Signature Description Parameters
#include <DataFrame/DataFrameStatsVisitors.h>

template<typename T, typename I = unsigned long>
struct CovVisitor;
        
This functor class calculates the covariance of two given columns. In addition, it provides the variances of both columns.
    explicit CovVisitor(bool bias = true, bool skipnan = true);
        
T: Column data type. T must be an arithmetic-enabled type
I: Index type.
    std::cout << "\nTesting Covariance Visitor ..." << std::endl;

    CovVisitor<double> cov_visitor;
    auto                fut10 = df.visit_async<double, double>("dbl_col", "dbl_col_2", cov_visitor);
    const double        cov = fut10.get().get_result();

    assert(fabs(cov - -0.358381) < 0.000001);
C++ DataFrame