[cfe-dev] Interesting clang behavior/bug? on Windows

[Edward Diener]

On 2/13/2015 11:17 PM, Nico Weber wrote:
> Hans would know for sure, but I think this is intentional: Since AClass
> is dllexported, all its implicit functions get generated (so that they
> can be exported from the dll), which means they need to be semantically
> checked. If you give the class a deleted copy constructor and assignment
> operator, it might work.

Where in the C++ standard is such behavior justified ?

The C++ standard does tell us I believe that if an object of a class is 
not copied there is never a need to specify a copy constructor and if a 
class is not assigned there is never a need to specify an assignment 
operator, whether or not one has a non-copyable or non-assignable member 
or not. Changing that basic rule to support something in clang related 
to "exported' classes cannot be correct IMO.

Why does '__attribute__((__visibility__("default")))' work but 
'__attribute__((dllexport))' does not ? Aren't they both the equivalent 
of "exporting" a class in clang ?

I realize that for clang on Windows targeting mingw/gcc and not VC++ 
that the correct "export" attribute is probably 
'__attribute__((__visibility__("default")))' and not 
'__attribute__((dllexport))'. But I heavily object to the idea that 
because I might be exporting a class the rules of copyability or 
assignability for that class must change contrary to the C++ standard. 
This also puts an extra conceptual burden on the design of a class.

>
> On Fri, Feb 13, 2015 at 8:07 PM, Edward Diener
> <eldlistmailingz at tropicsoft.com
> <mailto:eldlistmailingz at tropicsoft.com>> wrote:
>
>     I am compiling code with "-c -x c++ -O0 -g -fno-inline -Wall -g"
>     using clang targeting mingw/gcc on Windows.
>
>     This code compiles with no errors ( boost::scoped_ptr<T> is
>     non-copyable, non-assignable ):
>
>     #include <boost/scoped_ptr.hpp>
>     class
>     AClass { boost::scoped_ptr<int> sp_pointer; };
>     int main()  {  return 0;  }
>
>     This code compiles with no errors:
>
>     #include <boost/scoped_ptr.hpp>
>     class __attribute__((__visibility__(__"default")))
>     AClass { boost::scoped_ptr<int> sp_pointer; };
>     int main()  {  return 0;  }
>
>     but this code compiles with errors:
>
>     #include <boost/scoped_ptr.hpp>
>     class __attribute__((dllexport))
>     AClass { boost::scoped_ptr<int> sp_pointer; };
>     int main()  {  return 0;  }
>
>         test_clang_bug.cpp:3:1: error: field of type
>         'boost::scoped_ptr<int>' has private copy constructor
>         AClass { boost::scoped_ptr<int> sp_pointer; };
>         ^
>         ..\..\..\boost/smart_ptr/__scoped_ptr.hpp:47:5: note: declared
>         private here
>              scoped_ptr(scoped_ptr const &);
>              ^
>         test_clang_bug.cpp:3:1: note: implicit copy constructor for
>         'AClass' first required here
>         AClass { boost::scoped_ptr<int> sp_pointer; };
>         ^
>         test_clang_bug.cpp:3:1: error: 'operator=' is a private member
>         of 'boost::scoped_ptr<int>'
>         AClass { boost::scoped_ptr<int> sp_pointer; };
>         ^
>         ..\..\..\boost/smart_ptr/__scoped_ptr.hpp:48:18: note: declared
>         private here
>              scoped_ptr & operator=(scoped_ptr const &);
>                           ^
>         test_clang_bug.cpp:3:1: note: implicit copy assignment operator
>         for 'AClass' first required here
>         AClass { boost::scoped_ptr<int> sp_pointer; };
>         ^
>         2 errors generated.
>
>
>     Comments ?
>
>     Should I file a bug report ?
>
>     I do not see why, even "exporting" a class, clang should give an
>     error. If I am not copying/assigning an instance of a class I should
>     never get a compiler error telling me that some member is not
>     copyable or assignable.
>
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