Objective-C syntax

Question

I've been studying objective-c for a few days now. I keep coming across these two structures:

1. NSString * somestring

2. (NSString *) somestring

I understand the first simply sets a pointer to an NSString object, but what does the second construct do, and when should I use it? What's the deal with the asterix marks?

Sorry if this question doesn't make any sense, I am completely new to this language, and haven't even reached the level of asking proper questions.

Main purpose -- I'm trying to decipher this method:

-(NSString *)pickerView:(UIPickerView *)pickerView titleForRow:(NSInteger) row forComponent: (NSInteger)component

*Also, the classes I chose don't matter. *

Answer 1

The first one is variable declaration while second is type-casting. I.e. somestring is not of type NSString* so you type-cast it into being NSString*. I believe that the latter case is mostly (if not always) seen on the right side of assignment statement.

In ObjectiveC this comes from C and is also used in many other strictly-typed languages like Java, C# and so on.

In the updated question, your method signature defines that method returns pointer to NSString - (NSString *). And it accepts 3 parameters:

• Pointer to UIPickerView object;
• number of row;
• number of column;

Answer 2

mebFace - an asterisk represents a pointer. So whenever you see (eg) "NSString *" then it means you're dealing with a pointer to a NSString object. The method you're trying to decipher returns a pointer to a string as the result. It takes three parameters: a pointer to a UIPickerView, followed by a couple of integers. The reason that you don't use an asterisk in conjunction with the integers is because they are simple integers - not objects.

You should probably start off by reading a book on the C programming language. This is what Objective-C is built on top of.

Dave

Answer 3

What you want to understand is the method declaration syntax. In Objective-C, a method has a name which looks like this doSomethingWith:andAlso:inMode:. Each colon precedes an argument. When you declare or define a method, you also specify the types of the variables and their names; the type is given in parentheses. You also prepend things with a - for instance methods and a + for static methods, as well as the (parenthesized) return type. Now, you have

- (NSString*)   pickerView:(UIPickerView*)pickerView
               titleForRow:(NSInteger)row
              forComponent:(NSInteger)component
{
  ...
}

When we decipher this, we find that:

1. -: It's an instance method.
2. (NSString*): The return type, NSString*.
3. pickerView:: The first part of the method name.
4. (UIPickerView*)pickerView: The first argument; its name is pickerView and it has type UIPickerView*.
5. titleForRow:: The second part of the method name.
6. (NSInteger)row: The second argument; its name is row and its type is NSInteger.
7. forComponent:: The third part of the method name.
8. (NSInteger)component: The third argument; its name is component and its type is NSInteger.

Thus, putting it all together, this defines the instance method pickerView:titleForRow:forComponent:; it returns an NSString*, and the three arguments it takes are of type UIPickerView*, NSInteger, and NSInteger, respectively. This method would be called as [obj pickerView:myPV titleForRow:myRow forComponent:myComponent].

And just for further reference: in isolation, if you have NSString* str, it declares a variable of type NSString*; and if you have (NSString*)obj, it (forcefully) converts that object to have type NSString*. This has no connection to the method declaration syntax.

---

Edit 1: I also saw that you were asking about the asterisks. In (Objective-)C, this represents a pointer. If you have an int x, say, then when you write int y = x and then y = 3, the original value of x is unchanged. If, however, you have an int* px, then you can write px = &x. (Note that ints, declared as int, are a completely different data type than int pointers declared as int*. Writing int y = &x is garbage, as is int* py = x, and so on.) This & is the "address of" operator; it finds where x is in memory and returns it. Now, if you write int* py = px and then py = &y, this won't change px. But if you write *px, you access the value currently stored in x, and you can change it: *px = 42 sets x to 42. For various reasons, when working with objects, people tend to like to work with references to them instead of their actual values; thus, in Objective-C, you only handle objects through pointers. What this means is that you will never see NSMutableArray x, only NSMutableArray* x; and that if you have NSMutableArray* y = x, then x and y are, roughly speaking, the same, and calling [x addObject:obj] affects y as well. There are more comprehensive tutorials out there—it's worth checking them out if you don't understand pointers—but this should suffice as an overview.

---

Edit 2: In another comment, you say you're coming from Ruby and Python. In Ruby (and I think Python, but I've used it less), every variable is a reference. This means that the basic use of pointers for object types should be familiar; as long as you never use & or *, they'll function in pretty much the same way. The difference between pointers and references is that you can take references to objects and create pointers to pointers. For instance, many methods end in ...error:(NSDictionary**)error. This effectively provides an extra return value; in the method, if something goes wrong, they can write *error = myErrorInfo. Since function arguments are copied, error = myErrorInfo wouldn't be visible; however, the pointer's referent is still the same, and so it can be assigned to. If you then write code such as:

NSDictionary* errorDict = nil;
[obj unsafeOperation:@"http://stackoverflow.com" error:&errorDict];

You pass in a pointer to errorDict so that the unsafeOperation:error: method can write to errorDict and you can see it.

Answer 4

Confusingly, in Objective-c, both of your examples mean different things in different contexts.

When declaring a variable (and other places, but not everywhere else) the asterisk means 'pointer to', ie…

int foo;  // An int variable
int *bar; // A pointer to an int

When it comes to objective-c objects we mostly deal with pointers to them, ie.. you will not see this..

NSString myString;

you will always see this

NSString *myString;

The reason why is quite involved, but is to do with where the memory for the object is allocated (the heap) and what happens when you pass a variable to a function or method. A value passed as an argument to a method is copied. It turns out that copying our string when we pass it to a method is probably not what we intended, so we pass a pointer to the string instead, and that is copied. Imagine a string that contains the complete works of Shakesphere, roughly 5 million bytes. It is inefficient to copy it every time we pass it to a method, but a pointer to it is only 4 bytes, and a pointer gives us just as good access to the data because it, well, points to it. This approach wouldn't get us anything for the int argument tho, as the int and the pointer to the int are going to be the same size (or ballpark, at least).

Your second example is a method or a method declaration. It might help to compare it to function definition in something like javascript:

function pickerViewTitleForRowForComponent( pickerView, row, component ) {
    ...do something here
    return result;
}

Objective-c does things slightly better, sorry, i mean differently. The above would basically look like

- pickerView: pickerView titleForRow: row forComponent: component {
    ...do something here
    return result;
}

The arguments are the same: pickerView, row and component. However, the above isn't a valid method because it is missing type info. ie. We must explicitly say what type each argument is and also the type of the object returned from the method.

- (return type)pickerView:(argument type)pickerView titleForRow:(argument type)row forComponent:(argument type)component {
    ...do something here
    return result;
}

As you can probably see, in your example:

return type > a pointer to an NSString object -- (NSString *)

argument1 type > a pointer to a UIPickerView object -- (UIPickerView *)

argument2 type > an NSInteger -- (NSInteger)

argument3 type > an NSInteger -- (NSInteger)