Objectify

Objectify.h

#import <Foundation/Foundation.h>
#import <CoreGraphics/CoreGraphics.h>
#import <objc/message.h>
#import <string>
#import <map>
#import <typeinfo>

#define $(value) box<typeof(value)>(value)

using namespace std;

template <typename T> id box(T v) {
  typedef pair<Class,SEL> m;
  map<const char *,m> ops;
  
  ops[@encode(int)]          = m([NSNumber class], @selector(numberWithInt:));
  ops[@encode(float)]        = m([NSNumber class], @selector(numberWithFloat:));
  ops[@encode(double)]       = m([NSNumber class], @selector(numberWithDouble:));
  ops[@encode(BOOL)]         = m([NSNumber class], @selector(numberWithBool:));
  ops[@encode(const char *)] = m([NSString class], @selector(stringWithUTF8String:));
  ops[@encode(SEL)]          = m([NSString class], @selector(objectify_stringFromSelector:));
  ops[@encode(Class)]        = m([NSString class], @selector(objectify_stringFromClass:));
  
  ops[@encode(CGPoint)]      = m([NSValue class],  @selector(valueWithCGPoint:));
  ops[@encode(CGRect)]       = m([NSValue class],  @selector(valueWithCGRect:));
  ops[@encode(CGSize)]       = m([NSValue class],  @selector(valueWithCGSize:));
  ops[@encode(NSRange)]      = m([NSString class], @selector(objectify_stringFromRange:));
  ops[@encode(void *)]       = m([NSValue class],  @selector(valueWithPointer:));
  
  m method = ops[@encode(T)];
  Class klass = method.first ? method.first : [NSObject class];
  SEL s = method.second ? method.second : @selector(objectify_identity:);
  
  id result = objc_msgSend(klass, s, v);
  
  return result;
}

template <typename T> T unbox(id v) {
  map<const char *,SEL> ops;
  
  ops[@encode(int)]           = @selector(intValue);
  ops[@encode(float)]         = @selector(floatValue);
  ops[@encode(double)]        = @selector(doubleValue);
  ops[@encode(BOOL)]          = @selector(boolValue);
  ops[@encode(const char *)]  = @selector(UTF8String);
  ops[@encode(SEL)]           = @selector(objectify_selectorValue);
  ops[@encode(Class)]         = @selector(objectify_classValue);
  
  ops[@encode(CGPoint)]       = @selector(CGPointValue);
  ops[@encode(CGRect)]        = @selector(CGRectValue);
  ops[@encode(CGSize)]        = @selector(CGSizeValue);
  ops[@encode(NSRange)]       = @selector(objectify_rangeValue);
  ops[@encode(void *)]        = @selector(pointerValue);
  
  SEL method = ops[@encode(T)] ? ops[@encode(T)] : @selector(objectify_identity);
  
  typedef T (*SendFunctionType)(id, SEL);
  return ((SendFunctionType)objc_msgSend)(v, method);
}

Objectify.mm

#import "Objectify.h"

@implementation NSString (Objectify)

+ (id)objectify_stringFromSelector:(SEL)aSelector {
  return NSStringFromSelector(aSelector); 
}

- (SEL)objectify_selectorValue { 
  return NSSelectorFromString(self); 
}

+ (id)objectify_stringFromClass:(Class)aClass {
  return NSStringFromClass(aClass);
}

- (Class)objectify_classValue {
  return NSClassFromString(self);
}

+ (id)objectify_stringFromRange:(NSRange)aRange{
  return NSStringFromRange(aRange);
}

- (NSRange)objectify_rangeValue {
  return NSRangeFromString(self);
}

@end

@implementation NSObject (Objectify)
+ (id)objectify_identity:(id)object { return object; }
- (id)objectify_identity { return self; }
@end

Objectify_Example.mm

#import <Foundation/Foundation.h>
#import "Objectify.h"

int main (int argc, const char * argv[]) {
  NSAutoreleasePool *pool = [NSAutoreleasePool new];
  
  const char *str = "wut";
  id o = $<const char *>(str);
  
  NSLog(@"char: %s, string: %@, of class: %@", str, o, [o class]);
  // => char: wut, string: wut, of class: NSCFString
  
  [pool drain];
}

Yes, well said with respect to inference. As for autoboxing, I think you are right to a point. Haskell, at it's lowest implementation levels, does indeed involve autoboxing of primitives, as does Smalltalk; yet at the programmer's level, these are always values and objects respectively. From my extremely limited understanding, this sort of boxing is a performance measure, but I may be misunderstanding something.

(My Haskell bias demands I should point out the benefits of handling as much type-checking at compile-time as possible)

I can certainly believe it's a performance measure, since a main reason to use primitives over boxed values is for performance. Much as it's not collectable and annoying, int is faster than NSNumber ;)

Type checking at compile time seems like the best option, but in order to check, for example, a template enough to be sure it will work would involve traversing through the steps and types each place that template is used in the code. I would think that'd be a very resource-intensive operation. And it would also not allow such haxxery as truly dynamic typing — that is, if a type were read as input to the program and used (à la NSClassFromString or Ruby's const_get).

True. When working with a good static type system, the way you reason about solutions changes; problems that were solved previously using metaprogramming can be solved just as succinctly elsewise. But metaprogramming is so damned fun…

As for resource-intensive checking, just try compiling any Haskell program. Seems to work quickly enough for me… :)

not really following along completely, but are you calling the objc version that gave warnings like so?

id to_object( void * x )
{
    return [NSNumber numberWithInt: (int)x];
}

int a = 42;
id foo = to_object( a );

???

If you do it this way it doesn't give a warning:

id to_object( void * x )
{
    return [NSNumber numberWithInt: *(int*)x];
}

int a = 42;
id foo = to_object( &a );

requires sending in the address of the thing you want converted though...

Wow, Tyler, you're totally right. Should have thought of that. :)

maybe it prevents detecting the type though... didn't try to actually do what you were trying to do so not sure. Another other option would be to use C's var-args as the parameter because you can get their types moderately easily. I'm almost motivated enough to have a non-C++ version to sit down and write it, but a little pressed for time at the moment.