Omitted line of code on pg. 170 (Ch. 8) of physical book.

[Mike-D-Langdon]

The code in the book is missing the gainParamID constant. Later, on pg. 174, it is included (we added this line on pg. 167, it's supposed to be there), so it's only missing on pg. 170's listing. The code should read:

#pragma once

#include  <JuceHeader.h>

const juce::ParameterID gainParamID { "gain", 1 };  // <- This isn't in the book on pg. 170

class Parameters
{
public:
    Parameters(juce::AudioProcessorValueTreeState& apvts);
        
    static juce::AudioProcessorValueTreeState::ParameterLayout createParameterLayout();
        
    juce::AudioParameterFloat* gainParam;
};

[hollance]

Thanks for pointing this out, it's a mistake in the book indeed!