Program Listing for File set_to_map_transformer.h

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

// map_transformer: wraps a set-like construct so that its iterator returns
// a value_type where of the underlying set element as the key, with the value
// that same key transformed via a function.
template <typename BaseSet, typename Transform>
class set_to_map_transformer {
public:
    using key_type = typename BaseSet::value_type;
    using transformed_mapped_type = std::invoke_result_t<Transform, key_type>;
    using value_type = std::pair<key_type, transformed_mapped_type>;
    using mapped_type = transformed_mapped_type;

private:
    BaseSet* base_map_;
    Transform transform_;

public:
    set_to_map_transformer(BaseSet* base_set, Transform transform)
        : base_map_(base_set), transform_(transform) {}

    class iterator {
    public:
        using base_iterator = typename BaseSet::iterator;
        using iterator_category = std::forward_iterator_tag;
        using difference_type = typename std::iterator_traits<base_iterator>::difference_type;
        using key_type = typename BaseSet::key_type;
        using transformed_mapped_type = std::invoke_result_t<Transform, key_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 the set value and the mapped value
        // is the result of applying the transform to the underlying  value.
        value_type operator*() const {
            auto base_val = *base_it_;
            return { base_val, (*transform_ptr_)(base_val) };
        }

        // 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_);
    }

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