Struct forward list items disapearing? - c++

Question

This piece of code is getting quite on my nerves. Been debugging it for a while, cant believe how rusty i've got on c++.
I'm trying to model a graph to run some simple algorithms on, but that doesn't seem to work out so well. Every vertex contains a forward list to his neighbors, however when inserting the elements their obviously present.. Until I reach the print function; at that time the forward list is empty.
I've tried to allocate the forward_list using new aswell, as scoping could be an explication for it.. No luck there either..
#include <iostream>
#include <vector>
#include <set>
#include <forward_list>
#include <fstream>
using namespace std;
typedef struct Vertex Vertex;
struct Vertex {
unsigned id;
forward_list<Vertex*>_next;
bool operator < (const Vertex &other) const { return id < other.id; };
};
typedef set<Vertex> Graph;
typedef vector<Vertex*> Index;
typedef pair<unsigned, unsigned> Edge;
typedef forward_list<Vertex*> Neighbors;
// Function: process_line()
// Purpose: process a specific line from the file.
// Params: line to process
Edge process_line(string line){
unsigned vertex_from;
unsigned vertex_to;
int idx = line.find(" ");
vertex_from = (unsigned)stoul(line.substr(0, idx));
vertex_to = (unsigned)stoul(line.substr(idx+1, line.length()));
return make_pair(vertex_from, vertex_to);
}
// Function: load_graph()
// Purpose: load graph from file in relation
// Params: path, and reference to graph and index
bool load_graph(string file_path, Graph &graph, Index &index){
string line;
ifstream file(file_path);
bool foundEmptyLine = false;
if(file.is_open()){
while(getline(file, line)){
if(line.empty()){
foundEmptyLine = true;
continue;
}
if(!foundEmptyLine){
// processing vertexes
Vertex *vertex = new Vertex;
vertex->id = stoul(line);
graph.insert(*vertex);
index.emplace_back(vertex);
}else{
//Processing relations
Edge edge = process_line(line);
Vertex* neighbor = index.at(edge.second);
Vertex* source = index.at(edge.first);
// Lookup edge in index
source->_next.emplace_front(neighbor);
// ITEMS PRESENT! <----------------------
}
}
file.close();
}else{
cout << "Unable to open " << file_path;
return false;
}
return true;
}
void print_graph(Graph &graph){
for(Graph::iterator it = graph.begin(); it != graph.end(); ++it){
Neighbors neighs = it->_next;
cout << "Node: " << it->id << " neighbors: " neighs.empty();
cout << endl;
}
}
// Entry point.
int main() {
Graph graph;
Index index;
load_graph("graph_1.txt", graph, index);
print_graph(graph);
}

Answer 1

This is once again the same problem as yesterday.
Let's try to recapitulate the std::set
Since C++11 the iterator of a std::set is always an iterator to const value_type. This is because when we change an entry of a std::set this entry would need to be placed somewhere else in the data structure.
When we insert something into a std::set, two signatures are provided:
pair<iterator,bool> insert (const value_type& val);
pair<iterator,bool> insert (value_type&& val);
But in any case the insertion copies or moves the element into the container.
So in your case when you do
Vertex *vertex = new Vertex;
vertex->id = stoul(line);
graph.insert(*vertex);
index.emplace_back(vertex);
First you allocate memory (which by the way you never delete! You will leak much memory, which you can check using valgrind). Then you insert a copy of your vertex into the std::set and insert the pointer of your allocated memory into the std::vector.
When you then later do
Vertex* neighbor = index.at(edge.second);
Vertex* source = index.at(edge.first);
// Lookup edge in index
source->_next.emplace_front(neighbor);
You take the Vertex from your vector (remember, this is the vertex which you allocated with new). And insert another vertex (also dynamically allocated) into its std::forward_list. But: They have nothing to do with the vertex which are in your std::set.
So when you then later go through your std::set:
for (Graph::iterator it = graph.begin(); it != graph.end(); ++it)
This is completely unrelated to what you did when when inserting the edges - and all std::forward_lists are empty.
Side notes:
This is something you had to use in C, but not in C++!
typedef struct Vertex Vertex;
This one you should place above:
typedef forward_list<Vertex*> Neighbors;
It doesn't make sense to declare the type of Neighbors after you declared _next, because _next has this type.
Use const whereever you can, and cbegin / cend whereever you can (I already told you this yesterday), e.g.:
for(Graph::iterator it = graph.cbegin(); it != graph.cend(); ++it){
It doesn't make a difference here, but if you change the type of graph at some point, begin() may return a iterator to value_type instead of const value_type

Answer 2

Modified the graph to keep references to the existing vertices. I'm still not sure why this fixed it---but felt like giving a heads-up.