D and C++ undefined reference when namespace

[Steven Schveighoffer]

On 3/8/18 11:23 AM, Markus wrote:
> On Thursday, 8 March 2018 at 15:27:31 UTC, Markus wrote:
>> Hi
>>
>> I got the following c++ code [lib.cpp]:
>> namespace ns_a
>> {
>>   class class_a {
>>   };
>>   void some_function(class_a*) {;}
>> }
>>
>> and the following d code [main.d]:
>> extern (C++, namespace_a) {
>>     class class_a {}
>>     void some_function(class_a);
>> }
>> void main() {
>>     namespace_a.class_a instance_a;
>>     namespace_a.some_function(instance_a);
>> }
>>
>> I'm compiling lib.cpp to a shared library by:
>> g++ -shared lib.cpp -o libissue.so
>> and I'm building and linking it with main.d by
>> dmd main.d -L-lissue -L-L.
>>
>> Then I get the error:
>> main.o: In function `_Dmain':
>> main.d:(.text._Dmain[_Dmain]+0xa): undefined reference to 
>> `namespace_a::some_function(namespace_a::class_a*)'
>> collect2: error: ld returned 1 exit status
>> Error: linker exited with status 1
>>
>> When I move the c++ class_a to another namespace (eg. to namespace_b), 
>> I'm able to compile and link!
>> readelf -Ws libissue.so | grep some_function
>> gives me
>> _ZN4ns_a13some_functionEPNS_7class_aE
>> so I don't see any issue. Any idea what I'm missing? I guess some 
>> linker flag when compiling libissue.so
>>
>> Cheers,
>> Markus
> 
> https://forum.dlang.org/thread/mailman.2458.1448772039.22025.digitalmars-d@puremagic.com 
> 
> seems to describe my issue.
> To quote Walter Wright
>> D does not support C++ semantics. You cannot split namespaces into 
>> multiple files in D, nor can you add symbols to an existing namespace. 
>> For namespace NS, all the declarations in NS have to be in one file 
>> and between the { }, just like any other scope in D.
> in my opinion, that's really annoying for C++ wrapper devs.

Hm... that is over 2 years old. I would agree it would seem like a bad 
limitation.

And the error message doesn't seem like a linker error, whereas yours 
does. If it gets to the linker, D hasn't complained about it.

-Steve