Return multiple values from C++

[Scott Marley]

Hi Everyone. I'm struggling a little to get my head around how to
return multiple values from C++. Let's assume I have a struct in C++
that looks something like this:

struct details {
int index;
double distance;
...
};

I would like to return this from C++ to Java so that the members of my
struct are accessible. However, I don't need it to work the other way
(to put values from Java into my native struct). I have been reading
various articles regarding JNI but I have yet to find any really
simple examples of how this is done. I realise that I have to create a
Java class as a wrapper for my native struct but I'm still confused.
What would be extremely helpful is a simple code example including the
Java and Native code needed to achieve this. I know that once I've got
the principle it should be simple to extend.

Thank you everyone for your patience with a JNI novice.

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[fulanito]

You need to bundle all the output values as one class in the Java side
(as a public class), and then init it from the C++
code using JNI methods.
You need to explicitly call the constructor from the C++ code, even if
it is only a default constructor.

For example:
jclass javaCls = env->FindClass("com/myApp/JavaClass");
jmethodID cid = env->GetMethodID(javaCls,"<init>","()V");
jobject out = env->NewObject(javaCls, cid);
Here I assume the class name is JavaCls and the constructor is the
default constructor.

Then you can set the fields, again using JNI methods:
jfieldID fid = env->GetFieldID(javaCls , "res", "Z");
env->SetBooleanField( out, fid, TRUE);
Here I set a boolean field called "res".

Always check that the jclass, jmethodID and so on returned from JNI
methods are not null.

For more info, look at JNI documentation.
Fulanito.

[Scott Marley]

Fantastic, that's the first clear description I've seen! For anyone
else who needs this explaining at the basic level that I have, my code
is as follows:

####### Java class declaration: #######

public class JavaCls {
public boolean res;
}

####### Java native declaration: #######

public native JavaCls trueFalse();

############### C++: ###############

extern "C"{

jobject
Java_astroexplorer_scott_uk_AstroExplorer_trueFalse( JNIEnv* env,
jobject thiz)
{
jclass javaCls = env->FindClass("astroexplorer/scott/uk/JavaCls");

jmethodID cid = env->GetMethodID(javaCls,"<init>","()V");
jobject out = env->NewObject(javaCls, cid);

jfieldID fid = env->GetFieldID(javaCls , "res", "Z");

env->SetBooleanField( out, fid, true);

return out;
}
}

###### To get the native results: ######

JavaCls result = trueFalse();

Now I just need to add the code to check for null values being
returned which would inevitably cause the program to hang. Thank you,
thank you, thank you!