@"%@ in Objective C?

Question

I was following a tut and found a line of code like @"%@ button pressed.". I'm pretty sure the relevant part is the %@, but is the first @ an escape sequence or what?

Anyways, searching symbols doesn't go well in any search engine so I thought I'd ask. I think the %@ is like {0} in C#?

Answer 1

I found this page which might help - http://www.shrinkrays.net/code-snippets/csharp/objective-c-by-example-for-a-c-developer.aspx

It seems that you are on the right track.

Answer 2

@"some string" means this is an NSString literal.

The string as show in @"CupOverflowException", is a constant NSString object. The @ sign is used often in Objective-C to denote extentions to the language. A C string is just like C and C++, "String constant", and is of type char *

Answer 3

%@ is a format specifier. Functions such as NSLog and methods such as +stringWithFormat: will replace %@ with the description of the provided Objective-C or Core Foundation object argument.

For example:

NSString *myName = @"dreamlax";

NSLog (@"My name is: %@", myName);

This will log the output "My name is: dreamlax". See here for more information format specifiers.

The initial @ symbol at the beginning of the string tells the compiler to create a static instance of an NSString object. Without that initial @ symbol, the compiler will create a simpler C-style string. Since C-style strings are not Objective-C objects you cannot add them to NSArray or NSDictionary objects, etc.

Answer 4

I'm still fairly new to the language, but it looks like the @ specifies that the variable being passed/created is an NSObject, or a compiler directive.

As mentioned above, if you use it like this:

@"someText"

you're instantiating an NSString object, and setting the text of that object to someText. If you look at a good ol' C-style format specifier such as:

..."This is some text, and this is a float: %f", myFloat);

You're creating some text and telling the compiler to put the floating point string representation of myFloat into the string. %@ is a format specifier, just like %f, %d, %c, %s and any other format specifier you're used to. However, if you use %@ as follows:

... "This is some text, and this is an object:%@", myObject];

What you're doing is (I believe) telling the compiler that myObject is an object, and that you want it to include the output of the description method (ie. [myObject description]) in the string that you're creating.