Slider controlling gain/volume

Trying to control the gain/volume of audio being played in my audio app using a slider

AudioTransportSource transport;
but struggling with the exact syntax?
Ideas welcome

No offense but: Search a non-JUCE related C++ tutorial to learn the most basic C++ language syntax would be my first idea – otherwise you won’t get much further than just copy & pasting code. In January 2020 you asked what to learn in order to really understand JUCE – and I’d still give you the same reply as I did back then.

But regarding your specific question: transport.setGain(gainSlider.getValue) won’t work since getValue is a function – and to call a function with no arguments you need to add a pair of empty parentheses, so that it reads transport.setGain(gainSlider.getValue()).

Besides the syntax issues you should also make sure where the setGain call is placed. It needs to be called again on each slider value change. So a good place would be to put it into the onValueChanged callback of your slider, e.g. somewhere where you create the slider instance put this line:

gainSlider.onValueChanged = [this] () { transport.setGain (gainSlider.getValue()); };