Segmentation fault on list in C++ [closed] - c++

Question

Closed. This question needs debugging details. It is not currently accepting answers.
Edit the question to include desired behavior, a specific problem or error, and the shortest code necessary to reproduce the problem. This will help others answer the question.
Closed 8 years ago.
Improve this question
I'm using a class which holds a private list:
class Set
{
private:
list<long long unsigned> ways; //holds tags of addresses
and as part of the class's functionality I'm managing a LIFO on the list 'ways':
list<long long unsigned>::iterator it = ways.begin();
while (it!= ways.end()) //looks for the tag in the list
{
if ((*it) == tag) //tag is found in this set. moves the tag to the end of the list
{
ways.erase(it);
ways.push_back(tag);
return true;
}
it++;
}
return false;
and:
if (occupied < maxWays) //if the set is not all used up just pushes tag in the end
{
ways.push_back(tag);
occupied++;
return false;
}
else // if used up pops the front member (the least recently used one)
{
ways.pop_front();
ways.push_back(tag);
}
return true;
Nothing else touches 'ways' and nothing else erases the class 'set'.
Multiple instances of the class 'set' are created at the beginning.
During operation I'm getting Segmentation Fault for
list<long long unsigned>::iterator it = ways.begin();
which occurs after a long run. Trying to print the address of 'ways' before this line shows that at the point that I'm about to get Segmentation Fault the address of 'ways' changed dramatically.
All the previous times it was around 0x6000xxxxx for each instance, and at that time it was 0x23.
I don't have a clue what can cause that, please assist.

Answer 1

It might be that you delete an element from the list, and then increment the iterator, which points to the deleted element.
You probably need to forward the iterator first, and then remove the previous, to achieve what you want.
See:
Can you remove elements from a std::list while iterating through it?
EDIT: See also the return value of erase(), and similar operations that modify the iterator bag.
http://www.cplusplus.com/reference/list/list/erase/

Answer 2

Are you initiliazing 'occupied' and 'maxWays' ? If not, see example where it fails as we are calling ways.pop_front() on empty list ways
class Set
{
public:
Set(int max)
{
maxWays = max;
occupied = 10; // Say randomly stored value 10 is more than maxWays = 5
}
bool search(long long tag)
{
list<long long unsigned>::iterator it = ways.begin();
while (it!= ways.end()) {
if ((*it) == tag) {
ways.erase(it);
ways.push_back(tag);
return true;
}
it++;
}
return false;
}
bool add(long long tag)
{
if (occupied < maxWays) {
ways.push_back(tag);
occupied++;
return false;
}
else {
ways.pop_front(); // may fail here
ways.push_back(tag);
}
return true;
}
private:
list<long long unsigned> ways;
int maxWays;
int occupied;
};
int main()
{
Set set(5);
cout << set.add(100) << endl;
return 0;
}