Problem using pair with accumulate

Question

I am using a deque so I can generate rolling averages and variances for my data. I store n and n^2 as a pair in the deque and then use accumulate with my own operator+().

#include <deque>
#include <numeric>
#include <utility>

template <typename T1, typename T2>
std::pair<T1,T2> operator+(const std::pair<T1,T2>& lhs, const std::pair<T1,T2>& rhs)
{
   return std::pair<T1,T2>(lhs.first + rhs.first, lhs.second + rhs.second);
}

namespace resource 
{
template <typename T>
class rollingStats
{
public:
   rollingStats(unsigned int n, const T& val):
      xs(n, std::pair<T,T>(val, val*val))
   {;}
   ~rollingStats()
   {;}

   T getMean(void) const
   {
      std::pair<T,T> sum = std::accumulate(xs.begin(), xs.end(), std::pair<T,T>((T)0,(T)0));
      return sum.first / xs.size();
   }

   T getVar(void) const
   {
      const unsigned int n = xs.size();

      std::pair<T,T> sum = std::accumulate(xs.begin(), xs.end(), std::pair<T, T > ((T)0,(T)0));

      return ((n * sum.second - sum.first*sum.first) / (n * n));
   }

   void addValue(const T& val)
   {
      xs.pop_front();
      xs.push_back(std::pair<T,T>(val,val*val) );
   }

   const std::deque<std::pair<T,T> >& getXs(void) const {return xs;}
private:
   std::deque<std::pair<T,T> > xs;
};
}

I get a compilation error using g++ 4.1.2 which I can't resolve.

  [ CC         ]  resource/UnitTest: rollingStats_Test.o 
/usr/lib/gcc/i386-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_numeric.h: In function ‘_Tp std::accumulate(_InputIterator, _InputIterator, _Tp) [with _InputIterator = std::_Deque_iterator<std::pair<float, float>, const std::pair<float, float>&, const std::pair<float, float>*>, _Tp = std::pair<float, float>]’:
../rollingStats.hpp:45:   instantiated from ‘T resource::rollingStats<T>::getMean() const [with T = float]’
rollingStats_Test.cpp:98:   instantiated from here
/usr/lib/gcc/i386-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_numeric.h:89: error: no match for ‘operator+’ in ‘__init + __first.std::_Deque_iterator<_Tp, _Ref, _Ptr>::operator* [with _Tp = std::pair<float, float>, _Ref = const std::pair<float, float>&, _Ptr = const std::pair<float, float>*]()’
/usr/lib/gcc/i386-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_bvector.h:267: note: candidates are: std::_Bit_iterator std::operator+(ptrdiff_t, const std::_Bit_iterator&)
/usr/lib/gcc/i386-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/stl_bvector.h:353: note:                 std::_Bit_const_iterator std::operator+(ptrdiff_t, const std::_Bit_const_iterator&)
make: *** [rollingStats_Test.o] Error 1

What have I got wrong here? Do I need to add my own functor instead of relying on the STL alone?

thanks

Answer 1

std::pair doesn't have an operator+, and you haven't provided a way for std::accumulate to call your implementation of operator+.

I would wrap the functionality you provided in operator+ in a functor...

template <typename T1, typename T2> struct pair_sum : public std::binary_function< std::pair<T1,T2>, std::pair<T1,T2>, std::pair<T1,T2> >
{
    std::pair<T1,T2> operator()(const std::pair<T1,T2>& lhs, const std::pair<T1,T2>& rhs)
    {
       return std::pair<T1,T2>(lhs.first + rhs.first, lhs.second + rhs.second);
    }
};

...and use that by calling the version of std::accumulate that takes 4 arguments:

std::pair<T,T> sum = std::accumulate(xs.begin(), xs.end(), std::make_pair((T)0,(T)0), pair_sum<T,T>());