c++ - How to summarize the elements of std::set -

i'm trying find sum of elements in set , wondering ways find it. built 2 classes, 1 called customer , 1 called item, want write function calculate total payment customer needs pay products listed in std::set type item. here decleration of set:

set<item> _items; 

the class item:

private:     string _name;     string _serialnumber; //consists of 5 numbers     int _count=0; //default 1, can never less 1!     double _unitprice; //always bigger 0! 

function in class item summarize price of item:

double item :: totalprice() const {     return _count*_unitprice; }  

here function i'm trying write sum of elements:

#include <numeric> #include "customer.h" double customer::totalsum() const {     double sum = std::accumulate(_items.begin(), _items.end(), 0.0);     return sum; } 

but error: error c2893: failed specialize function template 'unknown-type std::plus<void>::operator ()(_ty1 &&,_ty2 &&) const'

improtant note: class customer includes header of item.

edit: added information class item.

assuming item this:

struct item {     double price; }; 

then can use following:

auto add_item_price = [](double sum, const item& item) {     return sum + item.price; };  double sum = std::accumulate(_items.begin(), _items.end(), 0.0, add_item_price); 

here's functional demo.

explanation:

std::accumulate lets provide function/functor accumulation. code i've posted uses lambda function accumulation. if aren't using c++11, can use regular function instead of lambda function.

avoid overloading operator+ item. adding 2 items doesn't make sense.