I'm studying Steven Kochan's "Programming in Objective-C 2.0". We created a Fraction object with two int instance variables. Later in the book Kochan uses the sizeof statement on a Fraction object's pointer myFract:
sizeof(*myFract)
When I do this, I receive a compile error:
Invalid application of 'sizeof' to interface 'Fraction' in non-fragile ABI
http://clang.llvm.org/compatibility.html#sizeof-interface states this error could occur for an object who's size can change but a Fraction instance only contains the two int instance variables (plus an "inherited isa member" mentioned in the book).
What am I doing wrong?
The object's size could still change because you're using the modern ABI. In older versions of Objective-C, objects were basically structs and that meant it was possible to sizeof() them. This is no longer the case, and it was never a particularly good idea in the first place. I'm not sure what Kochan was trying to teach with it, but FYI this is not necessary to program Objective-C. You should be able to get the old behavior by building as 32-bit on the Mac, but again, that isn't something you'll want to do in real programs.
To follow up on what Chuck said, you should use class_getInstanceSize() at runtime if you really need to know the size of an object.
来源：https://stackoverflow.com/questions/7219319/invalid-application-of-sizeof-to-interface-fraction-in-non-fragile-abi-in