There is no realy good reason to make the template class instatiation in an extern file. (#9)

To clearify the build output it will no be made in the src file.

Author: MaxSac

src/detail/BicubicSplines.cxx

@@ -289,3 +289,6 @@ template <typename T> T BicubicSplines<T>::double_prime(T x0, T x1) const {
       x0);
 }
 } // namespace cubic_splines
+
+template class cubic_splines::BicubicSplines<double>;
+template class cubic_splines::BicubicSplines<float>;

src/detail/BicubicSplines_imp.cxx

@@ -1,9 +1,7 @@
 target_sources(CubicInterpolation PRIVATE
     detail/Axis.cxx
     detail/BicubicSplines.cxx
-    detail/BicubicSplines_imp.cxx
     detail/CubicSplines.cxx
-    detail/CubicSplines_imp.cxx
     detail/InterpolantBuilder.cxx
     detail/Interpolant.cxx
     )

src/detail/CMakeLists.txt

@@ -101,5 +101,7 @@ template <typename T> T CubicSplines<T>::prime(T x) const {
 template <typename T> T CubicSplines<T>::double_prime(T x) const {
   return data->spline.double_prime(x);
 };
-
 } // namespace cubic_splines
+
+template class cubic_splines::CubicSplines<float>;
+template class cubic_splines::CubicSplines<double>;