如何从Android NDK中的另一个C ++文件读取常量

Question

I am using NDK in my android app . There was no problem . Here is the code for c++ file

#include <jni.h>
#include <string>
#include <stdio.h>


extern "C" JNIEXPORT jstring JNICALL
Java_com_examples_core_MyApplication_getKeyJNI(
        JNIEnv *env,
        jobject /* this */) {
    std::string secret_key = "mysecret";
    return env->NewStringUTF(secret_key.c_str());
}

Edit

Here is my approach

my native-lib.cpp

#include <jni.h>
#include <string>
#include <unistd.h> // for getcwd()
#include <iostream>
#include <stdio.h>
#include "constants.h"

extern "C" JNIEXPORT jstring JNICALL
Java_com_examples_core_MyApplication_getKeyJNI(
        JNIEnv *env,
        jobject /* this */) {
    std::string secret_key = secret_key;
    return env->NewStringUTF(secret_key.c_str());
}

my constants.h

#pragma once

#include <string>

extern const std::string secret_key;        // declaration

my constants.cpp

#include "constants.h"

const std::string secret_key = "mysecret";  // definition

When I compile I get the following error

native-lib.cpp:13: undefined reference to `secret_key'

Answer 1

You don't want to put the definition in a header file, as that could lead to multiple definitions of the same variable.

But you could do something like this:

constants.h

#pragma once

#include <string>

extern const std::string secret_key;        // declaration

constants.cpp

#include "constants.h"

const std::string secret_key = "mysecret";  // definition

main.cpp

#include <iostream>
#include "constants.h"

int main()
{
    std::cout << secret_key;               // usage
}