C++ - How to use a vector of reference_wrapper - c++

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;
}

Answer 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;
}

Answer 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*.