Why old usage (c++03) of auto does not compile und

问题:

I know that auto has a little usage before because it is the default for variables (opposite to static) - see question Consider however valid C++03 code where, maybe for self-explanatory, this keyword was used:

auto int foo2 = 8;

It compiles under C++03, and does not compile under C++11. Is there any reason for not being back-compatible with C++03? What was the source of standard committee opinion that this keyword was not used? Are there any statistics of keyword usage?

BTW i tested with gcc - maybe this is a compiler bug?

回答1:

It was known that this breaks compatibility and is mentioned in Appendix C 2.3 Clause 7 of the standard. Given how useless auto has been and that it is really easy to fix the cost was deemed acceptable.

As for keyword statistics: Some companies with huge code-bases are on the committee, they probably know if it was acceptable, but I could not dig up any full statistics.

I did some standards archeology and the oldest paper that talks about auto seems to be N1478, which refers to a reflector message for the first discussing of auto (all later papers don't talk about breaking compatibility or reasoning).

回答2:

This was removed from C++11 because they have a new use for auto, and it would be confusing to allow auto auto foo2 = 8; to work. Nothing of significance is lost by removing this ancient and pointless declaration.