This recipe looks really complicated because we are nesting lambda expressions a lot. In order to understand how this works, let's first have a look at the inner workings of std::accumulate. This is how it will look like in a typical STL implementation:
template <typename T, typename F>T accumulate(InputIterator first, InputIterator last, T init, F f){ for (; first != last; ++first) { init = f(init, *first); } return init;}
The function parameter, f, does the main work here, while the loop collects its results in the user provided init variable. In a usual example case, the iterator range may represent a vector of numbers, such as 0, 1, 2, 3, 4, and the init value is 0. The f function is then just a binary function that ...
