Get a reference to class object from uninitialized variable - not instantiated object

Question

Let's say I have an uninitialized variable:

UIViewController *vc;

From this variable, I want to reference UIViewController, such that I could call alloc or new on it to return an instantiated object.

Essentially, I want to do:

UIViewController *vc = [*typeof(vc) new];

... which does not compile because the compiler expects an expression rather than a type.

If @encode returned the actual type, I could do something like:

UIViewController *vc = [NSClassFromString(@(@encode(*typeof(vc)))) new];

...however, @encode returns '@', which just means "generic object".

I realize it's a little philosophical in nature, but I get tired of typing and would like to make a macro like NEW(x). I also realize a similar macro could be made if it involves the actual declaration, but I am not satisfied with that syntax/angle.

`#define NEW(class, varname) class * varname = [class new]` is unsatisfactory, you're saying? — jscs, Sep 07 '14 at 18:32
Yeah. I came up with something (see my answer), but it's not awesome. I probably won't end up doing this. — xtravar, Sep 07 '14 at 18:34
Similar problem, but not identical because you still have access to the local declaration: [ObjC can you test an uninitialized pointer for static class type?](http://stackoverflow.com/q/7680310) It explains why this isn't generally possible, however. — jscs, Sep 07 '14 at 18:38
Why are you trying to replace perfectly fine code with obfuscated code? — gnasher729, Sep 07 '14 at 18:45

xtravar · Accepted Answer · 2014-09-07T19:19:19.603

Here's what I have... it doesn't seem ideal, since it makes a performance hit. Still looking for better answers.

static Class classFromEncoding(const char *encoding) {
    char *className = strndup(encoding + 1, strchr(encoding, '=') - encoding - 1);
    Class retval = NSClassFromString(@(className));
    free(className);
    return retval;
}

#define NEW(variable) [classFromEncoding(@encode(typeof(*variable))) new]

Here's a macro-only version:

#define CLASS_FROM_VARIABLE(variable) \
^(const char *encoding) {             \
char *className = strndup(encoding + 1, strchr(encoding, '=') - encoding - 1); \
Class retval = NSClassFromString(@(className));      \
free(className);                         \
return retval;                              \
}(@encode(typeof(*variable)))

#define NEW(variable) [CLASS_FROM_VARIABLE(variable) new]
#define ALLOC(variable) [CLASS_FROM_VARIABLE(variable) alloc]

Variations can be made using objc_getClass() or NSString initWithBytes, perhaps with performance gains. Still, it's not a no-op, which is what I'd prefer.

I would suggest that everybody who reads your code using this macro is going to put a hex on your gerbil. — jscs, Sep 07 '14 at 18:34
I'd definitely limit my usage. :) I love Objective-C, but initializers are the most annoying part of my day (which is why I've moved to the +new camp instead of alloc]init]). — xtravar, Sep 07 '14 at 18:37

score -1 · Answer 2 · answered Sep 07 '14 at 18:31

-1

It's [obj class] or [obj className] depending on your needs.

answered Sep 07 '14 at 18:31

Gobra

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The object hasn't been initialized; there's nothing to respond to these messages. – jscs Sep 07 '14 at 18:33
True, my bad. NSClassFromString() seems to be the best way than. – Gobra Sep 07 '14 at 18:34

Get a reference to class object from uninitialized variable - not instantiated object

2 Answers2