Compare method

Either you implement a compare-method for your object:

- (NSComparisonResult)compare:(Person *)otherObject {
    return [self.birthDate compare:otherObject.birthDate];
}

NSArray *sortedArray = [drinkDetails sortedArrayUsingSelector:@selector(compare:)];

NSSortDescriptor (better)

or usually even better:

NSSortDescriptor *sortDescriptor;
sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"birthDate"
                                           ascending:YES];
NSArray *sortedArray = [drinkDetails sortedArrayUsingDescriptors:@[sortDescriptor]];

You can easily sort by multiple keys by adding more than one to the array. Using custom comparator-methods is possible as well. Have a look at the documentation.

Blocks (shiny!)

There's also the possibility of sorting with a block since Mac OS X 10.6 and iOS 4:

NSArray *sortedArray;
sortedArray = [drinkDetails sortedArrayUsingComparator:^NSComparisonResult(Person *a, Person *b) {
    return [a.birthDate compare:b.birthDate];
}];

Performance

The -compare: and block-based methods will be quite a bit faster, in general, than using NSSortDescriptor as the latter relies on KVC. The primary advantage of the NSSortDescriptor method is that it provides a way to define your sort order using data, rather than code, which makes it easy to e.g. set things up so users can sort an NSTableView by clicking on the header row.

People who come from other environments always seem to belive that something complicated is happening with the .c, .m, and .h files used in C and Objective-C programming.

Actually, its very, VERY simple.

For the purpose of buiding a project Integrated Development Environments - like XCode - ignore all the .h files. What they do do is to take each .c and .m file and compile it. If the programmer (thats you) has used any #include, or #import directives, the compiler inserts the entire text of the included/imported .h file where the directive was.

So, if you had a .h file - insert.h - that said:

in

And a .c file that said:

Alice
#include "insert.h"
Wonderland

The compiler would, after processing the #include & #import directives, see this:

Alice
in
Wonderland

It is this very VERY simple file merging behavior that we use to make complicated programs :)

.h is very simply a convention by which programmers can tell each other that the file is suitable to be merged in - potentially multiple times - using #include or #import.

The .c and .m files are not merged like that. Each .c and .m file is compiled seperately - to produce .o files. Each .o file is a collection of compiled functions. The .o files are then merged - or "linked" - to produce the final program. The linking step ensures that each function exists only once, and that all functions that are called do in fact exist somewhere.

C & Objctive-C define one special function that must exist somewhere - main(). Again, the language is very relaxed - it doesn't care which .c or .m file the main() function is in. Merely that it exists in some file somewhere.