How to insert values into single linked list - c++

Question

I'm trying to learn some basics about single linked lists, so I went with idea of creating some code. Sadly, I have given constructor to follow.
Untill now I've created all methods I've wanted. Unfortunately, seems like my insert doesn't work, so I can't even check if other methods works. Ofc role of insert method is to add number into sorted list L. This number should be put before first number, if it's bigger or put at the end of the list, if there is no such number.
#include <iostream>
#include <cassert>
using namespace std;
struct lnode
{
int key;
lnode* next;
lnode(int k, lnode* n=nullptr):key(k),next(n){}
};
void insert( lnode* &L, int x)
{
while(L)
{
if(x >= L->key)
{
L = L->next;
}
else
{
lnode* temp = L;
L = new lnode(x, nullptr);
L->next = temp;
break;
}
}
}
int main()
{
lnode* t = nullptr;
insert(t,3);
insert(t,4);
insert(t,1);
insert(t,7);
insert(t,-4);
insert(t,9);
insert(t,2);
while(L) {
std::cout << L->key << " ";
}
}
What do I expect? My expectations is to see elements of my list. At this moment there is nothing. No error, no result.

Answer 1

The trick to writing simple singly-linked list modifying code is to use a pointer to the pointer current node to indicate your position:
void insert( lnode* &L, int x)
{
lnode **pos = &L;
while (*pos && (*pos)->key <= x) {
pos = &((*pos)->next);
}
*pos = new lnode(x,*pos);
}
Since you are, as you say, a beginner, maybe you should start with the beginner version:
void insert( lnode* &L, int x)
{
if (!L || L->key > x) {
//insert at head
L = new lnode(x, L);
return;
}
lnode *previous=L;
lnode *current=L->next;
while(current && current->key <= x) {
previous = current;
current = current->next;
}
//insert between previous and current
previous->next = new lnode(x, current);
}
Comparing to the previous one shows the benefit of using lnode ** to track the insert position as you search:
no special case for inserting at the head
no separate variables for previous and next

Answer 2

This code should work. Fixed the compilation error and also the traversing logic while printing on top of Matt's code.
#include <iostream>
#include <cassert>
using namespace std;
struct lnode
{
int key;
lnode* next;
lnode(int k, lnode* n = nullptr) :key(k), next(n) {}
};
void insert(lnode* &L, int x)
{
lnode **pos = &L;
while (*pos && (*pos)->key <= x) {
pos = &((*pos)->next);
}
*pos = new lnode(x, *pos);
}
int main()
{
lnode * t = nullptr;
insert(t, 3);
insert(t, 4);
insert(t, 1);
insert(t, 7);
insert(t, -4);
insert(t, 9);
insert(t, 2);
while (t) {
std::cout << t->key << " ";
t = t->next;
}
}