Signature Description Parameters
template<typename T, typename I = unsigned long, typename = typename std::enable_if<std::is_arithmetic<T>::value, T>::type>
struct StdVisitor;
This functor class calculates the standard deviation of a given column.
          explicit StdVisitor (bool bias = true);
        
See this document and dataframe_tester.cc for examples.
T: Column data type.
I: Index type.