Question

I have two C functions that I’m exposing through SWIG to my Java layer and both have an input param with a const void * data type (“val) that needs to be a uint8_t for the addCategory function but a char for the addAttribute function. I’m currently, in the SWIG Interface file, using the %apply to map the const void * C type to a short on the Java side. Is there a way to modify the SWIG interface file to support both a char (String) and a uint8_t (short) for the const void * input parameter?

C Functions from header file:

int
addCategory(query_t *query, type_t type, const void *val);

int
addAttribute(query_t *query, type_t type, const void *val);

SWIG Interface File:

%module Example
%include "stdint.i"
void setPhy_idx(uint32_t value);
%include "arrays_java.i"
void setId(unsigned char *value);
%{
#include "Example.h"
%}
%apply char * { unsigned char * };
%apply char * { void * };
%apply uint8_t { const void * }
%apply int32_t { int32_t * }
%include "Example.h"

Answer 1

You can’t directly do this – what type would be used in this place in Java? You need to help SWIG decide that in some way.

You have (at least) three possible solutions:

1. Use a type hierarchy – The base type will be what the function takes, the subclasses will get wrapped also. You could do this on the C++ side, or on the Java side using SWIG’s typemap facilities. I think this is needlessly complicated though, so I’ve not made an example here.
2. Use overloads (or even different functions, with different names altogether – you could use %rename to make them back into overloads in Java even if they have different names in C)

3. Use a union. This will get wrapped with set and get functions by SWIG:

   %module test
   
   union values {
     unsigned char *string;
     void *generic;
     uint8_t someOtherThing;
     uint32_t number;
   };
   
   void func(values v);
   

   This results in a Java class called values, which func() takes and can pass one of the members of the union through. Clearly you’d want to %apply appropriate typemaps for the members of the union.