Static const member initialization in templated class

Question

I have a problem regarding 'static const' member initialization. In a templated class I define a const member and initialize it outside the class.
When I include the .h file where this class is implemented in multiple .cpp files, I get an LNK2005 error (I'm using VS2010) that says the constant is already defined.

// List.hpp
template <class T>
class List {
    static const double TRIM_THRESHOLD;
};

template <class T>
const double List<T>::TRIM_THRESHOLD = 0.8;

I tried putting the member initialization in a .cpp file, but then I get a linker error saying that the constant is not defined at all. If the list is not templated and I put the initialization in a .cpp file, all is fine.
Is there any solution for this situation? I have #ifdef/define clauses around the file already, and it's definitely not a solution.

Answer 1

You should define the constant in a source file not a header (so it only gets defined once) since this is a template which you need to keep in the header(and all instances have the same value) you can use a common base class.

class ListBase {
protected:
    ListBase() {} // use only as base 
    ~ListBase() { } // prevent deletion from outside
    static const double TRIM_THRESHOLD;    
};

template <class T>
class List : ListBase {  
};

// in source file
double ListBase::TRIM_THRESHOLD = 0.8;

Another option is to have it as a static function:

    static double trim_threashold() { return 0.8; }

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Edit: If your compiler supports C++11 you make your static method a constexpr function so that it has all the optimization opportunities that using the value directly has.