Using `std::equal_range` with `boost::transform_iterator`

Question

Say I have a struct of Items that I'm storing in an std::set and sorting like so:

struct Position
{
    int x;
    int y;
}

struct Item
{
    std::string id;
    Position position;

    // NOTE: only `position` should matter for equality
    operator==(const Item& other)
    {
        return position == position;
    }
};

inline bool operator<(const Item& lhs, const Item& rhs)
{
    if (lhs.position.x == rhs.position.x)
    {
        return lhs.position.y < rhs.position.y;
    }

    return lhs.position.x < rhs.position.x;
}

using ItemSet = std::set<Item>;

I want to use std::equal_range to search an ItemSet, except I want to search by Position. I know I could do something like:

ItemSet items;

Item tempItem;
tempItem.position = some_position;
auto result = std::equal_range(items.begin(), items.end(), tempItem);

But I would like to avoid the temporary Item.

I tried to use boost::transform_terator like so:

  auto tr = [](const Item& item) { return item.pos; };
  auto tr_begin = boost::make_transform_iterator(items.begin(), tr);
  auto tr_end = boost::make_transform_iterator(items.end(), tr);
  
  Position findme { 2, 1 };
  auto result = std::equal_range(tr_begin, tr_end, findme);

But this doesn't compile for reasons I don't understand, and also even if it did work, how would I get an iterator into the original collection from result? Or maybe there's a better way overall to do this?

Here is a test harness show the problem: http://cpp.sh/3hzsq

Any help would be appreciated!

Answer 1

You can use std::set::find with a different type to avoid constructing an Item. Note that your set can only contain one item with a specific position.

You can either make Position directly comparable with Item (Add Item{} < Position{} and Position{} < Item{}) or create a new proxy class:

struct ItemPosition {
    Position p;
};

inline bool operator<(const ItemPosition& l, const Item& r) {
    return l.position.x == r.position.x ? l.position.y < r.position.y : l.position.x < r.position.x;
};

inline bool operator<(const Item& l, const ItemPosition& r) {
    return l.position.x == r.position.x ? l.position.y < r.position.y : l.position.x < r.position.x;
};

// Change the comparator so it can compare with `ItemPosition` too
using ItemSet = std::set<Item, std::less<>>;

You can alternatively use a completely different comparator to make Position comparable with Item.

struct ItemComparator {
    bool operator()(const Position& l, const Position& r) const {
        return l.x == r.x ? l.y < r.y : l.x < r.x;
    }
    bool operator()(const Item& l, const Item& r) const {
        return operator()(l.position, r.position);
    }
    bool operator()(const Item& l, const Position& r) const {
        return operator()(l.position, r);
    }
    bool operator()(const Position& l, const Item& r) const {
        return operator()(l, r.position);
    }

    using is_transparent = void;
};

using ItemSet = std::set<Item, ItemComparator>;

And use it like so:

    Position findme { 2, 1 };
    // Or just `items.find(findme)` if using a custom comparator
    auto result = items.find(ItemPosition{ findme });
    if (result == items.end()) {
        // No item found
    } else {
        Item& item = *result;
        // found item
    }