bad receiver type 'const char *' when compiling ObjC++

[itsthejb]

Hi guys,

So, I'm writing a Tweak for an animation curve in an ObjC++ file (ComponentKit 😉), using the dictionary method. It's basically this:

[FBTweakValue(@"a", @"b", @"c",
                  @(UIViewAnimationOptionCurveEaseInOut),
                  (@{
                     @(UIViewAnimationOptionCurveEaseInOut) : @"Ease In-Out",
                     @(UIViewAnimationOptionCurveEaseIn) : @"Ease In",
                     @(UIViewAnimationOptionCurveEaseOut) : @"Ease Out",
                     @(UIViewAnimationOptionCurveLinear) : @"Linear"
                     })) unsignedIntegerValue]

However, this won't compile in an ObjC++ file with the following error:

<redacted>: error: bad receiver type 'const char *'
                FBTweakAnimationCurve(@"foo", @"Bar", @"Cheese");
                ^
In file included from <built-in>:351:
In file included from <command line>:26:
In file included from <redacted>:
<redacted>:18:2: note: expanded from macro 'FBTweakAnimationCurve'
[FBTweakValue(category_, collection_, name_, \
 ^
In file included from <built-in>:351:
In file included from <command line>:26:
In file included from  <redacted>:39:
In file included from  <redacted>:10:
 <redacted>/Headers/Public/Tweaks/FBTweakInline.h:38:58: note: expanded from macro 'FBTweakValue'
#define FBTweakValue(category_, collection_, name_, ...) _FBTweakValue(category_, collection_, name_, __VA_ARGS__)
                                                         ^
In file included from <built-in>:351:
In file included from <command line>:26:
In file included from  <redacted>:39:
In file included from  <redacted>:10:
In file included from  <redacted>/Headers/Public/Tweaks/FBTweakInline.h:10:
 <redacted>/Headers/Public/Tweaks/FBTweakInlineInternal.h:159:127: note: expanded from macro '_FBTweakValue'
#define _FBTweakValue(category_, collection_, name_, ...) _FBTweakDispatch(_FBTweakValueWithoutRange, _FBTweakValueWithRange, _FBTweakValueWithPossible, __VA_ARGS__)(category_, collection_, name_, __VA_ARGS__)
                                                                                                                              ^
 <redacted>/Headers/Public/Tweaks/FBTweakInlineInternal.h:67:174: note: expanded from macro '_FBTweakDispatch'
#define _FBTweakDispatch(__withoutRange, __withRange, __withPossible, ...) __FBTweakConcat(__FBTweakDispatch, __FBTweakIndexCount(__VA_ARGS__))(__withoutRange, __withRange, __withPossible)
                                                                                                                                                                             ^
 <redacted>/Headers/Public/Tweaks/FBTweakInlineInternal.h:65:78: note: expanded from macro '__FBTweakDispatch2'
#define __FBTweakDispatch2(__withoutRange, __withRange, __withPossible, ...) __withPossible
                                                                             ^
 <redacted>/Headers/Public/Tweaks/FBTweakInlineInternal.h:154:87: note: expanded from macro '_FBTweakValueWithPossible'
#define _FBTweakValueWithPossible(category_, collection_, name_, default_, possible_) \
                                                                                      ^
1 error generated.

It's possible to test this by pasting the macro into a plain ObjC file, then changing the type to ObjC++ or renaming to .mm. Since this works, I'm fairly sure this isn't my error...?

Of course, it's possible to work around this by returning the value from a function in an .h/.m helper pair, but raising here in case this is something that could be fixed.

Thanks!

[ashoemaker]

👍

[grp]

Sorry, I totally missed this one. I'll take a look as soon as I have time and add a unit test. Or, if you want to send a pull request :)

[itsthejb]

Hi there. Had a bit of a chance to look into this today, and appears that this is non-trivial, since FBTweakValue() is actually implemented using _Generic(). However, I notice the inclusion of extern "C" {..., so I'm a bit confused as to whether or not this was expected to work or not.

It seems to me that getting this to work with ObjC++ would require reimplementing FBTweakValue() to use C++ constructs rather than _Generic(). In which case, considering it's possible to just expose Tweaks values from another non-C++ compilation unit, I would say this isn't worthwhile. In which case, perhaps it would be better to just go with:

#ifdef __cplusplus
#error C++ unsupported error, etc... However, you can expose values from blah, blah, blah helpful advice
#endif

...but I'd be interested to know if this was intended to work first.

[itsthejb]

I did a quick PR if you think it's ok to just go with the build error "solution".