[C++-sig] [Py++] Undefined symbols for static const int member of class

Kun Hong kun.hong at uqconnect.edu.au
Fri May 21 13:53:39 CEST 2010


On 21/05/2010, at 8:05 PM, Brian O'Kennedy wrote:

> Hi,
>
> I'm wrapping a very simple class using PyPlusPlus.
>
> ---- snip ----
>  class testclass
>  {
>   public:
>    static const int a = 99;
>  };
> ---- /snip ----
>
> And Py++ exposes the static member with a def_readonly("a",  
> testclass::a ) call. When I import this module into python I get an  
> undefined symbol for testclass::a.
>
>   ImportError: ./testwrapper.so: undefined symbol: _ZN9testclass1aE
>
> I understand that the static member has no storage and thus boost  
> python cannot find the address of it.

I think your understanding of static member has no storage is not  
really correct. What I think is, you have
declaration of 'a' but no definition of it.  There is no linkage error  
unless you reference 'a'. The reason is
static const data member of integral type initialized with a constant  
value is treated as a constant expression.
So it means it is ok to use 'a' as a constant in your C++ code, but  
when you reference it in runtime, you need
the definition of it, which in your example, you don't have.

In your wrapper code put something like this:

const int testclass:a;


HTH,
Kun



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