This is again a newbie question. I want to create a graph based on some input file. I parse this file and in the code I want to check whether the link already exists int current=strToInt(tokens[0]); int numedges=strToInt(tokens[1]); for(int i=0;i<numedges;i++){ Vertex first=vertex(current,*g); int other=strToInt(tokens[3+i]); Vertex second=vertex(other,*g); pair<Edge, bool> result=boost::edge(first,second,*g); if(!result.second){ boost::add_edge(first,second,*g); } } tokens[0] is the id of the node, tokens[1] contains the number of edges this node is pointing to. From tokens[3] until the end, one gets the nodes to which the current node is connected (stored in other) As you can see I call the function boost::edge() to check whether there is already a link between first and second. As soon as I do this I get an error in Xcode. Does anyone know what I'm doing wrong here. It probably something very simple but I don't see it. Thanks in advance Tom