C++ “Access violation reading location” Error

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.