[NTP Snippets] Refactor home-grown container API

components/ntp_snippets/inner_iterator.h

-// Copyright 2015 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#ifndef COMPONENTS_NTP_SNIPPETS_INNER_ITERATOR_H_
-#define COMPONENTS_NTP_SNIPPETS_INNER_ITERATOR_H_
-
-#include <iterator>
-
-namespace ntp_snippets {
-
-// Build an iterator returning a dereference of an original iterator. For
-// example  an object with an instance variable of type
-// std::vector<std::unique_ptr<SomeClass>> instance_;
-// could serve a const_iterator (of const SomeClass&) by simply doing the
-// following:
-//
-// using const_iterator = InnerIterator<
-//     std::vector<std::unique_ptr<SomeClass>>::const_iterator,
-//     const Someclass>;
-// [...]
-// const_iterator begin() { return const_iterator(instance_.begin()); }
-//
-template <typename I,  // type of original iterator
-          typename R   // type returned by this iterator
-          >
-class InnerIterator {
- public:
-  using difference_type = typename std::iterator_traits<I>::difference_type;
-  using value_type = R;
-  using pointer = R*;
-  using reference = R&;
-  using iterator_category = std::random_access_iterator_tag;
-
-  // Construction from an iterator.
-  explicit InnerIterator(I from) : it_(from) {}
-
-  // Operators *, ->, and [] are first forwarded to the contained
-  // iterator, then extract the inner member.
-  reference operator*() const { return **it_; }
-  pointer operator->() const { return &(**it_); }
-  reference operator[](difference_type n) const { return *(it_[n]); }
-
-  // All operators that have to do with position are forwarded
-  // to the contained iterator.
-  InnerIterator& operator++() {
-    ++it_;
-    return *this;
-  }
-  InnerIterator operator++(int) {
-    I tmp(it_);
-    it_++;
-    return InnerIterator(tmp);
-  }
-  InnerIterator& operator--() {
-    --it_;
-    return *this;
-  }
-  InnerIterator operator--(int) {
-    I tmp(it_);
-    it_--;
-    return InnerIterator(tmp);
-  }
-  InnerIterator& operator+=(difference_type n) {
-    it_ += n;
-    return *this;
-  }
-  InnerIterator operator+(difference_type n) const {
-    I tmp(it_);
-    tmp += n;
-    return InnerIterator(tmp);
-  }
-  InnerIterator& operator-=(difference_type n) {
-    it_ -= n;
-    return *this;
-  }
-  InnerIterator operator-(difference_type n) const {
-    I tmp(it_);
-    tmp -= n;
-    return InnerIterator(tmp);
-  }
-
-  bool operator==(const InnerIterator<I, R>& rhs) const {
-    return it_ == rhs.it_;
-  }
-  bool operator!=(const InnerIterator<I, R>& rhs) const {
-    return it_ != rhs.it_;
-  }
-
- protected:
-  I it_;
-};
-
-}  // namespace ntp_snippets
-
-#endif  // COMPONENTS_NTP_SNIPPETS_INNER_ITERATOR_H_