folly: iterator classes (3/3): IOBuf's custom iterator should use new Iterator detail classes

Summary: IOBuf's custom iterator was changed so that it would no longer need `<boost/iterator_facade.hpp>`.  Change again so that it uses Folly's own (cheap) header file providing roughly the same functionality.

Reviewed By: yfeldblum

Differential Revision: D8345072

fbshipit-source-id: bbbfb4373e72444a5e54aeccafd2d316ebdbae63

folly/io/IOBuf.h

@@ -31,6 +31,7 @@
 #include <folly/FBVector.h>
 #include <folly/Portability.h>
 #include <folly/Range.h>
+#include <folly/detail/Iterators.h>
 #include <folly/lang/Ordering.h>
 #include <folly/portability/SysUio.h>
 
@@ -1552,13 +1553,9 @@ inline std::unique_ptr<IOBuf> IOBuf::maybeCopyBuffer(
   return copyBuffer(buf.data(), buf.size(), headroom, minTailroom);
 }
 
-class IOBuf::Iterator {
+class IOBuf::Iterator
+    : public detail::IteratorFacade<IOBuf::Iterator, ByteRange const> {
  public:
-  using difference_type = ssize_t;
-  using value_type = ByteRange;
-  using reference = ByteRange const&;
-  using pointer = ByteRange const*;
-  using iterator_category = std::forward_iterator_tag;
 
   // Note that IOBufs are stored as a circular list without a guard node,
   // so pos == end is ambiguous (it may mean "begin" or "end").  To solve
@@ -1585,31 +1582,20 @@ class IOBuf::Iterator {
     return *this;
   }
 
-  Iterator& operator++() {
-    increment();
-    return *this;
-  }
-
-  Iterator operator++(int) {
-    Iterator ret(*this);
-    ++*this;
-    return ret;
-  }
-
-  ByteRange const& operator*() const {
-    return dereference();
-  }
-
-  ByteRange const* operator->() const {
-    return &dereference();
+  const ByteRange& dereference() const {
+    return val_;
   }
 
-  bool operator==(Iterator const& rhs) const {
-    return equal(rhs);
+  bool equal(const Iterator& other) const {
+    // We must compare end_ in addition to pos_, because forward traversal
+    // requires that if two iterators are equal (a == b) and dereferenceable,
+    // then ++a == ++b.
+    return pos_ == other.pos_ && end_ == other.end_;
   }
 
-  bool operator!=(Iterator const& rhs) const {
-    return !equal(rhs);
+  void increment() {
+    pos_ = pos_->next();
+    adjustForEnd();
   }
 
  private:
@@ -1626,22 +1612,6 @@ class IOBuf::Iterator {
     }
   }
 
-  const ByteRange& dereference() const {
-    return val_;
-  }
-
-  bool equal(const Iterator& other) const {
-    // We must compare end_ in addition to pos_, because forward traversal
-    // requires that if two iterators are equal (a == b) and dereferenceable,
-    // then ++a == ++b.
-    return pos_ == other.pos_ && end_ == other.end_;
-  }
-
-  void increment() {
-    pos_ = pos_->next();
-    adjustForEnd();
-  }
-
   const IOBuf* pos_{nullptr};
   const IOBuf* end_{nullptr};
   ByteRange val_;