wpiutil UidVector: Implement clear() and forward iterator (#1293)

The forward iterator only iterates over "live" elements.

Also add a couple of unit tests for UidVector.

wpiutil/src/main/native/include/wpi/UidVector.h

@@ -8,11 +8,59 @@
 #ifndef WPIUTIL_WPI_UIDVECTOR_H_
 #define WPIUTIL_WPI_UIDVECTOR_H_
 
+#include <iterator>
 #include <utility>
 #include <vector>
 
 namespace wpi {
 
+namespace impl {
+template <typename It>
+class UidVectorIterator {
+ public:
+  using iterator_type = std::forward_iterator_tag;
+  using value_type = typename It::value_type;
+  using difference_type = typename It::difference_type;
+  using reference = typename It::reference;
+  using pointer = typename It::pointer;
+
+  UidVectorIterator() = default;
+  explicit UidVectorIterator(It it, It end) : m_it(it), m_end(end) {
+    // advance to first non-empty element
+    while (m_it != m_end && !*m_it) ++m_it;
+  }
+
+  reference operator*() const noexcept { return *m_it; }
+  pointer operator->() const noexcept { return m_it.operator->(); }
+
+  UidVectorIterator& operator++() noexcept {
+    // advance past empty elements
+    do {
+      ++m_it;
+    } while (m_it != m_end && !*m_it);
+    return *this;
+  }
+
+  UidVectorIterator operator++(int)noexcept {
+    UidVectorIterator it = *this;
+    ++it;
+    return it;
+  }
+
+  bool operator==(const UidVectorIterator& oth) const noexcept {
+    return m_it == oth.m_it;
+  }
+
+  bool operator!=(const UidVectorIterator& oth) const noexcept {
+    return m_it != oth.m_it;
+  }
+
+ private:
+  It m_it;
+  It m_end;
+};
+}  // namespace impl
+
 // Vector which provides an integrated freelist for removal and reuse of
 // individual elements.
 // @tparam T element type; must be default-constructible and evaluate in
@@ -22,7 +70,16 @@ namespace wpi {
 template <typename T, typename std::vector<T>::size_type reuse_threshold>
 class UidVector {
  public:
-  typedef typename std::vector<T>::size_type size_type;
+  using value_type = T;
+  using pointer = T*;
+  using const_pointer = const T*;
+  using reference = T&;
+  using const_reference = const T&;
+  using size_type = typename std::vector<T>::size_type;
+  using difference_type = typename std::vector<T>::difference_type;
+  using iterator = impl::UidVectorIterator<typename std::vector<T>::iterator>;
+  using const_iterator =
+      impl::UidVectorIterator<typename std::vector<T>::const_iterator>;
 
   bool empty() const { return m_active_count == 0; }
   size_type size() const { return m_vector.size(); }
@@ -56,6 +113,31 @@ class UidVector {
     --m_active_count;
   }
 
+  // Removes all elements.
+  void clear() {
+    m_vector.clear();
+    m_free.clear();
+    m_active_count = 0;
+  }
+
+  // Iterator access
+  iterator begin() noexcept {
+    return iterator(m_vector.begin(), m_vector.end());
+  }
+  const_iterator begin() const noexcept {
+    return const_iterator(m_vector.begin(), m_vector.end());
+  }
+  const_iterator cbegin() const noexcept {
+    return const_iterator(m_vector.begin(), m_vector.end());
+  }
+  iterator end() noexcept { return iterator(m_vector.end(), m_vector.end()); }
+  const_iterator end() const noexcept {
+    return const_iterator(m_vector.end(), m_vector.end());
+  }
+  const_iterator cend() const noexcept {
+    return const_iterator(m_vector.end(), m_vector.end());
+  }
+
  private:
   std::vector<T> m_vector;
   std::vector<size_type> m_free;