CODEWITHSUNDEEP
c++stl
nick
nick

Reputation: 862

Is there any risk if I call clear when loop a vector?

I want to call clear when loop to visit the vector. I think this will be dangerous, but in my experiment code below, it seems ok to run, it just stop the loop when i called clear.

#include <bits/stdc++.h>
using namespace std;

int main() {
  std::vector<int> a = {1,2,3,4,5};
  for (size_t i = 0; i < a.size(); ++i) {
    a.clear();  // just run once, then loop exited
    cout << "i = " << i << endl;
  }
}

Is there anything I don't know about the risk? could you help on this?

Upvotes: 1

Views: 83

Answers (1)

Sam Varshavchik
Sam Varshavchik

Reputation: 118340

There is a very, very major temptation that when you have a loop that looks like this:

for (size_t i = 0; i < a.size(); ++i) {

then you will be under the impression that looking at a[i] and/or modifying it, freely, inside the loop, whenever you have an urge to do so, is perfectly acceptable.

And it is. Unless you already did a

a.clear();

Alone, by itself, there's nothing wrong with just the code you showed. It's defined behavior. But as soon as a is clear()ed, doing something with a[i] becomes a no-no, inside the loop. But resistance is futile. At some point it's a near certainty that you'll forget that you clear()ed the vector, decide to take a peek at a[i], and have to deal with demons flying out of your nose.

If your goal is to stop the loop, a simple break will do. If there is a separate reason to clear() the vector, do so, but immediately break from the loop as well, to avoid giving in to the temptations of creating nasal demons.

Upvotes: 2

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