C++ Debugging Help- Graphs

I need help figuring out what is wrong with my code. I keep getting an error that says:

error: comparison between signed and unsigned integer expressions [-Werror=sign-compare] 

I get this everytime I use the .size() operator. I'm not sure why it is throwing me this error. Here is my code:

#include

#include

#include

#include

#include

#include

#include "Graph.hpp"

using namespace std;

void Graph::addVertex(std::string name){

vertex temp;

temp.name = name;

temp.district = -1;

//temp.distance = 50000000;

temp.visited = false;

vertices.push_back(temp);

}

void Graph::addEdge(std::string v1, std::string v2, int distance){

vertex * tempv1 = findVertex(v1);

vertex * tempv2 = findVertex(v2);

adjVertex adj1;

adj1.distance = distance;

adj1.v = tempv2;

tempv1 -> adj.push_back(adj1);

}

void Graph::displayEdges()

{

for(int i = 0; i < vertices.size(); i++)

{

cout << vertices[i].district << ":" <";

for(int j = 0; j < vertices[i].adj.size(); j++)

{

cout << vertices[i].adj[j].v->name;

cout << " (" << vertices[i].adj[j].distance << " miles)";

if (j != vertices[i].adj.size()-1)

cout << "***";

}

cout << endl;

}

}

void Graph::assignDistricts(){

int count = 0;

std::queue bft;

for(int i = 0 ; i < vertices.size() ; i++){

if(vertices[i].district < 0){

count ++;

bft.push(& vertices[i]);

while(!bft.empty()){

vertex *n = bft.front();

n -> district = count;

bft.pop();

for(int x = 0 ; x < n->adj.size() ; x++){

if(n -> adj[x].v -> district < 0){

bft.push(n-> adj[x].v);

}

}

}

}

}

}

vertex* Graph::findVertex(std::string name){

for(int i = 0; i < vertices.size(); i++){

if(vertices[i].name == name)

return &vertices[i];

}

return NULL;

}

void Graph::BFTraversalLabel(std::string startingCity, int distID){

queue temp;

for(int i = 0; i < vertices.size(); i++){

vertices[i].visited = false;

}

for(int i = 0; i < vertices.size(); i++){

if (vertices[i].name == startingCity){

vertices[i].visited = true;

temp.push(&vertices[i]);

vertices[i].district = distID;

break;

}

}

while (temp.empty() != true){

vertex * v1 = temp.front();

temp.pop();

for(int i = 0; i < v1 ->adj.size(); i++){

if (v1 -> adj[i].v -> visited == false){

v1 -> adj[i].v -> visited = true;

v1 -> adj[i].v -> district = distID;

temp.push(v1 -> adj[i].v);

}

}

}

}

--------------------------

Here is the .hpp file

#ifndef GRAPH_HPP

#define GRAPH_HPP

#include

#include

struct vertex;

/*This is the struct for the adjacent vertices for each

vertex in the graph. */

struct adjVertex{

vertex *v;

int distance;

};

/*this is the struct for each vertex in the graph. */

struct vertex{

int ID;

std::string name;

int district;

bool visited;

std::vector adj; //stores edges to adjacent vertices

};

class Graph{

public:

Graph();

~Graph();

void addEdge(std::string v1, std::string v2, int distance);

void addVertex(std::string name);

void displayEdges();

void assignDistricts();

private:

std::vector vertices; //stores vertices

vertex * findVertex(std::string name);

void BFTraversalLabel(std::string startingCity, int distID);

};

#endif // GRAPH_HPP

Will give thumbs up for correct corrections with good explanation. Thank you