Question

I’m currently trying to learn Objective-C by reading books and online tutorials, also referring to the Apple documentation but some things just don’t click. I have a question about classes, I have been using the NSString for a while now without putting too much attention on how it is used.

I was under the impression that in order for someone to be able to use a method from a certain class in Objective-C you first needed to instantiate it, something like…

ClasssName *varName = [[ClassName alloc]init];

Then you would call methods like…

[varName someMethod];

But looking at how the NSString is used I’m now I little confused, for instance here is how we would normally use it…

NSString *someString = @"some text here";
[someString stringByAppendingFormat:  @"some text = %d", 3];

Following what I have read about classes we would need to do something like the following instead.

NSString *someString  =  [[NSString alloc]initWithString: @"some text here"];
[someString stringByAppendingFormat:  @"some text = %d", 3];

Can someone explain why some classes do not require instantiation before using its methods?

Answer 1

Objective-C knows some abbreviations that are called literals. The compiler is aware of the special notation. A string literal is compiled into the binary and exits throughout the lifetime of an app.

For most cases it will behave like an object created on runtime.

• if two literals are identical, only one object will be created and live forever
• if you create NSString *string = [[NSString alloc] initWithString:@"My String"]; with @"My String" being used as a literal before, also string can point to it.

Since Apple LLVM Compiler 4.0 Objective-C knows some more literals. But in contrast to string literals these literals are created during runtime through convenient initializers.

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Note that

[someString stringByAppendingFormat:  @"some text = %d", 3];

does not change someString. It returns a new string object.

NSString *newString = [someString stringByAppendingFormat:  @"some text = %d", 3];