accumulate not a part of std

Question

I'm new to C++ (and OpenCV). Found some code and trying to compile it but get an error. What does it mean?

First of all accumulate was std::accumulate() but the compiler returned an error telling it was not a member of std. Then I changed it to cv::accumulate() but now I get a new error? Maybe it should be a memeber of std but I'm missing to include a header file?

In file included from txtbin.hpp:11,
                 from txtbin.cpp:12:
dewarp.hpp: In member function ‘std::vector<cv::Point_<double> > Dewarp::keypoints_from_samples(const std::vector<std::vector<cv::Point_<double> > >&, std::vector<double, std::allocator<double> >&, std::vector<std::vector<double, std::allocator<double> > >&)’:
dewarp.hpp:96:47: error: invalid initialization of reference of type ‘cv::InputArray’ {aka ‘const cv::_InputArray&’} from expression of type ‘std::vector<double, std::allocator<double> >::iterator’
   96 |   double mean = cv::accumulate(py_coords.begin(), py_coords.end(), 0.0) / py_coords.size();

code

std::vector<double> px_coords, py_coords;

for(const std::vector<cv::Point2d>& points : span_points){
        std::vector<double> py_coords, px_coords;
        for(const cv::Point2d& point : points){
            py_coords.push_back(point.dot(y_dir));
            px_coords.push_back(point.dot(x_dir) - px0);
        }
        double mean = cv::accumulate(py_coords.begin(), py_coords.end(), 0.0) / py_coords.size();
        y_coords.push_back(mean - py0);
        x_coords.push_back(px_coords);
    }