JUCE | source cross-platform C++ application framework for desktop | Build Tool library

juce::AudioFormatReaderSource has a method called getTotalLength() which returns the total amount of samples. Divide that by the sample rate of the file and you have the total length in seconds. Something like this:

You need something like this(forward declaration: because you didn't show us how you used forward decleration I show that again here):

Just like a functions can take default argument, template can have default parameters.

Install the OpenGL dev support:

Well that __index function would have to return BaseClass.foo if someone indexes DerivedClass.foo.

If __index is a table you basically do this:

You have not given us much to work with, but I think your problem stems from this Android JNI FAQ:

You can get into trouble if you create a thread yourself (perhaps by calling pthread_create and then attaching it with AttachCurrentThread). Now there are no stack frames from your application. If you call FindClass from this thread, the JavaVM will start in the "system" class loader instead of the one associated with your application, so attempts to find app-specific classes will fail.

So, in the code for my_app_helper.so, you should:

1. make sure to use a thread-specific env (obtained by calling AttachCurrentThread on the vm if needed);
2. make sure that all class references you need were obtained in advance. This can be as easy as calling FindClass from my_app.so and storing a GlobalReference in a static/global variable. (g_my_class = env->NewGlobalRef(env->FindClass("..."))) This is the easiest workaround, but see the linked post for other options.

In general, you should practice good JNI hygiene and always check for errors after a JNI call, and for Android specifically, look into turning on extended checking while developing.

EDIT: There is a second case in which FindClass will always work:

Any FindClass calls made as part of executing JNI_OnLoad will use the class loader associated with the function that called System.loadLibrary (this is a special rule, provided to make library initialization more convenient)."

You can see this in the Android Runtime source code here:

The code below does not compile because expressions like a.foo are not statements in Lua:

You have a mistake in the order of arguments for lua_resume
It should be lua_resume(coro, lua, instead of lua_resume(lua, coro,

concepts of k function ... Where should I use these stuffs?

There is a good example on using k-functions
https://stackoverflow.com/a/67961038/1847592

You're close...

Gray scale images need TWO components per pixel: brightness and alpha.

So, with just a couple changes (see the comments):