Passing cv::Mat by reference

I have a function where instead of returning a cv::Mat I want to pass in a reference to an existing data structure. I am trying to reduce unnecessary copying of cv::Mat Function currently looks like:

cv::Mat HSV::apply(const cv::Mat& input) {
  cv::Mat image, output;

  cv::cvtColor(input, image, cv::COLOR_BGR2HSV);
  cv::inRange(image, m_Low, m_High, output);
  return output;
}

I am trying to convert it to -

void HSV::apply(const cv::Mat& input) {
  cv::Mat image, output;

  cv::cvtColor(input, image, cv::COLOR_BGR2HSV);
  cv::inRange(image, m_Low, m_High, output);
}

Now in another node, I used to call this function as follows:

Detection::Detection(ros::NodeHandle& nh)
    : m_Filter_(std::make_unique<HSV>()), { 
  m_ImageSub = nh.subscribe("/image_raw", 10, &Detector::inputCb, this);
}

void Detector::inputCb(const sensor_msgs::Image::ConstPtr& msg) {
  cv::Mat image = ImageConverter::convertToCvMat(msg);
  cv::Mat finalImage = m_Filter_->apply(image);
}

How can I call the function now if I am passing it by reference?

I don't think your converted function is correct. It should be

void HSV::apply(const cv::Mat& input, cv::Mat& output) {
  cv::Mat image;

  cv::cvtColor(input, hsvImage, cv::COLOR_BGR2HSV);
  cv::inRange(image, m_Low, m_High, output);
}

That function would be called like this

void Detector::inputCb(const sensor_msgs::Image::ConstPtr& msg) {
  cv::Mat image = ImageConverter::convertToCvMat(msg);
  cv::Mat finalImage;
  m_Filter_->apply(image, finalImage);
}

However please note that named return value optimization (NRVO) means that no copying occurs when you return output; so this might be an unnecessary change.