Style: Dot notation vs. message notation in Objective-C 2.0

Question

In Objective-C 2.0 we got 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).

Answer 1

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

Answer 2

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.

Answer 3

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.

Recent blog post against dot syntax: http://weblog.bignerdranch.com/?p=83

Rebuttal to above post: http://eschatologist.net/blog/?p=226 (with original article in favor of dot syntax: http://eschatologist.net/blog/?p=160)

Answer 4

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!

Answer 5

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).

Answer 6

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.

Answer 7

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?

Answer 8

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.

Answer 9

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.

Answer 10

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¢.

Answer 11

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.

Answer 12

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.

Answer 13

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.