (or other integer value), despite YES and NO being 1 and 0 respectively. Consistency or purity isn't what the language was designed for; it was about getting things done.
So if you don't want to use it, don't use it. Get it done a different way. If you want to use it, and you understand it, it's fine if you use it. Other languages deal with the concepts of generic data structures (maps/structs) and object types (with properties), often using the same syntax for both despite the fact that one is merely a data structure and the other is a rich object. Programmers in Objective-C should have an equivalent ability to be able to deal with all styles of programming, even if it's not your preferred one.
I've mostly been raised in the Objective-C 2.0 age, and I prefer the dot notation. To me, it allows the simplification of code, instead of having extra brackets, I can just use a dot.
I also like the dot syntax because it makes me really feel like I'm accessing a property of the object, instead of just sending it a message (of course the dot-syntax really does translate into message sending, but for the sake of appearances, the dot feels different). Instead of "calling a getter" by the old syntax, it really feels like I'm directly getting something useful from the object.
Some of the debate around this is concerned with "But we already have dot-syntax, and it's for structs!". And that's true. But (and again, this is just psychological) it basically feels the same to me. Accessing a property of an object using dot-syntax feels the same as accessing a member of a struct, which is more or less the intended effect (in my opinion).
****Edit: As bbum pointed out, you can also use dot-syntax for calling any method on an object (I was unaware of this). So I will say my opinion on dot-syntax is only for dealing with properties of an object, not everyday message sending**
I use it for properties because
for ( Person *person in group.people){ ... }
is a little easier to read than
for ( Person *person in [group people]){ ... }
in the second case readability is interupted by putting your brain into message sending mode, whereas in the first case it is clear you are accessing the people property of the group object.
I will also use it when modifying a collection, for instance:
[group.people addObject:another_person];
is a bit more readable than
[[group people] addObject:another_person];
The emphasis in this case should be in the action of adding an object to the array instead of chaining two messages.
I much prefer the messaging syntax... but just because that is what I learned. Considering a lot of my classes and what not are in Objective-C 1.0 style, I wouldn't want to mix them. I have no real reason besides "what I'm used to" for not using the dot syntax... EXCEPT for this, this drives me INSANE
[myInstance.methodThatReturnsAnObject sendAMessageToIt]
I don't know why, but it really infuriates me, for no good reason. I just think that doing
[[myInstance methodThatReturnsAnObject] sendAMessageToIt]
is more readable. But to each his own!
Honestly, I think it comes down to a matter of style. I personally am against the dot syntax (especially after just finding out that you can use it for method calls and not just reading/writing variables). However, if you are going to use it, I would strong recommend not using it for anything other than accessing and changing variables.
One of the main advantages of object-oriented programming is that there is no direct access to internal state of the objects.
Dot syntax seems to me to be an attempt to make it look and feel as though state is being accessed directly. But in truth, it's just syntactic sugar over the behaviors -foo and -setFoo:. Myself, I prefer to call a spade a spade. Dot syntax helps readability to the extent that code is more succinct, but it doesn't help comprehension because failing to keep in mind that you're really calling -foo and -setFoo: could spell trouble.
Synthesized accessors seem to me to be an attempt to make it easy to write objects in which state is accessed directly. My belief is that this encourages exactly the kind of program design that object-oriented programming was created to avoid.
On balance, I would rather dot syntax and properties had never been introduced. I used to be able to tell people that ObjC is a few clean extensions to C to make it more like Smalltalk, and I don't think that's true any more.
In my opinion, the dot syntax makes Objective-C less Smalltalk-esque. It can make code look simpler, but adds ambiguity. Is it a struct, union, or object?
Many people seems to be mixing up 'properties' with 'instance variables'.
The point of the properties is to make it possible to modify the object without having to know its internals, I think. The instance variable is, most of the time, the 'implementation' of a property (which in turn is the 'interface'), but not always: sometimes a property does not correspond to an ivar, and instead it calculates a return value 'on the fly'.
That's why I believe the idea that "the dot syntax tricks you into thinking you're accessing the variable so it's confusing" is wrong. Dot or bracket, you shouldn't make assumptions about the internals: it's a Property, not an ivar.
I think I might switch to messaging instead of dot notation because in my head object.instanceVar is just instanceVar that belongs to object, to me it doesn't look at all like a method call, which it is, so there could be things going on in your accessor and whether you use instanceVar or self.instanceVar could have much more of a difference than simply implicit vs. explicit. Just my 2¢.
The dot notation tries to make messages look like accesses to a member of a struct, which they are not. Might work fine in some cases. But soon someone will come up with something like this:
NSView *myView = ...;
myView.frame.size.width = newWidth;
Looks fine. But is not. It's the same as
[myView frame].size.width = newWidth;
which doesn’t work. The compiler accepts the first, but rightfully not the second. And even if it emitted an error or warning for the first this is just confusing.
Call me lazy but if I had to type a single '.' vs. two [ ] each time to get the same results I would prefer a single . I hate verbose languages. The () in lisp drove me nuts. Elegant language such as mathematics are concise and effective, all others fall short.
Use dot notation (whenever you can)
On instance methods returning some value
Do not use dot notation
On instance methods returning void, on init methods or on Class method.
And my personal favorite exception
NSMutableArray * array = #[].mutableCopy;
I personally don't use dot-notation at all in code. I only use it in CoreData KVC binding expression when required.
The reason for not using them in code for me is that the dot-notation hides the setter semantics. Setting a property in dot-notation always looks like assignment regardless of the setter semantics (assign/retain/copy). Using the message-notation makes it visible that the receiving object has control over what happens in the setter and underlines the fact the those effects need to be considered.
I'm still considering whether I might want to use dot-notation when retrieving the value of a KVC compliant or declared property because it admittedly is a bit more compact and readable and there are no hidden semantics. Right now I'm sticking with message-notation for sake of consistency.
OK, dot notation in Objective-C looks strange, indeed. But I still can't do the following without it:
int width = self.frame.size.width;
Works fine, but:
int height = [[[self frame] size] height];
Gives me "Cannot convert to a pointer type". I'd really like to keep my code looking consistent with message notation, though.
This is a great question and I see many different answers to this. Although many have touched upon the topics, I will try to answer this from a different angle (some might have done it implicitly):
If we use the 'dot' notation, the resolution of the target for the method is done at compile time. If we use message passing, the resolution of the target is deferred to run-time execution. If the targets are resolved at compile time, the execution is faster, since resolving the targets at the run-time includes a few overheads. (Not that the time difference will matter much). Since we have defined the property in the interface of the object already, there is no point in differing the resolution of the target for a property to run-time and hence dot-notation is the notation that we should use for accessing property.
I've seen it claimed that the following are "pretty much equivalent":
foo([NSString stringWithString:#"blah"]) # version 1
foo([[[NSString alloc] initWithString:#"blah"] autorelease]) # version 2
Are the above in fact literally equivalent or are there any subtle differences?
What are reasons to prefer one or the other?
They are equivalent, but I prefer "stringWithString" since it is more concise.
The two are functionally equivalent, but as rpetrich observes, may operate ever-so-slightly differently internally. This shouldn't matter to you, use whichever seems more convenient to you. Furthermore, while there is a minute performance difference, it is highly unlikely to matter to your application in practice.
But all this misses a crucial point: both are pointless. By writing #"foo" you already have a fully functional NSString object. There is no need to mess around with extra methods to duplicate the string; it is quicker and simpler to just do:
foo(#"blah")
In most cases, the only difference is an extra call to objc_msgSend.
Decompiling NSString reveals that instead of sending +alloc it sends +allocWithZone:NSDefaultMallocZone()
Methods such as +stringWithString: or +array are simply convenience methods which always return autoreleased objects. These are mostly in place to reduce the amount of code written for classes that are created often, such as strings, arrays, dictionaries, numbers, etc. They strictly follow the basic memory management rules from which the one I mentioned above are derived.