Exposing Static Constant With Boost

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

Solution 1:

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

// ExposureSinusoid.hclassExposureSinusoid
{
    // ...public:
    staticconst UINT _min_exp = 20;   // declaration// ...
};

// ExposureSinusoid.cppconst 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.

Solution 2:

Static linkage means that the symbol is not visible outside the TU! Change static to extern (because global constants are implicitly static):

externconstUINT _min_exp = 20;

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Correction: I didn't see that _min_exp was a member variable, apologies. In that case the answer is, as ildjarn already said, to add the definition of the variable outside the class definition. To be sure, both the declaration and the initialization go inside, but the definition goes outside:

structFoo {
  staticconstint a = 1; // initialization can go inside the class definition
};
constint Foo::a;         // definition (this is what the linker sees)