Replace boost::iterator_adaptor with explicitly specified iterator definition for Usd_PrimData{Sibling,Subtree}Iterator

pxr/usd/usd/primData.h

@@ -41,7 +41,6 @@
 #include "pxr/usd/sdf/path.h"
 
 #include <boost/range/iterator_range.hpp>
-#include <boost/iterator/iterator_adaptor.hpp>
 #include <boost/intrusive_ptr.hpp>
 
 #include <atomic>
@@ -353,31 +352,53 @@ class Usd_PrimData
 };
 
 // Sibling iterator class.
-class Usd_PrimDataSiblingIterator : public boost::iterator_adaptor<
-    Usd_PrimDataSiblingIterator,                  // crtp.
-    Usd_PrimData *,                               // base iterator.
-    Usd_PrimData *,                               // value.
-    boost::forward_traversal_tag,                 // traversal.
-    Usd_PrimData *                                // reference.
-    >
-{
+class Usd_PrimDataSiblingIterator {
+    using _UnderylingIterator = Usd_PrimData*;
 public:
+    using iterator_category = std::forward_iterator_tag;
+    using value_type = Usd_PrimData*;
+    using reference = Usd_PrimData*;
+    using pointer = void;
+    using difference_type = std::ptrdiff_t;
+
     // Default ctor.
-    Usd_PrimDataSiblingIterator() {}
+    Usd_PrimDataSiblingIterator() = default;
+
+    reference operator*() const { return _underlyingIterator; }
+
+    // pre-increment
+    Usd_PrimDataSiblingIterator& operator++() {
+        increment();
+        return *this;
+    }
+
+    // post-increment
+    Usd_PrimDataSiblingIterator operator++(int) {
+        Usd_PrimDataSiblingIterator result = *this;
+        increment();
+        return result;
+    }
+
+    bool operator==(const Usd_PrimDataSiblingIterator& other) const {
+        return _underlyingIterator == other._underlyingIterator;
+    }
+
+    bool operator!=(const Usd_PrimDataSiblingIterator& other) const {
+        return _underlyingIterator != other._underlyingIterator;
+    }
 
 private:
     friend class Usd_PrimData;
 
     // Constructor used by Prim.
-    Usd_PrimDataSiblingIterator(const base_type &i)
-        : iterator_adaptor_(i) {}
+    Usd_PrimDataSiblingIterator(const _UnderylingIterator &i)
+        : _underlyingIterator(i) {}
 
-    // Core primitives implementation.
-    friend class boost::iterator_core_access;
-    reference dereference() const { return base(); }
     void increment() {
-        base_reference() = base_reference()->GetNextSibling();
+        _underlyingIterator = _underlyingIterator->GetNextSibling();
     }
+
+    _UnderylingIterator _underlyingIterator = nullptr;
 };
 
 // Sibling range.
@@ -412,33 +433,56 @@ Usd_PrimData::_GetChildrenRange() const
 
 
 // Tree iterator class.
-class Usd_PrimDataSubtreeIterator : public boost::iterator_adaptor<
-    Usd_PrimDataSubtreeIterator,                  // crtp.
-    Usd_PrimData *,                               // base iterator.
-    Usd_PrimData *,                               // value.
-    boost::forward_traversal_tag,                 // traversal.
-    Usd_PrimData *                                // reference.
-    >
-{
+class Usd_PrimDataSubtreeIterator {
+    using _UnderlyingIterator = Usd_PrimData*;
 public:
+    using iterator_category = std::forward_iterator_tag;
+    using value_type = Usd_PrimData*;
+    using reference = Usd_PrimData*;
+    using pointer = void;
+    using difference_type = std::ptrdiff_t;
+
     // Default ctor.
-    Usd_PrimDataSubtreeIterator() {}
+    Usd_PrimDataSubtreeIterator() = default;
+
+    reference operator*() const { return _underlyingIterator; }
+
+    // pre-increment
+    Usd_PrimDataSubtreeIterator& operator++() {
+        increment();
+        return *this;
+    }
+
+    // post-increment
+    Usd_PrimDataSubtreeIterator operator++(int) {
+        Usd_PrimDataSubtreeIterator result = *this;
+        increment();
+        return result;
+    }
+
+    bool operator==(const Usd_PrimDataSubtreeIterator& other) const {
+        return _underlyingIterator == other._underlyingIterator;
+    }
+
+    bool operator!=(const Usd_PrimDataSubtreeIterator& other) const {
+        return _underlyingIterator != other._underlyingIterator;
+    }
 
 private:
     friend class Usd_PrimData;
     friend class UsdPrimSubtreeIterator;
 
     // Constructor used by Prim.
-    Usd_PrimDataSubtreeIterator(const base_type &i)
-        : iterator_adaptor_(i) {}
+    Usd_PrimDataSubtreeIterator(const _UnderlyingIterator &i)
+        : _underlyingIterator(i) {}
 
-    // Core primitives implementation.
-    friend class boost::iterator_core_access;
-    reference dereference() const { return base(); }
     void increment() {
-        base_type &b = base_reference();
-        b = b->GetFirstChild() ? b->GetFirstChild() : b->GetNextPrim();
+        _underlyingIterator = _underlyingIterator->GetFirstChild() ?
+            _underlyingIterator->GetFirstChild() :
+            _underlyingIterator->GetNextPrim();
     }
+
+    _UnderlyingIterator _underlyingIterator = nullptr;
 };
 
 // Tree range.