Overloading C Functions with Clang

Clang, LLVM's compiler front-end for C-based languages, features a couple of interesting language extensions to C, C++ and Objective-C. You are probably familiar with auto-synthesis of properties or the new subscripting syntax.

One of the lesser known extensions, however, is the __attribute__((overloadable)) annotation.

While languages like Java and C++ allow you to define multiple functions with the same name but different arguments, this feature has been absent from C.

However, using recent versions of Clang you can now rectify this behavior and since Objective-C is a superset of C, knowing how to use this feature can be useful even if you rarely venture outside of Cocoa or Cocoa Touch.

Consider for example these function declarations:

#define OVERLOADABLE __attribute__((overloadable))

OVERLOADABLE NSArray *map(NSArray *array, id(^)(id obj));
OVERLOADABLE NSDictionary *map(NSDictionary *dictionary, id(^)(id key, id obj));

We declare two different versions of the classic map function.
Instances of NSArray will have a block applied to their elements that takes a single argument while instances of NSDictionary have both their keys and values sent to the block.

The compiler will figure out which map it needs to call simply based on the types of the arguments.

The implementation is pretty straightforward, too:
-[NSArray enumerateObjectsUsingBlock:] is used to iterate over all elements. If the block passed to map returns nil, the element is discarded.

OVERLOADABLE NSArray *map(NSArray *array, id(^block)(id))
    NSMutableArray *result = [NSMutableArray arrayWithCapacity:array.count];

    [array enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        id mapped = block(array);

        if (mapped) [result addObject:mapped];

    return result;

You may want to consider writing a parallel version of map that makes use of -[NSArray enumerateObjectsWithOptions:usingBlock:] and passes in NSEnumerationConcurrent.

The implementation for dictionaries looks similar:

OVERLOADABLE NSDictionary *map(NSDictionary *dict, id(^block)(id, id))
    NSMutableDictionary *result = [NSMutableDictionary dictionaryWithCapacity:dict.count];

    [dict enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
        id mapped = block(key, obj);

        if (mapped) result[key] = mapped;

    return result;

Now we have a nice and clean way to map over Cocoa's most prevalent data structures:

NSArray *numbers = @[ @1, @2, @3, @4 ];

NSArray *doubled = map(numbers, ^NSNumber *(NSNumber *number) {
    return @(2 * number.doubleValue);

NSDictionary *user = @{
  @"username": @"robb",
  @"website": @"http://robb.is"

// Implementing 'each' and its concurrent equivalent
// is left as an exercise to the reader.
each(user, ^(NSString *key, id obj) {
    NSLog(@"%@: %@", key, obj);

I think function overloading is a welcome addition to C and it goes to show how knowing the underpinnings of Objective-C can help you write better and more concise code.

Update: Nick Lockwood raised the point that map may be better suited for a category. When I came up with this example, I was also considering an each implementation that could deal with id<NSFastEnumeration>. E.g.:

OVERLOADABLE void each(id<NSFastEnumeration> list, void(^)(id obj));
OVERLOADABLE void each(NSArray *array, void(^)(id obj));
OVERLOADABLE void each(NSDictionary *dict, void(^)(id key, id obj));

That being said, if you're looking for a solid and well tested map category, check out BlocksKit.

Posted in thinking-about