Initializing std::list in freshly allocated memory [duplicate] - c++

Question

class graph
{
int v;
list<int> *adj;
void dfsutil(int v,bool visited []);
public:
graph(int v)
{
this->v=v;
//adj = new list<int>[v];
adj = (list<int> *)malloc(v*sizeof(list<int>));
}
void addedge(int v,int w);
void dfs(int v);
};
void graph::addedge(int v,int w)
{
adj[v].push_back(w);
}
void graph::dfsutil(int v,bool visited[])
{
list<int>::iterator i;
cout<<v<<" ";
visited[v]=true;
for(i=adj[v].begin();i!=adj[v].end();i++)
{
if(!visited[*i])
dfsutil(*i,visited);
}
}
void graph::dfs(int v)
{
int i=0;
bool visited[this->v];
for(i=0;i<this->v;i++)
visited[i]=false;
dfsutil(v,visited);
for(i=0;i<v;i++)//this loop is required if there are multiple component of the graph
if(!visited[i])
dfsutil(i,visited);
}
int main()
{
// Create a graph given in the above diagram
graph g(4);
g.addedge(0, 1);
g.addedge(0, 2);
g.addedge(1, 2);
g.addedge(2, 0);
g.addedge(2, 3);
g.addedge(3, 3);
cout << "Following is Depth First Traversal (starting from vertex 2) \n";
g.dfs(2);
return 0;
}
in the above code if try to allocate space for list *adj using malloc as written above,it does not work fine whereas if we use new,it works fine as its written in commented part above,i cant figure out why

Answer 1

You did not create an array of std::list objects when you used malloc. All malloc does is allocate memory from the heap -- no objects are created. Thus attempting to use your std::list's as if they are created correctly will result in undefined behavior.
You should use a container such as std::vector to store your list objects:
#include <vector>
#include <list>
class graph
{
int v;
std::vector<std::list<int>> adj;
void dfsutil(int v,bool visited []);
public:
graph(int num) : v(num), adj(num) {}
void addedge(int v,int w);
void dfs(int v);
};
Note there is no need to allocate memory. The rest of your code should stay the same, since vector has an overloaded operator [] to access the items.