Issue 224: define empty python string with Py_CONSTANT_EMPTY_STR (#228)

* define empty python string with Py_CONSTANT_EMPTY_STR

* support older versions of python

* update config

Author: jkotan

setup.py

@@ -150,8 +150,8 @@ def add_filters(filters, pkgs, pkgs_data):
 # ----------------------------------------------------------------------------
 # set compiler options
 # ----------------------------------------------------------------------------
-if sys.platform == "win32":
-    pass
+if sys.platform in ["win32", "win64"]:
+    arguments = ['/std:c++17']
 else:
     # old version
     # arguments = ['-std=c++17', '-Wall', '-Wextra',

src/cpp/h5cpp/numpy/hdf5_variable_strings_support.hpp

@@ -116,16 +116,27 @@ struct VarLengthStringTrait<numpy::ArrayAdapter>
     PyObject ***dataptr = (PyObject***)NpyIter_GetDataPtrArray(iter);
     for(auto string: buffer)
     {
+      PyObject *ptr;
       if(string == NULL)
 	{
 	  char empty[] = {'\0'};
 	  string = empty;
+#if PY_VERSION_HEX >= 0x030d0000
+	  ptr = Py_GetConstant(Py_CONSTANT_EMPTY_STR);
+#elif PY_MAJOR_VERSION >= 3
+	  ptr = PyUnicode_FromString(string);
+#else
+	  ptr = PyString_FromString(string);
+#endif
 	}
+      else
+	{
 #if PY_MAJOR_VERSION >= 3
-      PyObject *ptr = PyUnicode_FromString(string);
+	  ptr = PyUnicode_FromString(string);
 #else
-      PyObject *ptr = PyString_FromString(string);
+	  ptr = PyString_FromString(string);
 #endif
+	}
       if(ptr==NULL)
       {
         std::cerr<<"could not create python string!"<<std::endl;