• 不能向编译后得到的类中增加实例变量。
• 能向运行时创建的类中添加实例变量。

解释下：

• 因为编译后的类已经注册在 runtime 中，类结构体中的 objc_ivar_list 实例变量的链表和 instance_size 实例变量的内存大小已经确定，同时 runtime 会调用 class_setIvarLayout 或 class_setWeakIvarLayout 来处理 strong weak 引用。所以不能向存在的类中添加实例变量。
• 运行时创建的类是可以添加实例变量，调用 class_addIvar 函数。但是得在调用 objc_allocateClassPair 之后，objc_registerClassPair 之前，原因同上。

动态构建类的代码示例：

    #pragma mark - Runtime Class Construct
    int32_t testRuntimeMethodIMP(id self, SEL _cmd, NSDictionary *dic) {
        NSLog(@"testRuntimeMethodIMP: %@", dic);
        // Print:
        // testRuntimeMethodIMP: {
        //     a = "para_a";
        //     b = "para_b";
        // }
        
        return 99;
    }
    - (void)runtimeConstruct {
    #pragma clang diagnostic push
    #pragma clang diagnostic ignored "-Wundeclared-selector"
        Class cls = objc_allocateClassPair(SuperClass.class, "RuntimeSubClass", 0);
        // Method returns: "int32_t"; accepts: "id self", "SEL _cmd", "NSDictionary *dic". So use "i@:@" here.
        class_addMethod(cls, @selector(testRuntimeMethod), (IMP) testRuntimeMethodIMP, "i@:@");
        // You can only register a class once.
        objc_registerClassPair(cls);
        
        id sub = [[cls alloc] init];
        NSLog(@"%@, %@", object_getClass(sub), class_getSuperclass(object_getClass(sub))); // Print: RuntimeSubClass, SuperClass
        Class metaCls = objc_getMetaClass("RuntimeSubClass");
        if (class_isMetaClass(metaCls)) {
            NSLog(@"YES, %@, %@, %@", metaCls, class_getSuperclass(metaCls), object_getClass(metaCls)); // Print: YES, RuntimeSubClass, SuperClass, NSObject
        } else {
            NSLog(@"NO");
        }
        
        
        unsigned int outCount = 0;
        Method *methods = class_copyMethodList(cls, &outCount);
        for (int32_t i = 0; i < outCount; i++) {
            Method method = methods[i];
            NSLog(@"%@, %s", NSStringFromSelector(method_getName(method)), method_getTypeEncoding(method));
        }
        // Print: testRuntimeMethod, i@:@
        free(methods);
        
        
        int32_t result = (int) [sub performSelector:@selector(testRuntimeMethod) withObject:@{@"a":@"para_a", @"b":@"para_b"}];
        NSLog(@"%d", result); // Print: 99
        
        
        // Destroy instances of cls class before destroy cls class.
        sub = nil;
        // Do not call this function if instances of the cls class or any subclass exist.
        objc_disposeClassPair(cls);
        
    #pragma clang diagnostic pop
    }
    #pragma mark - Runtime Ivar&Property Construct
    NSString * runtimePropertyGetterIMP(id self, SEL _cmd) {
        Ivar ivar = class_getInstanceVariable([self class], "_runtimeProperty");
        
        return object_getIvar(self, ivar);
    }
    void runtimePropertySetterIMP(id self, SEL _cmd, NSString *s) {
        Ivar ivar = class_getInstanceVariable([self class], "_runtimeProperty");
        NSString *old = (NSString *) object_getIvar(self, ivar);
        if (![old isEqualToString:s]) {
            object_setIvar(self, ivar, s);
        }
    }
    - (void)aboutIvarAndProperty {
        
        
    #pragma clang diagnostic push
    #pragma clang diagnostic ignored "-Wundeclared-selector"
        // 1: Add property and getter/setter.
        Class cls = objc_allocateClassPair(SuperClass.class, "RuntimePropertySubClass", 0);
        BOOL b = class_addIvar(cls, "_runtimeProperty", sizeof(cls), log2(sizeof(cls)), @encode(NSString));
        NSLog(@"%@", b ? @"YES" : @"NO"); // Print: YES
        
        objc_property_attribute_t type = {"T", "@\"NSString\""};
        objc_property_attribute_t ownership = {"C", ""}; // C = copy
        objc_property_attribute_t isAtomic = {"N", ""}; // N = nonatomic
        objc_property_attribute_t backingivar  = {"V", "_runtimeProperty"};
        objc_property_attribute_t attrs[] = {type, ownership, isAtomic, backingivar};
        class_addProperty(cls, "runtimeProperty", attrs, 4);
        class_addMethod(cls, @selector(runtimeProperty), (IMP) runtimePropertyGetterIMP, "@@:");
        class_addMethod(cls, @selector(setRuntimeProperty), (IMP) runtimePropertySetterIMP, "v@:@");
        
        // You can only register a class once.
        objc_registerClassPair(cls);
        
        // 2: Print all properties.
        unsigned int outCount = 0;
        objc_property_t *properties = class_copyPropertyList(cls, &outCount);
        for (int32_t i = 0; i < outCount; i++) {
            objc_property_t property = properties[i];
            NSLog(@"%s, %s\n", property_getName(property), property_getAttributes(property));
        }
        // Print:
        // runtimeProperty, T@"NSString",C,N,V_runtimeProperty
        free(properties);
        
        
        // 3: Print all ivars.
        Ivar *ivars = class_copyIvarList(cls, &outCount);
        for (int32_t i = 0; i < outCount; i++) {
            Ivar ivar = ivars[i];
            NSLog(@"%s, %s\n", ivar_getName(ivar), ivar_getTypeEncoding(ivar));
        }
        // Print:
        // _runtimeProperty, {NSString=#}
        free(ivars);
        
        
        // 4: Use runtime property.
        id sub = [[cls alloc] init];
        [sub performSelector:@selector(setRuntimeProperty) withObject:@"It-is-a-runtime-property."];
        NSString *s = [sub performSelector:@selector(runtimeProperty)]; //[sub valueForKey:@"runtimeProperty"];
        NSLog(@"%@", s); // Print: It-is-a-runtime-property.
        
        
        // 5: Clear.
        // Destroy instances of cls class before destroy cls class.
        sub = nil;
        // Do not call this function if instances of the cls class or any subclass exist.
        objc_disposeClassPair(cls);
    #pragma clang diagnostic pop
    }

更多：iOS面试题合集