Re: boost.numpy functions are throwing unresolved extern symbol link errors

From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2021-01-20 08:52:50

On 1/20/21 11:48 AM, Andrey Semashev wrote:
> On 1/20/21 6:23 AM, Stefan Seefeld via Boost wrote:
>> Hello,
>>
>> I'm unfortunately not familiar enough with the platform (Windows) or 
>> this compiler (MSVC) to help, but I would like to add some notes in 
>> case someone else on this list has ideas:
>>
>> Compiling a trivial C++ applet based on code such as 
>> https://boostorg.github.io/python/doc/html/numpy/tutorial/ndarray.html 
>> on Linux simply requires a command such as
>>
>> ```
>>
>> g++ -I /usr/include/python3.8 -o np_test np_test.cpp -lboost_numpy38 
>> -lboost_python38 -lpython3.8
>> ```
>>
>> I find particularly confusing that the MSVC linker would generate a 
>> missing symbol error for the `ndarray::get_data()` function, given 
>> that this function (as defined in 
>> https://github.com/boostorg/python/blob/develop/include/boost/python/numpy/ndarray.hpp#L101) 
>> ought to be inlined, and not generate a symbol at all.
>>
>> Does anyone familiar with this platform have an idea under what 
>> circumstances this may result in a link error ?
> 
> The class is marked with BOOST_NUMPY_DECL, which, I assume, is 
> __declspec(dllexport)/(dllimport) on Windows. This means that when 
> compiling the user's code the compiler may look for the imported 
> function from the dll when inlining does not happen, which is usual in 
> debug builds, for example.
> 
> The fact that there is no such function probably means that the class is 
> never included when Boost.Python is compiled (i.e. when the class is 
> marked as dllexport).

Actually, no, it must be indirectly included in src/numpy/ndarray.cpp. 
But that file is a part of boost_numpy library, not boost_python. Does 
it get exported from boost_numpy?

> Marking the whole class as dllexport/dllimport has other consequences, 
> such as exporting type information for the class. If this is not needed, 
> I would suggest marking individual members to export/import and not the 
> whole class.
> 
>> (See the exact command-line in the previous post).
> 
> What previous post?