I thought that the double bracket notation was intended to replace __attribute__((foo)), but I'm not getting equivalence.
Clang toolchain set for C++14.
class [[packed]] test {};
class __attribute__((packed)) test2 {};
test has a warning about unknown attributed packed ignored. test2 has no warning. If I replace packed with a made-up attribute, both give warnings.
What's the best way to pack a struct in Clang?
I thought that the double bracket notation was intended to replace
__attribute__((foo)), but I'm not getting equivalence.
It's not meant to be entirely equivalent. The double bracket notation is for the C++ standard to introduce attributes with a more or less standard meaning. Those often exist already as compiler specific attributes, but it doesn't mean that all compiler specific attributes have a corresponding standard attribute. So you can't expect [[packed]] to be a thing just because __attribute__((packed)) is.
Nevertheless, the standard does give compilers an option to add custom attributes that use double brackets. But such attributes are usually scoped. In this case, the custom attribute is named [[gnu::packed]]. As the name implies, it's vendor specific.