C++ std::list with struct containing list with struct - c++

Question

I'm new to C++ (I did C before but never C++) I believe I have syntax problem.
I want to sort some orders by price level into a list.
So my list has positions with inside:
the price level
another std::list with all orders at that price. (Orders)
An Order is a struct with:
userid
quantity
So I ended with :
#include<iostream>
#include<list>
using namespace std;
typedef struct Order {
int userid;
int qty;
} Order;
typedef struct Bid {
int price;
list<Order> Orders;
} Bid;
typedef list<Bid> bids;
int main(void)
{
bids list;
Order order_to_insert;
list.begin();
list.front().price = 13000;
order_to_insert.userid = 3;
order_to_insert.qty = 20;
list.front().Orders.begin();
list.front().Orders.front().userid =3;
list.front().Orders.front().qty = 20;
// list.front().Orders.front() = order_to_insert; // compiles even if i uncomment this.
cout << "Liste : " << list.front().price << endl;
cout << list.front().Orders.front().qty << endl;
return 0;
}
The most intuitive way initially was to use the commented line, it compiles but gives seg fault.
I commented to assign values to fields directly and it seg fault also.
What is the proper way to do that?

Answer 1

First of all this line list.begin(); and that line list.front().Orders.begin(); don't do anything. Remove them.
Now the main problem. Elements in lists don't appear automagically. Lists are empty when created. Have a look at this line:
list.front().price = 13000;
At that point your list is empty, there is no .front(). So its an undefined behaviour, probably the cause of the segfault.
This will do:
Bid bid;
list.push_back(bid);
list.front().price = 13000;
// or if C++11
list.emplace_back();
list.front().price = 13000;
The same goes for list.front().Orders.front() and every other line using .front().
Side note: You may want to use list.emplace_back instead of push_back. Also you may want to use std::vector instead of std::list. There are several performance advantages over a list and it should be used by default unless you really know that you need lists.

Answer 2

Calling front() on an empty list has undefined behavior. An empty list doesn't have a first item, so you shouldn't be trying to access it.
You can use push_back() eg. to add an item into the list, which you can then access and modify.
list.push_back(Bid());

Answer 3

You do not insert elements into your list. std::list::front will return the first element of the list if it is non-empty, but behaviour is undefined if the list is empty.
std::list::begin returns an iterator to the first element, if the list is non-empty or the past-the-end iterator if the list is empty.
Use std::list::push_back to insert new elements into your list.

Answer 4

Thanks guys for clear explanations and advises.
I ended with the following working code :
#include <iostream>
#include <list>
using namespace std
typedef struct Order {
int userid;
int qty;
} Order;
typedef struct Bid {
int price;
list<Order> Orders;
} Bid;
typedef list<Bid> bids;
int main(void)
{
Bid bid;
bids bidList;
Order order_to_insert;
bidList.push_back(bid);
bidList.front().price =13000;
order_to_insert.userid = 3;
order_to_insert.qty = 20;
bidList.front().Orders.push_back(order_to_insert);
bidList.front().Orders.front() = order_to_insert;
cout << "Liste : " << bidList.front().price << endl;
cout << bidList.front().Orders.front().move << endl;
return 0;
}