Question

How should I initialize a static variable for a partial specialization?

template <bool A=true, bool B=false>
struct from {
    const static std::string value; 
};

// no specialization - works
template <bool A, bool B>
const std::string from<A, B>::value = "";

// partial specialization - does not compile -  
// Error: template argument list following class template name must list parameters in the order used in template parameter list
// Error: from<A,B>' : too few template arguments
template <bool B>
const std::string from<true, B>::value = "";

// full specialization - works
const std::string from<false, true>::value = "";

Why doesn’t the partial work?

EDIT: I found a solution based on Partial template specialization for initialization of static data members of template classes

I need to repeat the declaration for the partial specialization before it allowed me to initialize the static variable:

template <bool B>
struct from<true, B> {
    const static std::string value; 
};

Again, the question is why?

Answer 1

Partial specialization of members (whether they’re functions or static data) are not allowed without partial specialization of enclosing class template itself.

That is, you have to specialize the class template also. So the following should work:

//partial specialization of class template
template <bool B>
struct from<true, B> {
    const static std::string value; 
};

//now you can do this!    
template <bool B>
const std::string from<true, B>::value = ""

Also, this will not compile (have you tried compiling this?):

// full specialization - works (SORRY, IT WILL NOT WORK!)
const std::string from<false, true>::value = "";  //this should be an error

You’ve to write this:

// full specialization 
template<>   //<---- this is important!
const std::string from<false, true>::value = ""