Add README.md.

2026-01-19 04:42:12 +00:00 · 2019-08-10 18:39:04 -05:00
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+# iterator_facade
+
+An updated C++20-friendly version of the iterator_facade part of Boost.Iterator, targeting standardization.
+
+In short, if you need to write an iterator, iterator_facade turns this:
+
+```c++
+    struct repeated_chars_iterator
+    {
+        using value_type = char;
+        using difference_type = std::ptrdiff_t;
+        using pointer = char const *;
+        using reference = char const;
+        using iterator_category = std::random_access_iterator_tag;
+
+        constexpr repeated_chars_iterator() noexcept :
+            first_(nullptr),
+            size_(0),
+            n_(0)
+        {}
+        constexpr repeated_chars_iterator(
+            char const * first,
+            difference_type size,
+            difference_type n) noexcept :
+            first_(first),
+            size_(size),
+            n_(n)
+        {}
+
+        constexpr reference operator*() const noexcept
+        {
+            return first_[n_ % size_];
+        }
+
+        constexpr value_type operator[](difference_type n) const noexcept
+        {
+            return first_[(n_ + n) % size_];
+        }
+
+        constexpr repeated_chars_iterator & operator++() noexcept
+        {
+            ++n_;
+            return *this;
+        }
+        constexpr repeated_chars_iterator operator++(int)noexcept
+        {
+            repeated_chars_iterator retval = *this;
+            ++*this;
+            return retval;
+        }
+        constexpr repeated_chars_iterator & operator+=(difference_type n) noexcept
+        {
+            n_ += n;
+            return *this;
+        }
+
+        constexpr repeated_chars_iterator & operator--() noexcept
+        {
+            --n_;
+            return *this;
+        }
+        constexpr repeated_chars_iterator operator--(int)noexcept
+        {
+            repeated_chars_iterator retval = *this;
+            --*this;
+            return retval;
+        }
+        constexpr repeated_chars_iterator & operator-=(difference_type n) noexcept
+        {
+            n_ -= n;
+            return *this;
+        }
+
+        friend constexpr bool operator==(
+            repeated_chars_iterator lhs, repeated_chars_iterator rhs) noexcept
+        {
+            return lhs.first_ == rhs.first_ && lhs.n_ == rhs.n_;
+        }
+        friend constexpr bool operator!=(
+            repeated_chars_iterator lhs, repeated_chars_iterator rhs) noexcept
+        {
+            return !(lhs == rhs);
+        }
+        friend constexpr bool operator<(
+            repeated_chars_iterator lhs, repeated_chars_iterator rhs) noexcept
+        {
+            return lhs.first_ == rhs.first_ && lhs.n_ < rhs.n_;
+        }
+        friend constexpr bool operator<=(
+            repeated_chars_iterator lhs, repeated_chars_iterator rhs) noexcept
+        {
+            return lhs == rhs || lhs < rhs;
+        }
+        friend constexpr bool operator>(
+            repeated_chars_iterator lhs, repeated_chars_iterator rhs) noexcept
+        {
+            return rhs < lhs;
+        }
+        friend constexpr bool operator>=(
+            repeated_chars_iterator lhs, repeated_chars_iterator rhs) noexcept
+        {
+            return lhs <= rhs;
+        }
+
+        friend constexpr repeated_chars_iterator
+        operator+(repeated_chars_iterator lhs, difference_type rhs) noexcept
+        {
+            return lhs += rhs;
+        }
+        friend constexpr repeated_chars_iterator
+        operator+(difference_type lhs, repeated_chars_iterator rhs) noexcept
+        {
+            return rhs += lhs;
+        }
+        friend constexpr repeated_chars_iterator
+        operator-(repeated_chars_iterator lhs, difference_type rhs) noexcept
+        {
+            return lhs -= rhs;
+        }
+        friend constexpr repeated_chars_iterator
+        operator-(difference_type lhs, repeated_chars_iterator rhs) noexcept
+        {
+            return rhs -= lhs;
+        }
+        friend constexpr difference_type operator-(
+            repeated_chars_iterator lhs, repeated_chars_iterator rhs) noexcept
+        {
+            return lhs.n_ - rhs.n_;
+        }
+
+    private:
+        char const * first_;
+        difference_type size_;
+        difference_type n_;
+    };
+```
+
+into this:
+
+```c++
+struct repeated_chars_iterator : boost::iterator_facade::iterator_facade<
+                                     repeated_chars_iterator,
+                                     std::random_access_iterator_tag,
+                                     char,
+                                     char const>
+{
+    constexpr repeated_chars_iterator() noexcept :
+        first_(nullptr),
+        size_(0),
+        n_(0)
+    {}
+    constexpr repeated_chars_iterator(
+        char const * first, difference_type size, difference_type n) noexcept :
+        first_(first),
+        size_(size),
+        n_(n)
+    {}
+
+private:
+    friend boost::iterator_facade::access;
+    constexpr char const dereference() const noexcept
+    {
+        return first_[n_ % size_];
+    }
+    constexpr void advance(std::ptrdiff_t i) noexcept { n_ += i; }
+    constexpr auto compare(repeated_chars_iterator other) const noexcept
+    {
+        return n_ - other.n_;
+    }
+
+    char const * first_;
+    difference_type size_;
+    difference_type n_;
+};
+```
+
+if you don't ever write iterators, this is not for you.