What is the reason for this pointer conflicts? - c++

Question

Maybe I'm just confusing things a little bit.. But please consider this piece of C++ code:
#include <iostream>
#include <list>
using namespace std;
void printArray(int *arr, int n)
{
for(int i=0; i<n; ++i)
cout << arr[i] << " ";
cout << endl;
}
int main()
{
list<int*> arrays;
int times=20, v[9]={1,2,3,4,5,6,7,8,0};
arrays.push_back(v);
while(times--)
{
int *cur = arrays.front();
arrays.pop_front();
printArray(cur, 9);
int ta[9], tb[9];
for(int i=0; i<9; ++i)
{
ta[i] = *(cur+i)+1;
tb[i] = *(cur+i)-1;
}
arrays.push_back(&ta[0]);
arrays.push_back(&tb[0]);
}
return 0;
}
As you can see, the objective is to, beggining with a default array {1,2,3,4,5,6,7,8,0}, store (in times iterations) 2 variants of this array in a list of int pointers.
So, in the first iteration the 2 arrays {2,3,4,5,6,7,8,9,1} and {0,2,3,
,5,6,7,-1} should be stored in the list and thus, the first 3 printArray should be:
1 2 3 4 5 6 7 8 0
2 3 4 5 6 7 8 9 1
0 1 2 3 4 5 6 7 -1
Now, whats happening is that the first 3 printArray are:
1 2 3 4 5 6 7 8 0
2 3 4 5 6 7 8 9 1
2 3 4 5 6 7 8 9 1
I've already printed ta and tb at each iteration and I know that what's really being printed out is 1) default array, 2) first ta, 3) tb of the first ta. But I don't really know what's the reason for this behavior, I mean, aren't ta and tb new arrays (independent from the previous ones) in each iteration? If this is the case, my for is just assigning values for the new arrays positions, so why are they conflicting with each other?

Answer 1

The problem is quite simple, actually.
As #quimnuss stated in the comments, ta and tb were deallocating at some point and thus, the pointers saved in the list would point to something completely different from those arrays that were deallocated, leading to and undefined behavior of the program.
This statement makes complete sense since ta and tb are local variables of the while loop and so their validity will run out at each iteration completion.
We can workaround this problem by dynamically allocate memory at each iteration, like:
int *ta = new int[9];
int *tb = new int[9];
This away no loss of information will happen at every iteration end since the scope of this arrays are no longer local to the while loop.
Final code:
#include <iostream>
#include <list>
using namespace std;
void printArray(int *arr, int n)
{
for(int i=0; i<n; ++i)
cout << arr[i] << " ";
cout << endl;
}
int main()
{
list<int*> arrays;
int times=20;
int *v = new int[9];
for(int i=0; i<8; ++i)
v[i] = i+1;
v[8] = 0;
arrays.push_back(v);
while(times--)
{
int *cur = arrays.front();
arrays.pop_front();
printArray(cur, 9);
int *ta = new int[9];
int *tb = new int[9];
for(int i=0; i<9; ++i)
{
ta[i] = *(cur+i)+1;
tb[i] = *(cur+i)-1;
}
arrays.push_back(ta);
arrays.push_back(tb);
delete[] cur;
}
while(!arrays.empty())
{
int *p = arrays.front();
arrays.pop_front();
delete[] p;
}
return 0;
}