Vector iterator not dereferencable (vector inside vector)

Question

I am trying to write some data from these vectors onto text files. When I run the code it returns a run time error. Category, Product, Cart, Customerand Address are all struct with members that each get_member returns.

ofstream write_cats;
    write_cats.open("catprd.dat", ios::out, ios::trunc);
    vector<Category>::iterator i;
    write_cats << cats.size() << endl;
    for (i = cats.begin(); i < cats.end(); i++) {
        write_cats << i -> get_catid() << '\t';
        }
    vector<Product>::iterator j;
    write_cats << prods.size() << endl;
    for (j = prods.begin(); j < prods.end(); j++) {
        write_cats << j -> get_prodid() << '\t';
        write_cats << j -> get_prodprice() << endl;
        }
    write_cats.close();

    ofstream write_carts;
    write_carts.open("carts.dat", ios::out, ios::trunc);
    vector<Cart>::iterator k;
    write_carts << carts.size() << endl;
    for (k = carts.begin(); k < carts.end(); k++) {
        write_carts << k -> get_cartid() << '\t';
        write_carts << k -> get_day() << endl;
        }
    vector<Cart_item>::iterator l;
    write_carts << cart_items.size() << endl;
    for (l = cart_items.begin(); l < cart_items.end(); l++) {
        write_carts << l -> get_cartitemid() << '\t';
        write_carts << l -> get_qty() << endl;
        }
    write_carts.close();

    ofstream write_custs;
    write_custs.open("custs.dat", ios::out, ios::trunc);
    vector<Customer>::iterator m;
    vector<Address>::iterator n;
    write_custs << custs.size() << endl;
    for (m = custs.begin(); m < custs.end(); m++) {
        write_custs << m -> get_cust_id() << '\t';
        write_custs << n -> get_zip_code() << endl;
        }
    write_custs.close();

Returns run time error "Vector iterator not dereferencable"

Here is how struct Address looks like:

using namespace std;
#pragma once
#include <string>

struct Address {

public:

int get_st_number() const{return st_number;} 
int get_zip_code() const{return zip_code;} 
string get_st_name() const{return st_name;} 



Address(){}                                 
Address (int num, string name, int zip) 
    : st_number(num), st_name(name), zip_code(zip) {}

private:
int st_number;
int zip_code;
string st_name;


};

and struct Customer:

struct Customer {
public:

Address get_address() const{return addr;} 
int get_cust_id() const{return cust_id;}  customer id
string get_name() const{return cust_name;} 
Customer (int id, string n, Address a)  
    : cust_id(id), cust_name(n), addr(a) {}


string display_addr() const {
    std::cout<<setw(15)<<cust_name<<" ";
    std::cout<<setw(15)<<cust_id<<" ";

    return string();
}



private:

int cust_id;
string cust_name;
Address addr;           
};

Answer 1

You are declaring an iterator n, but not initialising it to a dereferencable value. From your update, it looks like you want to print the Address associated with the customer; so you'd access via the customer referred to by m, rather than a separate iterator:

write_custs << m -> get_cust_id() << '\t';
write_custs << m -> get_address().get_zip_code() << endl;

Also, it might be a good idea to scope each iterator inside its loop; that's less error-prone than declaring a new one in the outer scope each time:

for (vector<Whatever>::const_iterator i = stuff.begin(); i != stuff.end(); ++i) {
    // do stuff with "i"
}
// "i" is no longer available - no danger of accidentally using it again.

And a couple of other points:

• You should use != rather than < to compare with the end() iterator; < doesn't work for some types of iterator;
• You should write a new-line to the file as '\n' rather than endl; endl flushes the file buffer, forcing the file to be written to disk at that point, which can be extremely slow.

Answer 2

you forget to initialize vector<Address>::iterator n;