This is the latest version of this patch -- with some additional features as requested. Specifically, this is using the existing __has_attribute syntax in an extended style, and implements a feature-test macro (__has_feature(__has_attribute_syntax)) which allows you to determine whether the extended syntax is supported or not. It adds some additional documentation explaining this.
~Aaron On Thu, Jan 23, 2014 at 10:23 AM, Aaron Ballman <[email protected]> wrote: > 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!
HasAttribute.patch
Description: Binary data
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