Before I start in on this again, I wanted to make sure that there was a consensus that this functionality was desirable. I believe the answer (based on some conversations in IRC and here on the lists) was tentatively "yes."
As far as I can tell, the work left to be done on this is to add a feature test for __has_attribute_syntax, write the documentation for it and that's about it? The syntax-based form is the preferable nomenclature because it leaves the door open for testing parameters at some point in the future, and with the __has_attribute_syntax feature test, it is both backwards and forwards compatible. ~Aaron On Mon, Jan 13, 2014 at 8:49 PM, Richard Smith <[email protected]> wrote: > On Mon, Jan 13, 2014 at 5:44 PM, Aaron Ballman <[email protected]> > wrote: >> >> On Mon, Jan 13, 2014 at 8:39 PM, Richard Smith <[email protected]> >> wrote: >> > On Mon, Jan 13, 2014 at 5:31 PM, Aaron Ballman <[email protected]> >> > wrote: >> >> >> >> On Mon, Jan 13, 2014 at 8:20 PM, Richard Smith <[email protected]> >> >> wrote: >> >> > On Sun, Jan 12, 2014 at 4:49 PM, Aaron Ballman >> >> > <[email protected]> >> >> > wrote: >> >> >> >> >> >> On Sun, Jan 12, 2014 at 7:38 PM, Sean Silva <[email protected]> >> >> >> wrote: >> >> >> > >> >> >> > >> >> >> > >> >> >> > On Sun, Jan 12, 2014 at 6:53 PM, Aaron Ballman >> >> >> > <[email protected]> >> >> >> > wrote: >> >> >> >> >> >> >> >> > That's good news -- thanks for confirming. >> >> >> >> > >> >> >> >> > The feature detection macro itself will still need to have a >> >> >> >> > different >> >> >> >> > name >> >> >> >> > (or some other mechanism) so it can be used compatibly with >> >> >> >> > existing >> >> >> >> > clang >> >> >> >> > deployments, because _has_attribute() currently emits a parse >> >> >> >> > error >> >> >> >> > instead >> >> >> >> > of gracefully returning 0 when passed the new argument syntax: >> >> >> >> > >> >> >> >> > tmp/attr2.cpp:1:5: error: builtin feature check macro requires >> >> >> >> > a >> >> >> >> > parenthesized identifier >> >> >> >> > #if __has_attribute(__attribute__((weakref))) >> >> >> >> > ^ >> >> >> >> >> >> >> >> Good catch. Unfortunately, __has_attribute is really the best >> >> >> >> identifier for the macro, so I am loathe to let it go. >> >> >> >> >> >> >> >> Due to the current design of __has_attribute, we can't get away >> >> >> >> with >> >> >> >> " >> >> >> >> magic" by expanding the non-function-like form into a value that >> >> >> >> could >> >> >> >> be tested. So we would really have to pick a new name if we are >> >> >> >> worried about this. >> >> >> >> >> >> >> >> Suggestions on the name are welcome. >> >> >> > >> >> >> > >> >> >> > Ok, I'll bite: >> >> >> > >> >> >> > __has_attribute_written_as([[foo]]) >> >> >> > __has_attribute_syntax([[foo]]) >> >> >> > __has_attribute_spelling([[foo]]) >> >> >> >> >> >> I kind of like __has_attribute_syntax, truth be told. >> >> > >> >> > >> >> > Have we given up on using the name __has_attribute too soon? Users of >> >> > the >> >> > new syntax could write: >> >> > >> >> > // Probably already in project's porting header >> >> > #ifndef __has_feature >> >> > #define __has_feature(x) 0 >> >> > #endif >> >> > >> >> > #if __has_feature(__has_attribute_syntax) >> >> > #define MY_HAS_ATTRIBUTE(...) __has_attribute(__VA_ARGS__) >> >> > #else >> >> > #define MY_HAS_ATTRIBUTE(...) 0 >> >> > #endif >> >> > >> >> > If it's given a different name, they instead would write something >> >> > like: >> >> > >> >> > #ifdef __has_attribute_syntax >> >> > #define MY_HAS_ATTRIBUTE(...) __has_attribute_syntax(__VA_ARGS__) >> >> > #else >> >> > #define MY_HAS_ATTRIBUTE(...) 0 >> >> > #endif >> >> > >> >> > So I don't think the change-in-syntax argument holds water. >> >> >> >> So are you proposing that we would have a different name for the >> >> purposes of the __has_feature macro? Eg) >> >> __has_feature(__has_attribute_syntax) is 1 for the proposed >> >> functionality, and 0 otherwise? >> > >> > >> > It's a possibility. It could be that a new name is a better approach, >> > but >> > both directions seem to be feasible. >> >> I'll ponder; I rather like keeping the existing name. > > > By the same argument, it's possible to add extra arguments to > __has_attribute, if we have a __has_feature check for the new syntax. > >> >> >> >> >> > Also, supporting arguments in the attributes is useful in some cases >> >> > -- >> >> > it's >> >> > not true that they don't make sense in a feature-checking facility. >> >> > For >> >> > instance: >> >> > >> >> > __has_attribute( __attribute__((format)) ) >> >> > >> >> > ... doesn't tell me whether __attribute__((format, gnu_scanf, 1, 2) >> >> > will >> >> > work (and I'd expect that the format attribute will gain additional >> >> > archetypes in future). >> >> >> >> That's true, but the example also demonstrates why it's kind of >> >> nonsensical. What do the 1, 2 represent for the purposes of >> >> __has_attribute? >> > >> > >> > They represent themselves. Suppose we added support for a format >> > attribute >> > with negative indices, or with three indices, or something -- this >> > syntax >> > would allow the user to determine if that syntax is available. >> > >> >> Can they be elided? If so, can we come up with >> >> declarative rules as to when they can be elided? >> > >> > >> > If you could omit them, how would you tell whether an attribute could be >> > used without arguments? >> > >> > Again, I'm not saying we should go in this direction, but I don't think >> > we >> > should dismiss it without consideration -- we probably don't want to >> > find we >> > need a third form of __has_attribute later =) >> >> That's one of the reasons Alp's suggestion for forwards compatibility >> is so nice -- if implemented properly, we could add parameter support >> at a later date (presuming we stick with the attribute syntax style >> approach). >> >> I'd like to avoid attempting to preprocess parameters for this patch, >> but had intended to leave the door open for the future. So it wasn't >> entirely without consideration. ;-) > > > =) OK then! _______________________________________________ cfe-commits mailing list [email protected] http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
