How do I pass arrays from Java to C++ using Swig?

Question

I have a method in C++ that takes an array of doubles as an argument. I'm calling this method from Java and need to pass an array of doubles. The C++ routine reads and modifies the values of the array and I need those updated values in Java. How do I do this?

For example, take the C++ routine:

void myMethod( double *values, int size ) {
    for ( int i=0; i < size; i++ ) {
        values[i] = 2*values[i];
    }
}

And the Java code:

double[] values = { 1.3, 1.1 };
myMethod(values,values.length);
System.out.println(values[0]); // prints 2.6

I guess a call to myMethod cannot be made like the call above... or can it? And what is necessary in Swig to make this work. If I cannot make a call like the one above, how do I get my values to the C++ code?

Answer 1

Use carrays.i!

See Swig docs on carrays

%include carrays.i
%array_functions( double, double_array )

These two lines create the following code in my module:

 public static SWIGTYPE_p_double new_double_array(int nelements) {
    long cPtr = SimulatorModuleJNI.new_double_array(nelements);
    return (cPtr == 0) ? null : new SWIGTYPE_p_double(cPtr, false);
  }

  public static void delete_double_array(SWIGTYPE_p_double ary) {
    SimulatorModuleJNI.delete_double_array(SWIGTYPE_p_double.getCPtr(ary));
  }

  public static double double_array_getitem(SWIGTYPE_p_double ary, int index) {
    return SimulatorModuleJNI.double_array_getitem(SWIGTYPE_p_double.getCPtr(ary), index);
  }

  public static void double_array_setitem(SWIGTYPE_p_double ary, int index, double value) {
    SimulatorModuleJNI.double_array_setitem(SWIGTYPE_p_double.getCPtr(ary), index, value);
  }

Which allows me to use C arrays in Java... this solves my needs and is the best solution for my problem. Thanks everyone for your answers!

Answer 2

SWIG typemaps should allow this. The SWIG 2.0 documentation shows this example in Python.

With Java, you would use JNI code inside your SWIG typemap. JNI provides methods Get/ReleaseArrayElements. I'm guessing you could get a native array in a typemap(in), then release it in a typemap(argout).

Another approach would be to copy your array into a std::vector, using the existing SWIG typemap in std_vector.i.