C++ "Access violation reading location" Error

I have the following Vertex struct in a Graph class:

struct Vertex
{
    string country;
    string city;
    double lon;
    double lat;
    vector<edge> *adj;

    Vertex(string country, string city, double lon, double lat)
    {
        this->country = country;
        this->city = city;
        this->lon = lon;
        this->lat = lat;
        this->adj = new vector<edge>();
    }
};

When calling the method I've written called getCost(), I keep getting the same Unhandled Exception

Access violation reading location 0x00000048

and I cannot figure out why.

the getCost() method:

void Graph::getCost(string from, string to)
{

    Vertex *f = (findvertex(from));
    vector<edge> *v = f->adj;     // Here is where it gives the error
    vector<edge>::iterator itr = v->begin();

    for (; itr != v->end(); itr++)
    {
        if (((*itr).dest)->city == to)
            cout << "\nCost:-" << (*itr).cost;
    }
}

The method findvertex() returns a value of type Vertex*. Why do I keep receiving this error?

the findVertex method:

Vertex* Graph::findvertex(string s)
{
    vmap::iterator itr = map1.begin();
    while (itr != map1.end())
    {
        if (itr->first == s){

            return itr->second;
        }
        itr++;
    }
    return NULL;
}

Where map1 is defined:

typedef map< string, Vertex *, less<string> > vmap;
vmap map1;

You haven't posted the findvertex method, but Access Reading Violation with an offset like 0x00000048 means that the Vertex* f; in your getCost function is receiving null, and when trying to access the member adj in the null Vertex pointer (that is, in f), it is offsetting to adj (in this case, 72 bytes ( 0x48 bytes in decimal )), it's reading near the 0 or null memory address.

Doing a read like this violates Operating-System protected memory, and more importantly means whatever you're pointing at isn't a valid pointer. Make sure findvertex isn't returning null, or do a comparisong for null on f before using it to keep yourself sane (or use an assert):

assert( f != null ); // A good sanity check

EDIT:

If you have a map for doing something like a find, you can just use the map's find method to make sure the vertex exists:

Vertex* Graph::findvertex(string s)
{
    vmap::iterator itr = map1.find( s );
    if ( itr == map1.end() )
    {
        return NULL;
    }
    return itr->second;
}

Just make sure you're still careful to handle the error case where it does return NULL. Otherwise, you'll keep getting this access violation.