整数を各桁に分解するイテレータ。 過剰な抽象化をやってみた余興。 右の記事を見てやってみただけ → https://qiita.com/dsy_i07/items/5de9417ff235d5aa6a42

column_split.hpp

#include <cassert>
#include <iterator>
#include <type_traits>

template <class T, int base, typename std::enable_if<std::is_unsigned<T>::value, void*>::type = nullptr>
class column_splitted_iterator {
private:
  T data;
  bool endflag;

public:
  using difference_type = int;
  using value_type = T;
  using pointer = T*;
  using reference = T&;
  using iterator_category = std::input_iterator_tag;
  column_splitted_iterator(const T data) : data(data), endflag(false) {}
  column_splitted_iterator(void) : endflag(true) {}
  bool operator!=(column_splitted_iterator y) {
    if (this->endflag && y.endflag) return false;
    if (this->endflag) return y.data != 0;
    if (y.endflag) return this->data != 0;
    return this->data != y.data;
  }
  T operator*(void) { return data % base; }
  column_splitted_iterator& operator++(void) {
    assert(data);
    data /= 10;
    return *this;
  }
};

template <class T, int base = 10, typename std::enable_if<std::is_unsigned<T>::value, void*>::type = nullptr>
class column_splitted_adapter {
private:
  const T data;

public:
  column_splitted_adapter(const T data) : data(data) {}
  column_splitted_iterator<T, base> begin(void) { return column_splitted_iterator<T, base>(data); }
  column_splitted_iterator<T, base> end(void) { return column_splitted_iterator<T, base>(); }
};

template <class T, int base = 10, typename std::enable_if<std::is_unsigned<T>::value, void*>::type = nullptr>
column_splitted_adapter<T, base> make_column_splitted_adapter(const T obj) {
  return column_splitted_adapter<T, base>(obj);
}

main.cpp

#include <iostream>
#include <numeric>
#include "column_split.hpp"

int main(void) {
  auto adapted_number = make_column_splitted_adapter(123u);
  std::cout << std::accumulate(std::begin(adapted_number), std::end(adapted_number), 0u) << std::endl;
}