Transcoding audio file from one format to another format using FFmpeg

[Jaswinder]

Hello all,
I want to trans code audio file from one format (mp4) to other format like (mp3). I am using ffmpeg open source library. i build the library for Qt and it get plugged with qt. i am able to access all its function.
can someone tell me about transcending methods can someone tell me any example for this.
i found something on net
http://ffmpeg.org/doxygen/trunk/encoding-example_8c-source.html

But i want to conversion from a particular format to a particular format. i also found some command line tools like ffmpeg.exe, but i dont want to use this . i want to write c++ code to do this task.
can any one suggest me the best way.
ffmpeg -i abc.mp4 -acodec mp3 output.mp3