Proof that CFGetTypeID() doesn't call [_cfTypeID]

gistfile1.c

#include <CoreFoundation/CoreFoundation.h>
#include <objc/runtime.h>

typedef struct __CFRuntimeClass {
    CFIndex version;
    const char *className;
    void (*init)(CFTypeRef cf);
    CFTypeRef (*copy)(CFAllocatorRef allocator, CFTypeRef cf);
    void (*finalize)(CFTypeRef cf);
    Boolean (*equal)(CFTypeRef cf1, CFTypeRef cf2);
    CFHashCode (*hash)(CFTypeRef cf);
    CFStringRef (*copyFormattingDesc)(CFTypeRef cf, CFDictionaryRef formatOptions);  // str with retain
    CFStringRef (*copyDebugDesc)(CFTypeRef cf);	// str with retain
    void (*reclaim)(CFTypeRef cf);
} CFRuntimeClass;

typedef struct __CFRuntimeBase {
    uintptr_t _cfisa;
    uint8_t _cfinfo[4];
#if __LP64__
    uint32_t _rc;
#endif
} CFRuntimeBase;

extern CFTypeID _CFRuntimeRegisterClass(const CFRuntimeClass* const cls);

extern CFTypeRef _CFRuntimeCreateInstance(CFAllocatorRef allocator,
                                          CFTypeID typeID, CFIndex extraBytes,
                                          unsigned char* category);

// =============================================== _cfTypeID swizzling

static BOOL CFTypeIDCalled = NO;
static IMP OriginalCFTypeID = NULL;
static CFTypeID MyCFTypeID(id self, SEL _cmd)
{
    CFTypeIDCalled = YES;
    return ((CFTypeID(*)(id,SEL))OriginalCFTypeID)(self, _cmd);
}

static void SwizzleCFTypeID(Class class, BOOL swizzle)
{
    Method method = class_getInstanceMethod(class, @selector(_cfTypeID));
    NSCAssert(method, @"There is no _cfTypeID in %s", class_getName(class));

    if (swizzle)
    {
        OriginalCFTypeID = method_getImplementation(method);
        method_setImplementation(method, (IMP)&MyCFTypeID);
    }
    else
    {
        method_setImplementation(method, OriginalCFTypeID);
        OriginalCFTypeID = NO;
    }
}

// =============================================== tests

@interface NSObject (_cfTypeID)
- (CFTypeID)_cfTypeID;
@end

static void TestCFGetTypeID(CFTypeRef object, CFTypeID expectedTypeID)
{
    Class class = (Class)((const CFRuntimeBase*)object)->_cfisa;
    SwizzleCFTypeID(class, YES);

    CFTypeIDCalled = NO;
    CFTypeID typeID = CFGetTypeID(object);
    NSCAssert(typeID == expectedTypeID, @"CFGetTypeID() returned bad type ID %ld (expected %ld)",
              typeID, expectedTypeID);
    NSCAssert(!CFTypeIDCalled, @"_cfTypeID was called during CFGetTypeID() call on %p of objc type %s",
              object, class_getName(class));

    CFTypeIDCalled = NO;
    [(id)object _cfTypeID];
    NSCAssert(CFTypeIDCalled, @"_cfTypeID was NOT called during [_cfTypeID] message to %p of objc type %s",
              object, class_getName(class));

    SwizzleCFTypeID(class, NO);
}

typedef struct
{
    CFRuntimeBase base;
} MyObject;

static CFRuntimeClass MyObjectClass = {
    0,
    "MyObject"
};

void RunTests(void)
{
    // Custom object
    {
        CFTypeID myTypeID = _CFRuntimeRegisterClass(&MyObjectClass);
        NSCAssert(myTypeID != 0, @"Failed to register custom CF class");

        MyObject* object = (MyObject*)_CFRuntimeCreateInstance(
            kCFAllocatorDefault,
            myTypeID,
            sizeof(MyObject) - sizeof(CFRuntimeBase),
            NULL);

        TestCFGetTypeID(object, myTypeID);

        CFRelease(object);
    }

    // CFString
    {
        CFStringRef string = (CFStringRef)@"test";
        TestCFGetTypeID(string, CFStringGetTypeID());
    }

    // CFArray
    {
        CFArrayRef array = CFArrayCreate(kCFAllocatorDefault, NULL, 0, NULL);
        TestCFGetTypeID(array, CFArrayGetTypeID());
        CFRelease(array);
    }
}