Double pointer doesn't call object constructor when using new - c++

Question

My problem seems to be answered with only single pointers. I'm trying to dynamically allocate for a 2d linked list. The problem I ran into when I'm trying to use class llist constructor to set head and tail pointer to NULL :
//I've included only the parts that I see are neccesary
struct node {
int value ;
int weight ;
node* next ;
} ;
class llist{
private :
node* tail, *head ;
public :
llist(){tail = NULL ; head = NULL ;} /* this unneccesary retype is to make sure
that it wasn't a typo*/
}
class list_graph
{
private :
int size, s;
llist ** v ;
public :
list_graph(int s){
this -> s = s ;
size = 0 ;
v = new llist* [s] ;
}
}
I've use the debugger and ran through every steps and it seems that my constructor for llist is not called after I create a object of list_graph type, so every other function that relies on this fails and gives me segmentation fault. Is there anything I'm doing wrong, or is there any workarounds beside using the STL list, many thanks

Answer 1

This:
v = new llist* [s] ;
creates an array of pointers to type llist, but it doesn't create any llist objects. If you want an array of such things, then you want:
llist * v ;
and:
v = new llist[s] ;
Or better still, if this is not a homework assignment, use std::vector. And don't think of things like llist ** v as "double pointers"; think of them as what they are - pointers to pointers.

Answer 2

If you want to allocate a 2D pointer array, you can do it a couple ways:
Using a dynamic array of pointer arrays, given some width and height:
llist** data = new llist*[width];
for (int i = 0; i < width; ++i){
data[i] = new llist[height]; // constructors get called here
}
// accessing a linked list, make sure x is in [0, width) and y is in [0, height):
data[x][y]->value;
Using a single pointer array:
llist* data = new llist[width * height]; // constructors get called here
// accessing a linked list:
// again, make sure x is in [0, width) and y is in [0, height)
data[x + (y * width)]->value;

Answer 3

If you want to keep v as a array of pointers to llist objects, as opposed to an array of llist objects (suggested by Neil Butterworth), dynamically allocate each pointer by changing your list_graph constructor to
list_graph(int s) : size(0), s(s), v(new llist* [s]) {
for (int i(0); i < s; ++i)
v[i] = new llist();
}
EDIT: To avoid calling new s times in the for loop, you could pre-allocate all s llist objects at once. However, this would mean they can not be deleted individually. Your class would look like
class list_graph {
int size, s;
llist* buffer;
llist ** v ;
public:
list_graph(int s) : size(0), s(s),
buffer(new llist[s]), v(new llist* [s]) {
for (int i(0); i < s; ++i)
v[i] = &buffer[i];
}
~list_graph() {
delete[] v;
delete[] buffer;
}
};