bfsTree empty with unused value warning - c++

Question

I am trying to implement bfsTree. It should take a graph as input and give the bfs tree as output. However the line that adds the edge to the bfs tree seems to be not working. The bfstree is empty. I think it is related to this unused value warning. Why it gives the warning? And how to fix it? Thanks.
Here is the warning:
vertex.cpp:35:10: warning: expression result unused [-Wunused-value]
for (neighbor; neighbor != this->_neighbors.end(); neighbor++) {
Here is the bfs tree code snippet:
Graph Bicc::breadthFirstSearch(Graph& sparseGraph) {
Graph bfsTree;
auto lev = 0;
vector<Vertex> verticies = sparseGraph.getVerticies();
for(int i = 0; i < verticies.size(); i++){
verticies[i].color = "white";
verticies[i].level = lev;
}
Vertex start = verticies[0];
list<Vertex> VertexQueue;
VertexQueue.push_back(start);
while (!VertexQueue.empty())
{
Vertex current = VertexQueue.front();
current.color = "gray"; //current vertex is being processed
auto neighbors = current.getNeighbors();
//process all neighbors of current vertex
for(auto n = neighbors.begin(); n != neighbors.end(); n++) {
if (n->color == "white") { // This is an unvisited vertex
n->level = lev + 1; // Set level
n->parent = &current; // Set parent
n->color = "gray"; // Set color visited
bfsTree.add(current, *n); //add the edge to bfsTree
VertexQueue.push_back(*n); // Add it to the queue
}
}
VertexQueue.pop_front(); // Pop out the processed vertex
++lev; // The next level
}
return bfsTree;
}
Here is the vertex.cpp code snippet that was called by the warning:
bool Vertex::hasNeighbor(const Vertex& vertex) const {
auto neighbor = this->_neighbors.begin();
for (neighbor; neighbor != this->_neighbors.end(); neighbor++) {
if (*neighbor == vertex) {
break;
}
}
return neighbor != this->_neighbors.end();
}