iOS Detecting if a class is Objective-C or Swift

If, for academic purposes, you absolutely must know if a class was created in swift code or not (and not in a way that can easily be fooled via objc_allocateClassPair), then you can utilize the information that can be found in objc-runtime-new.h, specifically, the flags related to FAST_IS_SWIFT.

To extract these flags without having to make your source code C++ or fight with including a ton of private headers, you can use something similar to the following, but please note: THIS IS SUPER FRAGILE.

This is probably not ABI-enforced anywhere, and any future version can change this without consequence. Bear that in mind, etc., etc.

Without further ado, the crazy hack ensues:

#define FAST_IS_SWIFT         (1UL<<0)
#define FAST_HAS_DEFAULT_RR   (1UL<<1)
#define FAST_DATA_MASK        0xfffffffcUL

uintptr_t getClassBits(Class kls) {
#if __LP64__
    typedef uint32_t mask_t;
#else
    typedef uint16_t mask_t;
#endif

    return ((const struct {
        /* struct objc_class */
        Class isa;
        Class superclass;

        /* struct cache_t */
        void *bucket_t;
        mask_t mask;
        mask_t occupied;

        /* struct class_data_bits_t */
        uintptr_t bits;
    } *) (__bridge const void *) kls)->bits;
}

This is recreating the structure format from objc-runtime-new.h, without all of the additional C++ overhead.

Once you have the bits for a class, simply compare it it with FAST_IS_SWIFT (e.g. bits & FAST_IS_SWIFT), and you should have your answer.

Swift class names are prefixed with the name of the module. If one uses:

NSStringFromClass(SomeClass.self)

The result will be

MyApp.SomeClass

For my purpose, this works perfectly.