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// clang -fobjc-arc -framework Foundation runtime-class.m
#import <Foundation/Foundation.h>
#import <objc/runtime.h>
@interface Person : NSObject
- (id)initWithFirstName: (NSString *)firstName lastName: (NSString *)lastName age: (NSUInteger)age;
@property (readonly) NSString *firstName;
@property (readonly) NSString *lastName;
@property (readonly) NSUInteger age;
int main(int argc, char **argv)
Class c = objc_allocateClassPair([NSObject class], "Person", 0);
class_addIvar(c, "firstName", sizeof(id), log2(sizeof(id)), @encode(id));
class_addIvar(c, "lastName", sizeof(id), log2(sizeof(id)), @encode(id));
class_addIvar(c, "age", sizeof(NSUInteger), log2(sizeof(NSUInteger)), @encode(NSUInteger));
Ivar firstNameIvar = class_getInstanceVariable(c, "firstName");
Ivar lastNameIvar = class_getInstanceVariable(c, "lastName");
ptrdiff_t ageOffset = ivar_getOffset(class_getInstanceVariable(c, "age"));
IMP initIMP = imp_implementationWithBlock(^(id self, NSString *firstName, NSString *lastName, NSUInteger age) {
object_setIvar(self, firstNameIvar, firstName);
object_setIvar(self, lastNameIvar, lastName);
char *agePtr = ((char *)(__bridge void *)self) + ageOffset;
memcpy(agePtr, &age, sizeof(age));
return self;
const char *initTypes = [[NSString stringWithFormat: @"%s%s%s%s%s%s", @encode(id), @encode(id), @encode(SEL), @encode(id), @encode(id), @encode(NSUInteger)] UTF8String];
class_addMethod(c, @selector(initWithFirstName:lastName:age:), initIMP, initTypes);
const char *objectGetterTypes = [[NSString stringWithFormat: @"%s%s%s", @encode(id), @encode(id), @encode(SEL)] UTF8String];
IMP descriptionIMP = imp_implementationWithBlock(^(id self) {
return [NSString stringWithFormat: @"<%@ %p: %@ %@ age %zd>", [self class], self, [self firstName], [self lastName], [self age]];
class_addMethod(c, @selector(description), descriptionIMP, objectGetterTypes);
IMP firstNameIMP = imp_implementationWithBlock(^(id self) {
return object_getIvar(self, firstNameIvar);
class_addMethod(c, @selector(firstName), firstNameIMP, objectGetterTypes);
IMP lastNameIMP = imp_implementationWithBlock(^(id self) {
return object_getIvar(self, lastNameIvar);
class_addMethod(c, @selector(lastName), lastNameIMP, objectGetterTypes);
IMP ageIMP = imp_implementationWithBlock(^(id self) {
char *agePtr = ((char *)(__bridge void *)self) + ageOffset;
NSUInteger age;
memcpy(&age, agePtr, sizeof(age));
return age;
const char *ageTypes = [[NSString stringWithFormat: @"%s%s%s", @encode(NSUInteger), @encode(id), @encode(SEL)] UTF8String];
class_addMethod(c, @selector(age), ageIMP, ageTypes);
Class PersonC = NSClassFromString(@"Person");
Person *alex = [[PersonC alloc] initWithFirstName: @"Alex" lastName: @"Trebek" age: 29];
Person *sean = [[PersonC alloc] initWithFirstName: @"Sean" lastName: @"Connery" age: 42];
NSArray *people = @[ alex, sean ];
NSLog(@"%@", people);
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