Question

The SWIG documentation explains how a variety of input types in C, like this:

void spam1(Foo *x);      // Pass by pointer
void spam2(Foo &x);      // Pass by reference
void spam3(Foo x);       // Pass by value
void spam4(Foo x[]);     // Array of objects

… would all take a single type of argument in Java, like this:

Foo f = new Foo();  // Create a Foo
example.spam1(f);   // Ok. Pointer
example.spam2(f);   // Ok. Reference
example.spam3(f);   // Ok. Value.
example.spam4(f);   // Ok. Array (1 element)

Similarly, for return types in C:

Foo *spam5();
Foo &spam6();
Foo  spam7();

… all three functions will return a pointer to some Foo object that will be assigned to a Java object variable, the final one requiring an allocation of a value type that the Java garbage collection will take care of upon release.

But suppose spam5() returns a pointer to an array. In Java, I have to use array semantics to access the individual elements, but I don’t think that I can just do this:

Foo foo[] = spam5();

I don’t even think the compiler would accept a cast to (Foo[]), so how does this work in SWIG?

Answer 1

This problem does not have a simple or automatic solution. Believe me, I looked.

The problem is that SWIG doesn’t know how big the array you’re returning is supposed to be, so it can’t generate a Java array. You can’t supply the size as an argument to the function, either (gross as that would be) – typemaps don’t work that way.

In the general case you have to write another wrapper function, which takes a C array and a length as an out parameter, and use a typemap to turn those two parameters into a Java array. Or, if you don’t mind using carrays.i, you can skip the second step and just work with C arrays directly from Java.