[cfe-dev] Adding namespaces to Objective-C

[Chris Lattner]

On Nov 9, 2010, at 9:47 AM, Owen Shepherd wrote:
> As someone interested in contributing to Clang - and, in particular, contributing to its Objective-C(++) support, I would like to hear people's thoughts with regards to adding support for namespaces to Objective-C.

Hi Owen,

We've considered adding namespaces to ObjC many times in the past.  It hasn't happened so far largely due to the fact that we really want a unified way to get selectors and classes into namespaces.  Just getting classes into namespaces doesn't solve the whole name conflict issue.

> Some thoughts:
> 1.	 Do we make this an extension to Objective-C, or to C itself? From my point of view, each way primarily boils 
> 	down to a matter of syntax: As an Objective-C extension, the keyword would likely be "@namespace"; while 
> 	as a C extension it would probably be "__namespace__" (or maybe just "namespace" in a hypothetical 
> 	"clang99"  mode)

This is another big issue.  There aren't a lot of good answers here.  If you decide that it should be added to C, then it is logical that C and C++ namespaces work the same way.  If you do this, then you're changing the behavior of existing ObjC++ code that does:

namespace abc {
@interface xyz

If you make it *objc only* (e.g. "@namespace") then it really can only apply to classes, which makes the extension fit even more poorly into the language (why classes but not globals, functions, or selectors?)

Depending on how crazy you get, you could imagine coming up with some sort of way to retroactively make NS::Object resolve to NSObject or something.

> 2.	Function name mangling. Function names follow the platform's C++ ABI (Much like the existing 	
> 	__attribute__((overloadable)) does*
> 3.	When class names are involved in Objective-C symbols, then namespaced classes are mangled similarly to 
> 	C++ names are; a hypothetical MyNS::MyClass is mangled as 4MyNS7MyClass. This is backwards 	
> 	compatible; traditional class names cannot start with numbers
> 4.	The added namespace declarations are equivalent to and interchangeable with C++ " namespace" 
> 	declarations. 

Yes, making it as similar as possible to c++ namespaces is the only way to go.  However, you probably don't want argument dependent lookup (aka Koenig lookup), right?  What other things would be different?

Adding namespaces to ObjC is a huge minefield, not something to be done lightly.  If you'd really really like to pursue it, please start by writing out a spec/proposal of how you think it should work along with the corner cases enumerated.

-Chris