[SWIG] added typemap for string arrays in SWIG interface (#2118)

* added typemap for string arrays in SWIG interface

* added voidp pointer casts

swig/lightgbmlib.i

@@ -10,11 +10,18 @@
 #include "../include/LightGBM/c_api.h"
 %}
 
+%include "various.i"
+%include "carrays.i"
+%include "cpointer.i"
+
+/* Note: instead of using array_functions for string array we apply a typemap instead.
+   Future char** parameter names should be added to the typemap.
+*/
+%apply char **STRING_ARRAY { char **feature_names, char **out_strs }
+
 /* header files */
 %include "../include/LightGBM/export.h"
 %include "../include/LightGBM/c_api.h"
-%include "cpointer.i"
-%include "carrays.i"
 
 %typemap(in, numinputs=0) JNIEnv *jenv %{
    $1 = jenv; 
@@ -199,14 +206,21 @@
 %pointer_cast(int32_t *, int *, int32_t_to_int_ptr)
 %pointer_cast(long *, int64_t *, long_to_int64_t_ptr)
 %pointer_cast(double *, int64_t *, double_to_int64_t_ptr)
-%pointer_cast(double *, void *, double_to_voidp_ptr)
 %pointer_cast(int *, int32_t *, int_to_int32_t_ptr)
+
+%pointer_cast(double *, void *, double_to_voidp_ptr)
 %pointer_cast(float *, void *, float_to_voidp_ptr)
+%pointer_cast(int *, void *, int_to_voidp_ptr)
+%pointer_cast(int32_t *, void *, int32_t_to_voidp_ptr)
+%pointer_cast(int64_t *, void *, int64_t_to_voidp_ptr)
 
 %array_functions(double, doubleArray)
 %array_functions(float, floatArray)
 %array_functions(int, intArray)
 %array_functions(long, longArray)
+/* Note: there is a bug in the SWIG generated string arrays when creating
+   a new array with strings where the strings are prematurely deallocated
+*/
 %array_functions(char *, stringArray)
 
 /* Custom pointer manipulation template */
@@ -256,4 +270,3 @@ TYPE *NAME##_handle();
 
 /* Allow retrieving handle to void** */
 %pointer_handle(void*, voidpp)
-