On Nov 15, 2011, at 9:55 AM, Kyle Sluder wrote: >> FWIW I check on the llvm IRC channel and the response was "I wouldn't >> be surprised if there are annoying edge cases, but offhand I don't see >> any reason it couldn't be done." > > If it could've been done, they would have done it. The fragile base class > problem is a well-understood aspect of language design, and the compiler team > is full of smart people.
Yup. Rather, to be pedantic, it can be done, and they did do it, but at the cost of changes to object layout and the ABI. So to avoid breaking binary compatibility with all existing apps, they didn’t make these changes in the 32-bit x86 runtime, only newer runtimes like 64-bit and iOS. (In a nutshell: if a class doesn’t know the size of its base class’s instance data at compile time, it can’t look up its own instance variables as constant offsets from ‘self’, so this changes the machine instructions for getting and setting instance variables.) —Jens
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