Custom Reading Function for FFMPEG I/O - c++

Question

I need to create a custom reading callback function that can read contents of a file in the form of a std::string into a uint8_t * buf. I tried multiple different methods found around the internet and on stackoverflow but sometimes it works and other the the program infinitely loops or stops execution half way.
I have no problems with amr/3gp files but all wav/pcm files are causing some problems for some reason. All I know its something to do with the reading function I have so far.
Ideally I would like to be able to give the program any type of file and then it converts it.
This is how I am calling the readCallback function from the code:
//create the buffer
uint8_t * avio_ctx_buffer = NULL;
//allocate space for the buffer using ffmpeg allocation method
avio_ctx_buffer = (uint8_t *) av_malloc(avio_ctx_buffer_size);
//Allocate and initialize an AVIOContext for buffered I/O.
//audio variable contains the contents of the audio file
avio_ctx = avio_alloc_context(avio_ctx_buffer, avio_ctx_buffer_size,0, &audio, &readCallback, NULL, NULL);
Here is the callback function that works on some types of files:
static int readCallback(void* opaque, uint8_t * buf, int buf_size){
std::string * file =static_cast<std::string *>(opaque);
if(file->length() == 0){
return AVERROR_EOF; //if we reach to the end of the string, return
// return End of file
}
// Creating a vector of the string size
std::vector<uint8_t> array(file->length());
//Copying the contents of the string into the vector
std::copy(file->begin(),file->end(),array.begin());
//Copying the vector into buf
std::copy(array.begin(),array.end(),buf);
return file->length();
}

Answer 1

After tyring some stuff for awhile, I got a solution using std::stringstream and it works well with several formats I tested with so far: 3gp/amr,wav/pcm,mp3.
Here a the snippet of code:
//Create a string stream that contains the audio
std::stringstream audio_stream(audio);
//create the buffer
uint8_t * avio_ctx_buffer = NULL;
//allocate space for the buffer using ffmpeg allocation method
avio_ctx_buffer = (uint8_t *) av_malloc(avio_ctx_buffer_size);
//Allocate and initialize an AVIOContext for buffered I/O.
//Pass the stringstream audio_stream
avio_ctx = avio_alloc_context(avio_ctx_buffer, avio_ctx_buffer_size,0,&audio_stream, &readCallback, NULL, NULL);
The callback function:
static int readFunction1(void* opaque, uint8_t * buf, int buf_size){
//Cast the opaque pointer to std::stringstream
std::stringstream * me =static_cast<std::stringstream *>(opaque);
//If we are at the end of the stream return FFmpeg's EOF
if(me->tellg() == buf_size){
return AVERROR_EOF;
}
// Read the stream into the buf and cast it to char *
me->read((char*)buf, buf_size);
//return how many characters extracted
return me->tellg();
}