How to decrement iterator in C++?

Question

I am using c++ stl list of custom defined structs and I have an iterator that goes through that list but at one point I need to decrement and go back a few entries in the list. I tried itt-- but that just gave me a segmentation fault. Is there any other way of doing it.

I also tried this std::advance(cur_instruc, a);

Here is what the iterator looks like:

#include <iostream>
#include <string>
#include <list>

using namespace std;

typedef struct {
    int instruction_ptr;
    int token;
    std::string instruction;
    std::string value;
} decoraded_nodes;

int main () {
    std::list<decoraded_nodes> instruction_list;
    std::list<decoraded_nodes>::iterator it;
    it = instruction_list.begin();

    decoraded_nodes a;
    a.instruction_ptr = 1;
    a.token = 1;
    a.instruction = "1";
    a.value = "2";

    decoraded_nodes b;
    b.instruction_ptr = 1;
    b.token = 1;
    b.instruction = "1";
    b.value = "1";

    decoraded_nodes c;
    c.instruction_ptr = 1;
    c.token = 1;
    c.instruction = "1";
    c.value = "3";

    instruction_list.insert(it, a);
    instruction_list.insert(it, b);
    instruction_list.insert(it, c);

    bool aa = false;
    int bb = 0;

    for (std::list<decoraded_nodes>::iterator cur_instruc = instruction_list.begin(), 
        end = instruction_list.end(); 
        cur_instruc != end; ++cur_instruc) {
        cout << cur_instruc->value;

        bb++;

        if (bb == 1) {
            aa = true;
        }

        if (aa) {
            cur_instruc--;
            aa = false;
        }

    }

    return 0;
}

Answer 1

It's a bad idea to change loop "for" variable (in this case - an iterator) inside the loop. Try to crete new iterator inside the loop, when you need to go back and work with this newly created iterator.