文字列受け渡し

class Hello {
    companion object {
        init {
            System.loadLibrary("Hello")
        }
    }

    external fun printHello()
    external fun printNative(str:String, len:Int);
}

fun main(args: Array<String>) {
    Hello().printHello()
    val str = "hello form kotoln"
    Hello().printNative(str, str.length)
}

でcpp側は

#include "Hello.h"
#include <iostream>
#include <string>
extern "C" {
    JNIEXPORT void JNICALL Java_Hello_printHello
    (JNIEnv *env, jclass obj) {
        std::cout << "hello cpp" << std::endl;
    }
    
    JNIEXPORT void JNICALL Java_Hello_printNative
    (JNIEnv *env, jclass obj, jstring str, jint len) {
        /// charに変換
        jboolean iscopy;
        const char* msgStr = env->GetStringUTFChars(str, &iscopy);
        char* copiedChars = strdup(msgStr);
        env->ReleaseStringUTFChars(str, msgStr);
        env->DeleteLocalRef(str);

        printf("%s\n", copiedChars);
    }
}

でいけた。 が、おそらくこの辺の文字コードの話がいるはず…。

Java とか Android (DEX) の MUTF-8 (Modified UTF-8) って何者よ？っていう話 - bearmini's blog

とか

  java, a unicode char will be encoded with 4 bytes (utf16).
  jstring will container characters  utf16
  std::string in c++ is essentially a string of bytes, not characters,not characters
 we have convert utf16 to bytes.

とか

• 返却値

class Hello {
    companion object {
        init {
            System.loadLibrary("Hello")
        }
    }

    external fun printHello()
    external fun printNative(str:String, len:Int): Boolean;
}

fun main(args: Array<String>) {
    Hello().printHello()
    val str = "hello form kotoln"
    val ret = Hello().printNative(str, str.length)
    println("ret = " + ret)
}

#include "Hello.h"
#include <iostream>
#include <string>
extern "C" {
    JNIEXPORT void JNICALL Java_Hello_printHello
    (JNIEnv *env, jclass obj) {
        std::cout << "hello cpp" << std::endl;
    }
    
    JNIEXPORT jboolean JNICALL Java_Hello_printNative
    (JNIEnv *env, jclass obj, jstring str, jint len) {
        /// charに変換
        jboolean iscopy;
        const char* msgStr = env->GetStringUTFChars(str, &iscopy);
        char* copiedChars = strdup(msgStr);
        env->ReleaseStringUTFChars(str, msgStr);
        env->DeleteLocalRef(str);

        printf("%s\n", copiedChars);
        
        return true;
    }
}