Question

I am using Boost 1.44.0 to cross-compile C++ code for Python. I’m trying to expose a static constant defined in ExposureSinusoid.h like this:

static const UINT _min_exp = 20;

And in the file extending.cpp, I am trying to expose them like this, according to documentation from the Boost team:

.def_readonly("_min_exp", &ExposureSinusoid::_min_exp)

The library compiles fine but when the program runs I get the following error:

ImportError: ../linux/appfs/master/usr/lib/python2.6/pa/vision/_vision.so: undefined symbol: _ZN16ExposureSinusoid8_min_expE

To rule out the possibility the Boost wasn’t finding the constant, I tried changing its name in extending.cpp and the library failed to compile. So it seems like the constant is being found during compilation but it isn’t being properly exposed.

Answer 1

In addition to being declared, static data members must also be defined.

// ExposureSinusoid.h
class ExposureSinusoid
{
    // ...
public:
    static const UINT _min_exp = 20;   // declaration
    // ...
};

// ExposureSinusoid.cpp
const UINT ExposureSinusoid::_min_exp; // definition

The only scenario where the definition of a static data member may be omitted is when the data member in question is a const fundamental integral type, and it is initialized with a constant expression, and it is never ODR-used (see the C++03 standard, §9.4.2/4); however, taking the address of a variable qualifies as ODR-usage, so in your case a definition must be supplied.