Hi all, i ' ve written a piece of code to get the class sum_accumulator working which is described in the user's guide http://boost-sandbox.sourceforge.net/libs/accumulators/doc/html/accumulators.... The function result must be provided with a struct dont_care. And the constructor of dont_care must be provided with a value to get an instanciation. In my opinion a default constructor taking no arguments for the struct dont_care must exists so the user can write code like dont_care(). To get the example working i must write code like sumacc.result(*dont_care(0.0)*) to get the result from my accumulator. And this is a little bit unnecessary. with regards, Kim Tang the code below is tested with visual studio 2008. #include <iostream> #include #include using namespace boost::accumulators; namespace boost { // Putting your accumulators in the namespace accumulators { // impl namespace has some namespace impl { // advantages. See below. template<typename Sample> struct sum_accumulator // All accumulators should inherit from : accumulator_base // accumulator_base. { typedef Sample result_type; // The type returned by result() below. template<typename Args> // The constructor takes an argument pack. sum_accumulator(Args const & args) : sum(args[sample | Sample()]) // Maybe there is an initial value in the { // argument pack. ('sample' is defined in } // sample.hpp, included above.) template<typename Args> // The accumulate function is the function void operator ()(Args const & args) // call operator, and it also accepts an { // argument pack. this->sum += args[sample]; } result_type result(dont_care) const // The result function will also be passed { // an argument pack, but we don't use it here, return this->sum; // so we use "dont_care" as the argument type. } private: Sample sum; }; }}} int main() { try { boost::accumulators::impl::sum_accumulator<double> sumacc(sample=0.0); sumacc(sample=1.2); std::cout<