demo.cpp

#include <cstdio>
#include <vector>

#define inline __attribute__((always_inline))

namespace {

template <typename Iterator, typename Value, typename Checker>
class EasyIterator {
 public:
  inline Iterator& begin() {
    this->operator++();
    return static_cast<Iterator&>(*this);
  }

  inline Iterator const& end() const { return static_cast<Iterator const&>(*this); }

  inline bool operator!=(Iterator&) const { return !Checker()(value_); }

  inline void operator++() { value_ = static_cast<Iterator&>(*this).Next(); }

  inline Value operator*() const { return value_; }

 private:
  Value value_;
};

struct IsNull {
  inline bool operator()(int x) const { return x == 0; }
};

class MyIterator : public EasyIterator<MyIterator, int, IsNull> {
 public:
  MyIterator(std::vector<int> const& objects) : objects_(objects) {}
  int Next() { return objects_[index_++]; }

 private:
  int index_ = 0;
  std::vector<int> const& objects_;
};

}  // namespace

void foo(std::vector<int> const& a) {
  for (int i : MyIterator(a)) {
    printf("%d\n", i);
  }
}

void foo2(std::vector<int> const& a) {
  MyIterator mi(a);
  for (int i = mi.Next(); i >= 0; i = mi.Next()) {
    printf("%d\n", i);
  }
}