vector<struct> as returntype: possible? Mapping correct?

[ochafik]

From @apiening on September 25, 2012 22:37

Hi bridj users,

after a successful hello world test, I'm running more and more in problems using BridJ in my "real world" app. I wonder if I'm doing something wrong, my type of mapping is not supported or my platform support (Mac OS X, Mountain Lion) is still buggy.

I want to return a container with structs from a C++ function. I used vector but I'm not specifically bound to this container.
I created a simple demo project and a mapping with JNAerator which compiles, but fails with this error:

Exception in thread "main" java.lang.RuntimeException: Invalid return value type !
    at bridjtest_java.bridj.Test.getStruct(Native Method)
    at bridjtest_java.BridJTest_Java.main(BridJTest_Java.java:35)
Java Result: 1

Here is my C++ header that feeds JNAerator:

#include <vector>

struct SimpleStruct {
    int number;
};

class Test {
public:
    Test();
    Test(const Test& orig);
    virtual ~Test();

    std::vector<SimpleStruct> getStruct();
private:

};

This is the mapping I get from JNAerator:

package bridjtest_java.bridj;
import org.bridj.Pointer;
import org.bridj.ann.Library;
import org.bridj.ann.Name;
import org.bridj.cpp.CPPObject;
import org.bridj.cpp.std.vector;

@Library("test")
public class Test extends CPPObject {
    public Test() {
        super();
    }
    /// Original signature : <code>int Test()</code>
    @Name("Test")
    public native int Test$2();
    /// Original signature : <code>int Test(const Test&)</code>
    public native int Test(Pointer<Test > orig);
    /**
     * char* getGaps(const char* outfile);<br>
     * Original signature : <code>std::vector getStruct()</code><br>
     * <i>native declaration : line 50</i>
     */
    public native vector<SimpleStruct> getStruct();
}

And here's the mapping class for the struct:

package bridjtest_java.bridj;
import org.bridj.Pointer;
import org.bridj.StructObject;
import org.bridj.ann.Field;
import org.bridj.ann.Library;

@Library("test")
public class SimpleStruct extends StructObject {
    public SimpleStruct() {
        super();
    }
    @Field(0)
    public int number() {
        return this.io.getIntField(this, 0);
    }
    @Field(0)
    public SimpleStruct number(int number) {
        this.io.setIntField(this, 0, number);
        return this;
    }
    public SimpleStruct(Pointer pointer) {
        super(pointer);
    }
}

Can someone please confirm if this mapping is supported/possible and if it should work or not?
What may cause my error? How can I dig into that?

Thank you very much!

Andreas Piening

Copied from original issue: nativelibs4java/nativelibs4java#341

[ochafik]

Hi @apiening ,

Returning a vector by value is not recommended in C++ (might incur a copy upon return), people usually pass it by reference:

void getValues(std::vector& out);

BridJ's org.bridj.cpp.std.vector support is currently very flaky at best (STL has different layouts on each platform / runtime library, which even changes between release and debug modes), but you might manage to use it if you provide creation / destruction methods (vector newSimpleStructVector(); void deleteSimpleStructVector(vector);).

The other alternative is to use the C style:

void getValues(SimpleStruct* out, size_t outSize, size_t *elementCount);

Which you first call with:

Pointer pElementCount = allocateSizeT();
getValues(null, 0, pElementCount);

Then:

long elementCount = pElementCount.getSizeT();
Pointer pStructs = allocateArray(SimpleStruct.class, elementCount);
getValues(pStructs, elementCount, null);

Hope it helps :-)

Cheers