[C++-sig] undefined symbols with def_readonly

[Neal Becker]

Stefan Seefeld wrote:

> Neal Becker wrote:
>> I have:
>> 
>> struct turbo_enc_1_15 {
>> ...
>>   static const int TAIL_BITS = 2*MEMORY;
>> ...
>> 
>> Then this wrapper:
>> BOOST_PYTHON_MODULE(turbo_enc_1_15)
>> {
>>   class_<turbo_enc_1_15> ("turbo_enc_1_15", no_init)
>> ...
>>     .def_readonly ("TAIL_BITS", &turbo_enc_1_15::TAIL_BITS)
>>     ;
>> 
>> At runtime I get this:
>> ImportError: ../mod/turbo_enc_1_15.so: undefined symbol:
>> _ZN14turbo_enc_1_159TAIL_BITSE
>> 
>> Anything obviously wrong here?
> 
> The code above only shows the declaration of turbo_enc_1_15::TAIL_BITS,
> but not its definition. It seems the linker agrees with me that it is
> missing. (See 9.4.2/4 of the spec:
> 
> "...The member shall still be defined in a namespace scope if it is used
> in the program..."
> 
> It seems you can get away without the definition if you ever only take
> its value, but not its address.
> Note that in your code you pass the member's address to def_readonly().
> May be what you really want is simply add an attribute to your class,
> using the (compile-time evaluated) value ?
> 

Yes, I just want to get the (compile-time evaluated) value.  What is the
suggested way to do this?