C++ - How to use a vector of reference_wrapper

Question

I'm trying to refactor part of a pathfinding algorithm I had that used pointers to not use pointers. Unfortunately I'm not that knowledgable about references. I get the error: Invalid operands to binary expression ('std::__1::reference_wrapper<Tile>' and 'const std::__1::reference_wrapper<Tile>')

I also have no idea what that means. My code is below, and I can gather that it comes from the line: openList.erase(std::find(openList.begin(), openList.end(), current)); but I'm not sure how to fix this.

bool TileMap::tilesBetween(Tile& p_start, Tile& p_end)
{
    std::vector<std::reference_wrapper<Tile>> openList;
    std::vector<std::reference_wrapper<Tile>> closedList;

    openList.push_back(p_start);

    do
    {
        std::sort(openList.begin(), openList.end(), sortF());
        Tile& current = openList[0];
        closedList.push_back(current);
        openList.erase(std::find(openList.begin(), openList.end(), current));
        if(std::find(closedList.begin(), closedList.end(), p_end) != closedList.end())
        {
            return true;
        }

        std::vector<std::reference_wrapper<Tile>> adjacentTiles;
        if (current.m_coordinates.x > 0)
        {
            adjacentTiles.push_back(m_tiles[current.m_coordinates.y * m_width + (current.m_coordinates.x - 1)]);
        }
        if (current.m_coordinates.x < m_width)
        {
            adjacentTiles.push_back(m_tiles[current.m_coordinates.y * m_width + (current.m_coordinates.x + 1)]);
        }
        if (current.m_coordinates.y > 0)
        {
            adjacentTiles.push_back(m_tiles[(current.m_coordinates.y - 1) * m_width + current.m_coordinates.x]);
        }
        if (current.m_coordinates.y < m_height)
        {
            adjacentTiles.push_back(m_tiles[(current.m_coordinates.y + 1) * m_width + current.m_coordinates.x]);
        }

        for(auto t : adjacentTiles)
        {
            if(std::find(closedList.begin(), closedList.end(), t) != closedList.end())
            {
                continue;
            }

            if(std::find(openList.begin(), openList.end(), t) == closedList.end())
            {
                openList.push_back(t);
            }
        }
    }
    while(!openList.empty());

    return false;
}

EDIT: posted sortF

struct sortF
{
    bool operator()(const Tile* p_a, const Tile* p_b) const
    {
        return p_a->f < p_b->f;
    }
};

UPDATE: As per the suggestion, I've changed the function to use pointers instead of references. It SEEMS to be working, but I have more to implement before it's finished.

bool TileMap::tilesBetween(Tile* p_start, Tile* p_end)
{
    std::vector<Tile*> openList;
    std::vector<Tile*> closedList;

    std::cout << p_start << ", ";

    openList.push_back(p_start);

    do
    {
        std::sort(openList.begin(), openList.end(), sortF());
        Tile* current = openList[0];
        closedList.push_back(current);
        openList.erase(std::find(openList.begin(), openList.end(), current));
        if(std::find(closedList.begin(), closedList.end(), p_end) != closedList.end())
        {
            return true;
        }

        std::vector<Tile*> adjacentTiles;
        if (current->m_coordinates.x > 0)
        {
            adjacentTiles.push_back(&m_tiles[current->m_coordinates.y * m_width + (current->m_coordinates.x - 1)]);
        }
        if (current->m_coordinates.x < m_width)
        {
            std::cout << &m_tiles[current->m_coordinates.y * m_width + (current->m_coordinates.x + 1)] << std::endl;
            adjacentTiles.push_back(&m_tiles[current->m_coordinates.y * m_width + (current->m_coordinates.x + 1)]);
        }
        if (current->m_coordinates.y > 0)
        {
            adjacentTiles.push_back(&m_tiles[(current->m_coordinates.y - 1) * m_width + current->m_coordinates.x]);
        }
        if (current->m_coordinates.y < m_height)
        {
            adjacentTiles.push_back(&m_tiles[(current->m_coordinates.y + 1) * m_width + current->m_coordinates.x]);
        }

        for(auto t : adjacentTiles)
        {
            if(std::find(closedList.begin(), closedList.end(), t) != closedList.end())
            {
                continue;
            }

            if(std::find(openList.begin(), openList.end(), t) == openList.end())
            {
                openList.push_back(t);
            }
        }
    }
    while(!openList.empty());

    return false;
}

Solution 1

I can gather that it comes from the line: openList.erase(std::find(openList.begin(), openList.end(), current)); but I'm not sure how to fix this.

std::find iterates over std::reference_wrapper<Tile> and not Tile& itself. Hence

    Tile& current = openList[0];
    openList.erase(std::find(openList.begin(), openList.end(), current));

is incorrect. Change this to

    openList.erase(std::find_if(openList.begin(), openList.end(), [&](const std::reference_wrapper<Tile> &i)
    {
        return i.get() == current;
    }));

std::reference_wrapper::get returns the underlying reference.

A simple, working example to demonstrate this

#include <algorithm>
#include <list>
#include <vector>
#include <iostream>
#include <functional>

struct S
{
    int val;
    S(int i) : val(i) {}
};

int main()
{
    std::list<S> l = {-4, -3, -2, -1, 0, 1, 2, 3, 4};
    std::vector<std::reference_wrapper<S>> v(l.begin(), l.end());

    auto& x = l.front();
    v.erase(std::find_if(v.cbegin(), v.cend(), [&](const std::reference_wrapper<S> &i)
    {
        return i.get().val == x.val;
    }));
    std::cout << v[0].get().val;
}

Solution 2

Your problem is here:

std::sort(openList.begin(), openList.end(), sortF());

Your sortF is an invalid comparison object. The operator() must look like this:

bool operator()(const Tile& lhs, const Tile& rhs) const
//              ^^^ ref ^^^      ^^^ ref ^^^
{
    return lhs.f < rhs.f;
}

Instead of:

bool operator()(const Tile* p_a, const Tile* p_b) const
//              ^^^ ptr ^^^      ^^^ ptr ^^^

You have a vector of reference_wrapper<Tile>, not a vector of Tile*.

Comments

[Evan Ward]

Sorry, I was updating the whole function when I posted this so it didn't match. I did have it the way you suggested when it used references though.

[Praetorian]

That works, but manually unwrapping the reference_wrapper is not required because it has an implicit conversion operator to T&. So you can avoid the lambda and use find as long as S is equality comparable - coliru.stacked-crooked.com/a/4dcd9b9164a93645

[legends2k]

I tried without the manual unwrapping - lambda - in vain, but I'd implemented operator== as a member function, when I changed it to a external friend function it worked! Thanks to you I'd learned about the symmetric assumption made in std's generic algorithms.

[legends2k]

Symmetric Assumption - Replacement for broken link.