使用 Android 中的 C++ std::ifstream.getline() 加载文件

Question

I'm struggling with reading file stream inside Android environment using C++ library. I believe I set all the permissions correctly (1st figure) and I'm getting the file path using Android internal library. Would you please give me a snippet to correctly read a file using std::ifstream.getline()? For instance, I get "/document/1EF7-1509:Download/015_1440.jpg" for the image file existing in the 2nd figure under "Download" folder. This path is a raw value returned by "Intent.getData().getPath()" with "ACTION_GET_CONTENT".

extern "C" JNIEXPORT jstring JNICALL Java_com_example_testpplication_MainActivity_testGeneralEncryption(JNIEnv* env, jobject, jstring myString)
{
    const char *nativeString = env->GetStringUTFChars(myString, nullptr);
    std::string filePath = std::string(nativeString);
    std::string buffer;
    std::ifstream fStreamIn(filePath);
    if(fStreamIn.is_open())
    {
        std::getline(fStreamIn, buffer);
    }
    else
    {
        bool exists = fStreamIn.good();
        if(exists)
        {
            buffer = "Exists";
        }
        else
        {
            buffer = "Non-existing";
        }
    }
    return env->NewStringUTF((buffer + ":" + filePath).c_str());
}

Answer 1

Thanks to @blackapps, I did a google with a different keyword and found below answer. Basically, my question is a duplicate.

Inspired by this answer: https://stackoverflow.com/a/49221353/1770003

The idea is, Android doesn't directly return the absolute path as other OS reports like Windows. A different approach is needed.

The approach I took is,

1. Add this in my solution: https://android-arsenal.com/details/1/8142#!package
2. Edit the project: https://github.com/onimur/handle-path-oz/wiki/Java-Single-Uri

In my case, I moved the caller into onRequestHandPathOz which is add by implementing "HandlePathOzListener.SingleUri".

@Override
public void onRequestHandlePathOz(PathOz pathOz, Throwable throwable)
{
    txt_pathShow.setText(testGeneralEncryption(pathOz.getPath()));
}

The method "testGeneralEncryption" is defined in the original question and pathOz.getPath() is passed as an argument to the parameter "jstring myString".