Question

I have a class

template <unsigned int N> class StaticVector { // stuff }; 

How can I declare and define in this class a static factory method returning a StaticVector<3> object, sth like

StaticVector<3> create3dVec(double x1, double x2, double x2); 

?

Answer 1

‘How can I declare and define in this class’

In what class? You’ve defined a class template, not a class. You can’t call a static function of a class template itself, you have to call a particular version of the static function that’s part of a real class.

So, do you want the template (and hence all instantiations of it) to have a function returning a StaticVector<3>, or do you want one particular instantiation of that template to have a function returning a StaticVector<3>?

If the former:

   template <unsigned int N>   struct SV {     int contents[N];     static SV<3> get3dVec(int x, int y, int z) {       SV<3> v;       v.contents[0] = x;       v.contents[1] = y;       v.contents[2] = z;       return v;     }   };    int main() {     SV<3> v = SV<1>::get3dVec(1,2,3);   } 

works for me.

If the latter (you only want get3dVec to be a member of SV<3>, not of all SV<whatever>), then you want template specialisation:

   template <unsigned int N>   struct SV {     int contents[N];   };    template<>   struct SV<3> {     int contents[3]; // must be re-declared in the specialization     static SV<3> get3dVec(int x, int y, int z) {       SV<3> v;       v.contents[0] = x;       v.contents[1] = y;       v.contents[2] = z;       return v;     }   };    int main() {     SV<3> v = SV<1>::get3dVec(1,2,3); // compile error     SV<3> v = SV<3>::get3dVec(1,2,3); // OK   } 

If for no other reason than to make the calling code look nicer by omitting the basically irrelevant template parameter, I agree with Iraimbilanja that normally a free function (in a namespace if you’re writing for re-use) would make more sense for this example.

C++ templates mean that you don’t need static functions as much in C++ as you do in Java: if you want a ‘foo’ function that does one thing for class Bar and another thing for class Baz, you can declare it as a function template with a template parameter that can be Bar or Baz (and which may or may not be inferred from function parameters), rather than making it a static function on each class. But if you do want it to be a static function, then you have to call it using a specific class, not just a template name.