4.6创建一个迭代器生成器.cpp

#include"print.h"

class fib_generator {
	using fib_t = unsigned long;
	fib_t stop_{};
	fib_t count_{ 0 };
	fib_t a_{ 0 };
	fib_t b_{ 1 };
	constexpr void do_fib() {
		const fib_t old_d = b_;
		b_ += a_;
		a_ = old_d;
	}
public:
	using iterator_concept = std::forward_iterator_tag;
	using iterator_category = std::forward_iterator_tag;
	using value_type = std::remove_cv_t<fib_t>;
	using difference_type = std::ptrdiff_t;
	using pointer = const fib_t*;
	using reference = const fib_t&;

	explicit fib_generator(fib_t stop = 0) :stop_{ stop } {}

	fib_t operator*()const { return b_; }
	constexpr fib_generator& operator++() {
		do_fib();
		++count_;
		return *this;
	}
	fib_generator operator++(int) {
		auto tmp{ *this };
		++* this;
		return tmp;
	}
	bool operator==(const fib_generator& o)const {
		return count_ == o.count_;
	}
	const fib_generator& begin()const { return *this; }
	const fib_generator end()const {
		auto sentinel = fib_generator();
		sentinel.count_ = stop_;
		return sentinel;
	}
	fib_t size() { return stop_; }
};

int main() {
	printc(fib_generator(10));
	fib_generator fib(10);
	auto x = std::ranges::views::transform(fib, [](auto x) {return x * x; });
	printc(x, "squared:");
}