Issue using for loop and goto statement

Question

I am using C++11 std::list and list iterators. I use nested loops and test a condition and if it succeeds, I use goto statement to get out. A pseudo code is shown below:

std::list<myclass>::iterator tmpItr;
std::list<myclass>::iterator tmpItr2;
std::list<std::list<myclass>::iterator>::iterator radItr;
double cutoffVal = someVal;
double currVal = 0.0;

for (radItr=radList.begin();radItr!=radList.end();radItr++) {
    tmpItr2 = *radItr;
    for (tmpItr=pList.begin();tmpItr!=pList.end();tmpItr++) {
         if (tmpItr == tmpItr2)
             continue;

         /* Some Operation Here */
         currVal += NewVal;
         if (currVal >= cutoffVal)
             goto BREAKLOOP;
    }
}

BREAKLOOP:
// Use tmpItr and tmpItr2

My problem is that sometime the value of tmpItr goes to list.end() and I get a segmentation fault. Does the goto statement somehow does post increment as well here?

I also have tried adding another class iterator between continue and operations as tmpItr3 = tmpItr. In the cases, where I get segmentation fault i.e. tmpItr is pointing to list.end(), tmpItr3 does not. Which suggests the while breaking the loop, tmpItr changed — GuessMe
– GuessMe, Commented Sep 22, 2015 at 16:29
This is unrelated to goto, and it shouldn’t generally be a problem if/when your iterator equals list.end(). — Konrad Rudolph
– Konrad Rudolph, Commented Sep 22, 2015 at 16:32
What's the point of comparing two iterators to different lists??? — SergeyA
– SergeyA, Commented Sep 22, 2015 at 16:32
The list of iterator holds pointer to few elements which I need to use frequently (and they change once in a while) .. So instead of searching through the whole list, I minimize the calculations by doing so !! — GuessMe
– GuessMe, Commented Sep 23, 2015 at 19:15

dufresnb · Accepted Answer · 2015-09-22 16:33:14Z

1

I'm not positive it will fix the problem with your iterators, but most people I talk to consider goto's in c++ to be bad code. Why not replace goto with:

  (bool)breakout=true;
  break;
  }
if(breakout)break;

answered Sep 22, 2015 at 16:33

dufresnb

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2 Comments

Konrad Rudolph Over a year ago

Yeah, goto is pretty bad — but I’m not convinced this solution is any better, especially considering the tools C++11 now offers us (here, for instance, we could handily use a lambda to nest the loops into, and then early exit out of that.

GuessMe Over a year ago

For nested loops, I think goto is not too bad .. is it?

Toby Speight · Accepted Answer · 2015-09-23 09:16:18Z

0

I think your problem occurs when the goto is not reached:

for (...;  radItr != radList.end();  ...) {
    for (...;  tmpItr != pList.end();  ...) {
         if (...)
             goto BREAKLOOP;
    }
}

// At this point, radItr==radList.end() && tmpItr==pList.end()

BREAKLOOP:

answered Sep 23, 2015 at 9:16

Toby Speight

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1 Comment

GuessMe Over a year ago

I figured it out .. you are correct .. I was by mistake doing calculation using a not existing vector element. Unlike array, I guess vector holds additional spaces and which is why I was not getting segmentation fault at that location but when calculation get out of loops (without going to goto) and the iterator pointing to non-existing element .. Thank you for your comments :)

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