Question

I have a new object inherited from NSObject

Car *myCar = [[Car alloc] init];

This returns 16, the size of a Car object

printf("myCar object: %d\n", sizeof(*myCar));

plz help me to make it clear, thanks

Answer 1

sizeof in this case works like C.

As bbum said, you should not use sizeof on NSObject types (but pointers to NSObjects are ok). Clang forbids requesting the sizeof and object. The objc runtime should be your reference if you need an object’s size because the actual size of an objc object is not a static (compile-time) value, it is determined at runtime.

Given these types:

@interface A : NSObject
@end

@interface B : A
{
    uint64_t ivar;
}
@end

And these messages:

printf("Class size is %lu\n", sizeof(Class)); // << for NSObject.isa
printf("NSObject size is %lu\n", sizeof(NSObject));
printf("A size is %lu\n", sizeof(A));
printf("B size is %lu\n", sizeof(B));

On 64 bit, we get:

Class size is 8
NSObject size is 8
A size is 8
B size is 16

And on 32 bit, we get:

Class size is 4
NSObject size is 4
A size is 4
B size is 12

There’s a little example which shows you why sizes grow. For more detail, here is the wikipedia page.