How to map Java ArrayList<float[]> to C++ Vector<array<float,size>> in JniWrapper?

Question

I have an ArrayList of float arrays as ArrayList<float[]> which I want to map to C++ Vector<array<float,size>> in JniWrapper.

I followed this link:

"Returning an arraylist of string from Native java to JNI"

and made few changes to the code as per my requirement.

static jclass java_util_ArrayList;
static jmethodID java_util_ArrayList_;
jmethodID java_util_ArrayList_size;
jmethodID java_util_ArrayList_get;
jmethodID java_util_ArrayList_add;
static thread_local JNIEnv *env;

void java2cpp(jobject arrayList, vector<array<float, 320>> &result) {
    java_util_ArrayList = static_cast<jclass>(env->NewGlobalRef(
            env->FindClass("java/util/ArrayList")));
    java_util_ArrayList_ = env->GetMethodID(java_util_ArrayList, "init", "(I)V");
    java_util_ArrayList_size = env->GetMethodID(java_util_ArrayList, "size", "()I");
    java_util_ArrayList_get = env->GetMethodID(java_util_ArrayList, "get", "(I)Ljava/lang/Object;");
    java_util_ArrayList_add = env->GetMethodID(java_util_ArrayList, "add", "(Ljava/lang/Object;)V");


    jint len = env->CallIntMethod(arrayList, java_util_ArrayList_size);
    result.reserve(len);
    for (jint i = 0; i < len; i++) {
        jfloatArray element = static_cast<jfloatArray>(env->CallObjectMethod(arrayList,
                                                                             java_util_ArrayList_get,
                                                                             i));
        const float *pchars = env->GetFloatArrayElements(element, nullptr);
        result.emplace_back(pchars);
        env->ReleaseFloatArrayElements(element, const_cast<jfloat *>(pchars), 0);
        env->DeleteLocalRef(element);
    }
}

jfloatArray cpp2java(array<float, 320> output) {
    jfloatArray result;
    result = env->NewFloatArray(320);

    float *data;
    data = output.data();

    env->SetFloatArrayRegion(result, 0, 320, data);
    free(data);
    return result;

}

Error:

note: in instantiation of function template specialization 'std::__ndk1::vector<std::__ndk1::array<float, 320>, std::__ndk1::allocator<std::__ndk1::array<float, 320> > >::emplace_back<const float *&>' requested here
        result.emplace_back(pchars);
           ^

I am new to the development of JNI Wrappers. I need help in mapping ArrayList<float[]> to Vector<array<float,320>>.

Your vector holds array<float, 320> but you are passing it a const float *. You need to create the array of the correct type and fill it with appropriate data. — ChrisMM
– ChrisMM, Commented Oct 18, 2019 at 16:25

Oblivion · Accepted Answer · 2019-10-18 16:39:50Z

0

the type of result is: vector<array<float, 320>> which is passed by reference. However you emplace_back a const float*:

const float *pchars = env->GetFloatArrayElements(element, nullptr);
 result.emplace_back(pchars);

You can solve the problem by creating an array out of the data pointed by pchar e.g.

std::array<float, 320> res;
for (int i = 0; i < end; ++i)
     res[i] = pchars[i];
result.emplace_back(res);

Make sure to range check pchar (end) you may access out of bounds.

edited Oct 18, 2019 at 16:39

answered Oct 18, 2019 at 16:33

Oblivion

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4 Comments

Khubaib Ahmad Over a year ago

So what should I do..??

Khubaib Ahmad Over a year ago

Okay Thanks alot. Let me try the solution.

Jesper Juhl Over a year ago

@KhubaibAhmad "My problem has been solved" - Then you should Accept the answer that solved the problem, rather than leaving a "thank you" comment.

Khubaib Ahmad Over a year ago

@JesperJuhl My apologies.. there were some technical issues and I was unable to do it.

ChrisMM · Accepted Answer · 2019-10-18 16:42:01Z

0

Your vector holds array<float, 320> but you are passing it a const float *. You need to create the array of the correct type and fill it with appropriate data. At no point do you create the array<float, 320>, nor do you fill it. It's been a while since I've used JNI so I don't really remember the syntax, but you would have to copy the elements in the jfloatarray over into a new array<float, 320>.

Assuming pchars contains the appropriate data, then you should be able to do something like:

const float *pchars = env->GetFloatArrayElements(element, nullptr);
array<float, 320> newArray;
for ( size_t i = 0; i < 320; ++i ) newArray[i] = pchars[i];
result.emplace_back(newArray);

edited Oct 18, 2019 at 16:42

answered Oct 18, 2019 at 16:34

ChrisMM

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1 Comment

Khubaib Ahmad Over a year ago

Thanks alot @ChrisMM. I think this is the right solution.

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