[ADT] Add a generic iterator utility for adapting iterators much like

Boost's iterator_adaptor, and a specific adaptor which iterates over
pointees when wrapped around an iterator over pointers.

This is the result of a long discussion on IRC with Duncan Smith, Dave
Blaikie, Richard Smith, and myself. Essentially, I could use some subset
of the iterator facade facilities often used from Boost, and everyone
seemed interested in having the functionality in a reasonably generic
form. I've tried to strike a balance between the pragmatism and the
established Boost design. The primary differences are:

1) Delegating to the standard iterator interface names rather than
   special names that then make up a second iterator-like API.
2) Using the name 'pointee_iterator' which seems more clear than
   'indirect_iterator'. The whole business of calling the '*p' operation
   'pointer indirection' in the standard is ... quite confusing. And
   'dereference' is no better of a term for moving from a pointer to
   a reference.

Hoping Duncan, and others continue to provide comments on this until
we've got a nice, minimal abstraction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207069 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/iterator.h

@@ -0,0 +1,145 @@
+//===- iterator.h - Utilities for using and defining iterators --*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_ITERATOR_H
+#define LLVM_ADT_ITERATOR_H
+
+#include <iterator>
+
+namespace llvm {
+
+/// \brief CRTP base class for adapting an iterator to a different type.
+///
+/// This class can be used through CRTP to adapt one iterator into another.
+/// Typically this is done through providing in the derived class a custom \c
+/// operator* implementation. Other methods can be overridden as well.
+///
+/// FIXME: Factor out the iterator-facade-like aspects into a base class that
+/// can be used for defining completely custom iterators.
+template <typename DerivedT, typename WrappedIteratorT, typename T,
+          typename PointerT = T *, typename ReferenceT = T &,
+          // Don't provide these, they are mostly to act as aliases below.
+          typename WrappedTraitsT = std::iterator_traits<WrappedIteratorT>>
+class iterator_adaptor_base
+    : public std::iterator<typename WrappedTraitsT::iterator_category, T,
+                           typename WrappedTraitsT::difference_type, PointerT,
+                           ReferenceT> {
+protected:
+  WrappedIteratorT I;
+
+  iterator_adaptor_base() {}
+
+  template <
+      typename U,
+      typename = typename std::enable_if<
+          !std::is_same<typename std::remove_cv<
+                            typename std::remove_reference<U>::type>::type,
+                        DerivedT>::value>::type>
+  explicit iterator_adaptor_base(U &&u)
+      : I(std::forward<U &&>(u)) {}
+
+public:
+  typedef typename iterator_adaptor_base::iterator::difference_type
+  difference_type;
+
+  DerivedT &operator+=(difference_type n) {
+    I += n;
+    return *static_cast<DerivedT *>(this);
+  }
+  DerivedT &operator-=(difference_type n) {
+    I -= n;
+    return *static_cast<DerivedT *>(this);
+  }
+  DerivedT operator+(difference_type n) const {
+    DerivedT tmp = *this;
+    tmp += n;
+    return tmp;
+  }
+  friend DerivedT operator+(difference_type n, const DerivedT &i) {
+    return i + n;
+  }
+  DerivedT operator-(difference_type n) const {
+    DerivedT tmp = *this;
+    tmp -= n;
+    return tmp;
+  }
+  difference_type operator-(const DerivedT &RHS) const { return I - RHS.I; }
+
+  DerivedT &operator++() {
+    ++I;
+    return *static_cast<DerivedT *>(this);
+  }
+  DerivedT &operator--() {
+    --I;
+    return *static_cast<DerivedT *>(this);
+  }
+  DerivedT operator++(int) {
+    DerivedT tmp = *static_cast<DerivedT *>(this);
+    ++*this;
+    return tmp;
+  }
+  DerivedT operator--(int) {
+    DerivedT tmp = *static_cast<DerivedT *>(this);
+    --*this;
+    return tmp;
+  }
+
+  bool operator==(const DerivedT &RHS) const { return I == RHS.I; }
+  bool operator!=(const DerivedT &RHS) const {
+    return !static_cast<const DerivedT *>(this)->operator==(RHS);
+  }
+
+  bool operator<(const DerivedT &RHS) const { return I < RHS.I; }
+  bool operator>(const DerivedT &RHS) const {
+    return !static_cast<const DerivedT *>(this)->operator<(RHS) &&
+           !static_cast<const DerivedT *>(this)->operator==(RHS);
+  }
+  bool operator<=(const DerivedT &RHS) const {
+    return !static_cast<const DerivedT *>(this)->operator>(RHS);
+  }
+  bool operator>=(const DerivedT &RHS) const {
+    return !static_cast<const DerivedT *>(this)->operator<(RHS);
+  }
+
+  ReferenceT operator*() const { return *I; }
+  PointerT operator->() const {
+    return static_cast<const DerivedT *>(this)->operator*();
+  }
+  ReferenceT operator[](difference_type n) const {
+    return *static_cast<const DerivedT *>(this)->operator+(n);
+  }
+};
+
+/// \brief An iterator type that allows iterating over the pointees via some
+/// other iterator.
+///
+/// The typical usage of this is to expose a type that iterates over Ts, but
+/// which is implemented with some iterator over T*s:
+///
+/// \code
+///   typedef pointee_iterator<SmallVectorImpl<T *>::iterator> iterator;
+/// \endcode
+template <
+    typename WrappedIteratorT,
+    typename T = typename std::remove_pointer<
+        typename std::iterator_traits<WrappedIteratorT>::value_type>::type>
+struct pointee_iterator
+    : iterator_adaptor_base<pointee_iterator<WrappedIteratorT>,
+                            WrappedIteratorT, T> {
+  pointee_iterator() {}
+  template <typename U>
+  pointee_iterator(U &&u)
+      : pointee_iterator::iterator_adaptor_base(std::forward<U &&>(u)) {}
+
+  T &operator*() const { return **this->I; }
+};
+
+}
+
+#endif

unittests/Support/CMakeLists.txt

@@ -15,12 +15,13 @@ add_llvm_unittest(SupportTests
   EndianTest.cpp
   ErrorOrTest.cpp
   FileOutputBufferTest.cpp
+  IteratorTest.cpp
   LEB128Test.cpp
   LineIteratorTest.cpp
   LockFileManagerTest.cpp
+  MD5Test.cpp
   ManagedStatic.cpp
   MathExtrasTest.cpp
-  MD5Test.cpp
   MemoryBufferTest.cpp
   MemoryTest.cpp
   Path.cpp

unittests/Support/IteratorTest.cpp

@@ -0,0 +1,59 @@
+//===- IteratorTest.cpp - Unit tests for iterator utilities ---------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/iterator.h"
+#include "llvm/ADT/SmallVector.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+namespace {
+
+TEST(PointeeIteratorTest, Basic) {
+  int arr[4] = { 1, 2, 3, 4 };
+  SmallVector<int *, 4> V;
+  V.push_back(&arr[0]);
+  V.push_back(&arr[1]);
+  V.push_back(&arr[2]);
+  V.push_back(&arr[3]);
+
+  typedef pointee_iterator<SmallVectorImpl<int *>::const_iterator> test_iterator;
+
+  test_iterator Begin, End;
+  Begin = V.begin();
+  End = test_iterator(V.end());
+
+  test_iterator I = Begin;
+  for (int i = 0; i < 4; ++i) {
+    EXPECT_EQ(*V[i], *I);
+
+    EXPECT_EQ(I, Begin + i);
+    EXPECT_EQ(I, std::next(Begin, i));
+    test_iterator J = Begin;
+    J += i;
+    EXPECT_EQ(I, J);
+    EXPECT_EQ(*V[i], Begin[i]);
+
+    EXPECT_NE(I, End);
+    EXPECT_GT(End, I);
+    EXPECT_LT(I, End);
+    EXPECT_GE(I, Begin);
+    EXPECT_LE(Begin, I);
+
+    EXPECT_EQ(i, I - Begin);
+    EXPECT_EQ(i, std::distance(Begin, I));
+    EXPECT_EQ(Begin, I - i);
+
+    test_iterator K = I++;
+    EXPECT_EQ(K, std::prev(I));
+  }
+  EXPECT_EQ(End, I);
+}
+
+} // anonymous namespace