C++ compiler converting list to const list - c++

Question

This might be something really simple but I can't seem to work it out. Within my Vertex I have a std::list<Edge> but when I try to call methods on it like push_front I get an error saying the list is const and I can't push into it. I think for some reason the compiler is converting the std::list<Edge> to a const std::list<Edge>. I know my code isn't set up very well but it's just homework so I'm taking a few shortcuts.
Header file:
#ifndef GRAPH_H
#define GRAPH_H
#include <set>
#include <list>
class Edge{
public:
unsigned int to;
unsigned int weight;
};
class Vertex{
public:
unsigned int id;
std::list<Edge> edges;
bool operator<(const Vertex& other) const{
return id < other.id;
}
};
class Graph{
public:
void add_vertex(unsigned int id);
void add_edge(unsigned int from, unsigned int to, unsigned int weight);
std::set<Vertex> get_vertices();
std::list<Edge> get_edges(unsigned int id);
private:
std::set<Vertex> _vertices;
unsigned int size = 0;
};
Lines causing the error:
void Graph::add_edge(unsigned int from, unsigned int to, unsigned int weight)
{
Vertex find_vert;
find_vert.id = from;
set<Vertex>::iterator from_v = _vertices.find(find_vert);
Edge new_edge;
new_edge.to = to;
new_edge.weight = weight;
from_v->edges.push_front(new_edge); // ERROR HERE
}
Compiler Error message from running g++ -c Graph.cpp:
Graph.cpp:23:38: error: passing ‘const std::list<Edge>’ as ‘this’ argument of ‘void std::list<_Tp,
_Alloc>::push_front(const value_type&) [with _Tp = Edge; _Alloc = std::allocator<Edge>; std::list<_Tp,
_Alloc>::value_type = Edge]’ discards qualifiers [-fpermissive]

Answer 1

The contents of a std::set are implicitly const, because changing the contents could invalidate their sort order.
That makes from_v implicitly const here.
set<Vertex>::iterator from_v = _vertices.find(find_vert);
And your error is telling you that you're trying to modify a const object.
from_v->edges.push_front(new_edge);
// ^^^^^^ const ^^^^^^^^^^ non-const behavior