Is there an equivalent to C++'s dynamic cast in Objective-C?

Question

If I have two classes, SubClass and SuperClass:

SuperClass *super = new SuperClass();
SubClass *sub = new SubClass();
SubClass *sub_pointer;

// **The nice one-line cast below**
sub_pointer = dynamic_cast<SubClass*> super;
// Prints NO
printf("Is a subclass: %s\n", sub_pointer ? "YES" : "NO");

sub_pointer = dynamic_cast<SubClass*> sub;
// Prints YES
printf("Is a subclass: %s\n", sub_pointer ? "YES" : "NO");

I can accomplish the same thing in objective-C with isMemberOfClass as follows:

SuperClass *super = [[SuperClass alloc] init];
SubClass *sub = [[SubClass alloc] init];
SubClass *sub_pointer;
id generic_pointer;

// Not as easy:
generic_pointer = super;
if ([generic_pointer isMemberOfClass:[SubClass class]]) {
  sub_pointer = generic_pointer;
} else {
  sub_pointer = nil;
}
// Logs NO
NSLog(@"Is a subclass: %@", sub_pointer ? @"YES" : @"NO");

generic_pointer = sub;
if ([generic_pointer isMemberOfClass:[SubClass class]]) {
  sub_pointer = generic_pointer;
} else {
  sub_pointer = nil;
}
// Logs YES
NSLog(@"Is a subclass: %@", sub_pointer ? @"YES" : @"NO");

Is there an easier way than this?

(P.S. I know I don't have to use the extra id variable, but then I would have to force cast super to SubClass*, which would sometimes result in an invalid reference that I would have to clean up afterwards. That implementation, however, is less wordy, and it's below)

SuperClass *super = [[SuperClass alloc] init];
SubClass *sub = [[SubClass alloc] init];
SubClass *sub_pointer;

// Not as easy:
sub_pointer = (SubClass*) super;
if (![sub_pointer isMemberOfClass:[SubClass class]]) {
  sub_pointer = nil;
}
// Logs NO
NSLog(@"Is a subclass: %@", sub_pointer ? @"YES" : @"NO");

sub_pointer = (SubClass*) sub;
if (![sub_pointer isMemberOfClass:[SubClass class]]) {
  sub_pointer = nil;
}
// Logs YES
NSLog(@"Is a subclass: %@", sub_pointer ? @"YES" : @"NO");

Answer 1

You could add a category on NSObject to add the functionality you want.

//NSObject+DynamicCast.h
@interface NSObject (DynamicCast)
-(id)objectIfMemberOfClass:(Class)aClass;
@end

//NSObject+DynamicCast.m
@implementation NSObject (DynamicCast)
-(id)objectIfMemberOfClass:(Class)aClass;
{
  return [self isMemberOfClass:aClass] ? self : nil;
}
@end

Then you could do this:

SuperClass *super = [[SuperClass alloc] init];
SubClass *sub = [[SubClass alloc] init];
SubClass *sub_pointer;
id generic_pointer;

// **The nice one-line cast below**
sub_pointer = [super objectIfMemberOfClass:[SubClass class]];
// Prints NO
printf("Is a subclass: %s\n", sub_pointer ? "YES" : "NO");

sub_pointer = [sub objectIfMemberOfClass:[SubClass class]];
// Prints YES
printf("Is a subclass: %s\n", sub_pointer ? "YES" : "NO");

Answer 2

I use a macro:

#define DYNAMIC_CAST(x, cls)                                \
  ({                                                        \
    cls *inst_ = (cls *)(x);                                \
    [inst_ isKindOfClass:[cls class]] ? inst_ : nil;        \
  })

I marginally prefer it to using a category on NSObject because the returned object is the correct type (rather than id), although I realise that in most cases you’re simply going to assign it to a variable of the same type anyway.

Answer 3

If you're allowed to throw any C++ into the mix you can avoid macros and get the correct type with a template routine:

template <typename T, typename U>
inline T* objc_cast(U* instance)
{
    return [instance isMemberOfClass:[T class]] ?
               static_cast<T*>(instance) :
               nil;
}

Then the call would simply look like:

sub_pointer = objc_cast<SubClass>(super);

Answer 4

More recent versions of xCode provide instancetype and allow a very neat category based solution. Using it looks like this:

TypeIWant *const thingIWant = [TypeIWant tryCast: thingIWantToCast];

Category Declaration on NSObject

@interface NSObject (DynamicCast)
// Try a dynamic cast. Return nil if the class isn't compatible.
+(instancetype) tryCast: (id) toCast;

// Check a dynamic cast. Throw if the class isn't compatible.
+(instancetype) checkCast: (id) toCast;
@end

Category Implementation

@implementation NSObject (DynamicCast)

+(instancetype) tryCast: (id) toCast
{
    return [toCast isKindOfClass: self] ? toCast : nil;
}

+(instancetype) checkCast:(id)toCast
{
    const id casted = [self tryCast: toCast];
    if(!casted)
    {
        [NSException raise: NSInvalidArgumentException format: @"Can't cast %@ to be an %@", toCast, NSStringFromClass(self)];
    }
    return casted;
}

@end

Usage

This is a class method callable on any class having NSObject as a super class (so, basically, any class).

You send the tryCast: method to the class you hope to cast to, with the object you want casted as the parameter. Like this [ClassIWant tryCast: thingIWantCasted].

Example Of Use

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    UIPopoverController *const popoverDestination =
    [UIStoryboardPopoverSegue tryCast: segue].popoverController;
    if(popoverDestination)
    {
        UITableViewCell *const tableViewSender =
        [UITableViewCell tryCast: sender];
        if(tableViewSender)
        {
            // Things you need to do in this case.
            ...

Answer 5

It occurs to me that I can use the ternary operator to put this all on one line, but it's still a bit of a mess:

SuperClass *super = [[SuperClass alloc] init];
SubClass *sub = [[SubClass alloc] init];
SubClass *sub_pointer;

// One line, but still a bit wordy
sub_pointer = [super isMemberOfClass:[SubClass class]] ? (SubClass*) super : nil;
// Logs NO
NSLog(@"Is a subclass: %@", sub_pointer ? @"YES" : @"NO");

sub_pointer = [sub isMemberOfClass:[SubClass class]] ? (SubClass*) sub : nil;
// Logs YES
NSLog(@"Is a subclass: %@", sub_pointer ? @"YES" : @"NO");

If I'm getting the intended variable back from a function, I'll have to cache it in an id variable for this version to work.

SubClass *sub_pointer;
id generic_pointer;

// One line, but still a bit wordy
generic_pointer = (id) mySuperFunc();
sub_pointer = [generic_pointer isMemberOfClass:[SubClass class]] ? generic_pointer : nil;
// Logs NO
NSLog(@"Is a subclass: %@", sub_pointer ? @"YES" : @"NO");

generic_pointer = (id) mySubFunc();
sub_pointer = [generic_pointer isMemberOfClass:[SubClass class]] ? generic_pointer : nil;
// Logs YES
NSLog(@"Is a subclass: %@", sub_pointer ? @"YES" : @"NO");

Answer 6

basically..

id sub_pointer = [foo isMemberOfClass:AClass] ? foo : nil;
NSLog(@"Is a subclass: %i", sub_pointer!=nil ); 

doesn't seem alot more wordy.