Compile-time constant classes in Objective-C

ConstantClass.m

// 
// ConstantClass.m
//
// An example for how to create compile-time constant Objective-C objects that
// can be assigned to any static const variable.
//
// Compile with -fno-objc-arc to allow overriding of retain/release methods.
//

#import <Foundation/Foundation.h>
#import <objc/objc.h>
#import <objc/runtime.h>

// Use pointer authentication if available.
#if defined(__has_include) && __has_include(<ptrauth.h>)
#import <ptrauth.h>
#endif

// Define the pointer authentication attribute as nothing if it doesn't exist.
#ifndef __ptrauth_objc_isa_pointer
#define __ptrauth_objc_isa_pointer
#endif

#pragma mark - Rectangle

@interface Rectangle : NSObject
@property (nonatomic, readonly) NSPoint origin;
@property (nonatomic, readonly) NSSize  size;
- (instancetype)initWithOrigin:(NSPoint)origin size:(NSSize)size;
@end

#pragma mark - Constant Rectangle Structure

#if __OBJC2__
/// The _ConstantRectangle class object, stored as a text section symbol.
/// https://stackoverflow.com/questions/24968690/objective-c-mangled-names-objc-class-vs-objc-class-name
extern void const OBJC_CLASS_$__ConstantRectangle;
#elif __OBJC__
/// The _ConstantRectangle class object, stored as an absolute global symbol.
/// https://stackoverflow.com/questions/24968690/objective-c-mangled-names-objc-class-vs-objc-class-name
extern void const OBJC_CLASS_$__ConstantRectangle __asm(".objc_class_name__ConstantRectangle");
#endif

/// The structure we'll use for instances of our _ConstantRectangle class.
struct _ConstantRectangle {
    Class __ptrauth_objc_isa_pointer isa;
    NSRect rect;
};

#if __OBJC2__
/// A convenience macro for creating our compile-time constant object.
#define ConstRect(x, y, width, height)                              \
    ((__bridge Rectangle *)(void *)&(struct _ConstantRectangle){    \
        (__bridge Class)&OBJC_CLASS_$__ConstantRectangle,           \
        {{ x, y }, { width, height }}                               \
    })
#elif __OBJC__
/// https://lists.llvm.org/pipermail/llvm-dev/2007-July/010004.html
#error "Static class references in const expressions are only reliable in the Objective-C 2.0 runtime."
#endif

#pragma mark - Main Program

/// A compile-time constant rectangle object.
static Rectangle *const rect = ConstRect(0, 0, 100, 100);

int main(int argc, char **argv) {
    @autoreleasepool {
        printf("%s", [[rect description] UTF8String]);
    }
    return 0;
}

#pragma mark - Rectangle Implementation

@implementation Rectangle
@synthesize origin = _origin;
@synthesize size = _size;

- (instancetype)initWithOrigin:(NSPoint)origin size:(NSSize)size {
    if (self = [self init]) {
        _origin = origin;
        _size = size;
    }
    return self;
}

- (NSString *)description {
    return [NSString stringWithFormat:@"[Rectangle origin:%@ size:%@]",
            NSStringFromPoint(self.origin), NSStringFromSize(self.size)];
}

- (BOOL)isEqual:(id)object {
    if (self == object) { return YES; }
    if ([object isKindOfClass:[Rectangle class]]) {
        Rectangle *other = object;
        return NSEqualPoints(self.origin, other.origin)
            && NSEqualSizes(self.size, other.size);
    }
    return NO;
}

@end

#pragma mark - Constant Rectangle Implementation

@interface _ConstantRectangle : Rectangle
@end

@implementation _ConstantRectangle

- (NSPoint)origin {
    return ((struct _ConstantRectangle *)(__bridge void *)self)->rect.origin;
}
- (NSSize)size {
    return ((struct _ConstantRectangle *)(__bridge void *)self)->rect.size;
}

#if defined(__has_feature) && __has_feature(objc_arc)

/// We can't deallocate constant objects.
static void Dealloc(id self, SEL _cmd) {}

/// No use retaining constant objects.
static void *Retain(void *self, SEL _cmd) { return self; }

/// No use releasing constant objects.
static void Release(void *self, SEL _cmd) {}

/// No use autoreleasing constant objects.
static void *Autorelease(void *self, SEL _cmd) { return self; }

/// No use keeping track of a constant object's retain count.
static NSUInteger RetainCount(void *self, SEL _cmd) { return 1; }

/// Override an existing method on the given class.
static BOOL AddMethodOverride(Class self, SEL selector, IMP function) {
    Method method = class_getInstanceMethod(self, selector);
    char const *types = method_getTypeEncoding(method);
    return class_addMethod(self, selector, function, types);
}

/// If ARC is enabled, we can't simply override the existing retain/release
/// methods, so instead we manually swizzle in our overrides when the class
/// is loaded into the runtime.
+ (void)load
{
    AddMethodOverride(self, sel_getUid("dealloc"), (IMP)Dealloc);
    AddMethodOverride(self, sel_getUid("retain"), (IMP)Retain);
    AddMethodOverride(self, sel_getUid("release"), (IMP)Release);
    AddMethodOverride(self, sel_getUid("autorelease"), (IMP)Autorelease);
    AddMethodOverride(self, sel_getUid("retainCount"), (IMP)RetainCount);
}

#else /* if !defined(__has_feature) || !__has_feature(objc_arc) */

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wobjc-missing-super-calls"
/// We can't deallocate constant objects.
- (void)dealloc {}
#pragma clang diagnostic pop

/// No use retaining constant objects.
- (instancetype)retain { return self; }

/// No use releasing constant objects.
- (oneway void)release {}

/// No use autoreleasing constant objects.
- (instancetype)autorelease { return self; }

/// No use keeping track of a constant object's retain count.
- (NSUInteger)retainCount { return 1; }

#endif /* !defined(__has_feature) || !__has_feature(objc_arc) */

@end