Program Listing for File map_transformer.h

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#include <iterator>
#include <type_traits>
#include <utility>
#include <functional>

// map_transformer: wraps a map-like construct so that its iterator returns
// a value_type where the mapped element is transformed via a function.
template <typename BaseMap, typename Transform, typename TransformBack = void>
class map_transformer {
public:
    using key_type = typename BaseMap::key_type;
    using base_mapped_type = typename BaseMap::mapped_type;
    using transformed_mapped_type = std::invoke_result_t<Transform, base_mapped_type>;
    using value_type = std::pair<key_type, transformed_mapped_type>;
    using mapped_type = transformed_mapped_type;

    static constexpr bool read_only = std::is_void<TransformBack>::value;
    struct back_storage_fallback {};
    using back_storage = typename std::conditional<read_only, back_storage_fallback, TransformBack>::type;

  private:
    BaseMap* base_map_;
    Transform transform_;

    back_storage transform_back_;

public:
    // read-only constructor with TransformBack=void
    template <typename TB = TransformBack,
              typename std::enable_if<std::is_void<TB>::value, int>::type = 0>
    map_transformer(BaseMap* base_map, Transform transform)
        : base_map_(base_map), transform_(std::move(transform)), transform_back_{} {}

    // read-write constructor with TransformBack provided
    template <typename TB = TransformBack,
              typename std::enable_if<!std::is_void<TB>::value, int>::type = 0>
    map_transformer(BaseMap* base_map, Transform transform, TB transform_back)
        : base_map_(base_map), transform_(std::move(transform)), transform_back_(std::move(transform_back)) {}

    class iterator {
    public:
        using base_iterator = typename BaseMap::iterator;
        using iterator_category = std::forward_iterator_tag;
        using difference_type = typename std::iterator_traits<base_iterator>::difference_type;
        using key_type = typename BaseMap::key_type;
        using transformed_mapped_type = std::invoke_result_t<Transform, typename BaseMap::mapped_type>;
        using value_type = std::pair<key_type, transformed_mapped_type>;
    private:
        base_iterator base_it_;
        Transform* transform_ptr_;

        mutable value_type cached_value_;


    public:
        iterator() : base_it_(), transform_ptr_(nullptr) {}
        iterator(base_iterator base_iterator, Transform* transform)
            : base_it_(base_iterator), transform_ptr_(transform) {}

        // On dereference, return a pair where the key is unchanged and the mapped value
        // is the result of applying the transform to the underlying mapped value.
        // @todo should these also all be const references so that key/value can be non-copyable (i.e unique_ptr)
        value_type operator*() const {
            auto base_val = *base_it_;
            return { base_val.first, (*transform_ptr_)(base_val.second) };
        }

        // operator-> uses a mutable cache to return a pointer to the current value.
        value_type* operator->() const {
            cached_value_ = **this;
            return &cached_value_;
        }

        iterator& operator++() {
            ++base_it_;
            return *this;
        }

        iterator operator++(int) {
            iterator tmp(*this);
            ++(*this);
            return tmp;
        }

        bool operator==(const iterator& other) const {
            return base_it_ == other.base_it_;
        }

        bool operator!=(const iterator& other) const {
            return !(*this == other);
        }
    };

    iterator begin() {
        return iterator(base_map_->begin(), &transform_);
    }

    iterator end() {
        return iterator(base_map_->end(), &transform_);
    }

    iterator find(const key_type& key) {
        return iterator(base_map_->find(key), &transform_);
    }

    // @todo still not sure if this is a good idea, do we want to insert into the transformed map?
    std::pair<iterator, bool> insert(const value_type& value) {
        static_assert(!read_only, "insert() requires TransformBack");
        auto result = base_map_->insert({ value.first, transform_back_(value.second) });
        return { iterator(result.first, &transform_), result.second };
    }

    size_t erase(const key_type& key) {
        return base_map_->erase(key);
    }
};