Can't return unique_pointer to slider

std::unique_ptr<Slider>sliderInit(String name, Slider::SliderStyle style, float min, float max, float step) {
	auto sliderPtr = std::unique_ptr<Slider>{new Slider(name)};
	sliderPtr->setRange(min, max, step); 
	return sliderPtr; 

ClipperGUI::ClipperGUI (ClipperAudio& p): AudioProcessorEditor (&p), processor (p)
	setSize(400, 300);//Set the size of my plugin to be 400 by 300.
    GainSlider = sliderInit("Gain", Slider::SliderStyle::LinearBarVertical, 0.0f, 1.0f, 0.0001f);
void ClipperGUI::paint(Graphics& g)
	g.drawFittedText("Drive", 0, 0, getWidth(), 30, Justification::centred, 1);

void ClipperGUI::resized()
	GainSlider->setBounds(40, 30, 20, getHeight() - 60);
void ClipperGUI::sliderValueChanged(Slider* slider)
	processor.mGain = GainSlider->getValue();

	GainSlider = NULL; 

A bit more of context description beneath the code and the topic would be really nice, like: What goes wrong? A compiler error (what does it say) a runtime error…?

What looks suspicious:
std::unique_ptr<Slider>sliderInit(String name, Slider::SliderStyle style, float min, float max, float step)
So sliderInit is a free function declared outside the class in your compile unit? I wouldn’t consider this as best practice. Better make it a private static member function of the ClipperGUI class. Furthermore, is the missing space in std::unique_ptr<Slider>sliderInit a copy & paste error? Otherwise I’m not sure if this is valid, I guess not.

Then: Looking at the documentations for the operator= of std::unique_ptr here shows us that all the methods there look like this:

unique_ptr& operator=( unique_ptr&& r)

What’s important here is the && before the argument. It tells you, that a unique_ptr can only be move-assigned. So assumed GainSlider is a std::unique_ptr, the line GainSlider = sliderInit (...) won’t work. Instead you need to write GainSlider = std::move (sliderInit (...)) .

Two code-style related things: It’s good practise to begin member variables with a lowercase letter, and types with an upper case one, so you should consider naming your slider member gainSlider. Second: As you use auto for sliderPtr (which is good!), it’s considered clean and modern C++ to write auto sliderPtr = std::make_unique<Slider> (name);.