Delete elements from a vector in C++11 while iterating over it

Question

My applications requires to iterate over a vector and delete certain elements which doesnt satisfy the requirements. Which is the most correct way? I believe the following way is incorrect.Reason: I am getting segmentation fault.

std::vector<ObjectX> vec1;
//Fill in vec1
std::vector<ObjectX>::iterator itVec1 = vec1.begin();

for(;itVec1 != vec1.end(); ++itVec1) {
   if (Oracle(*itVec1)) vec1.erase(itVec1);
}

When you're using `erase`, it invalidates iterators and references at or after the point of the erase, including the end() iterator. — Andreas DM, Mar 31 '16 at 23:11
I usually iterate it in reverse order and use index access instead of vector iterator — Asier Sánchez Rodríguez, Mar 31 '16 at 23:13
I feel like the two most frequent C++ questions I see: how do I get numbers out of `std::string`s and why is my program segfaulting when invalidating my iterator. — erip, Mar 31 '16 at 23:17

R Sahu · Accepted Answer · 2016-03-31T23:54:37.410

4

When you call

vec1.erase(itVec1);

you invalidate itVec1. After that, ++itVec1 is not right. It leads to undefined behavior. You need to change your code a little bit.

for( ; itVec1 != vec1.end(); ) {
   if (Oracle(*itVec1))
   {
      itVec1 = vec1.erase(itVec1);
   }
   else
   {
      ++itVec1;
   }
}

You can remove all the boiler plate code by using the Erase-Remove Idiom:

vec1.erase(std::remove_if(vec1.begin(), vec1.end(), Oracle), vec1.end());

edited Mar 31 '16 at 23:54

answered Mar 31 '16 at 23:15

R Sahu

204,454
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2

this loop is anachronistic. teach him the right way - erase/remove_if. – Richard Hodges Mar 31 '16 at 23:39

score 0 · Answer 2 · answered Mar 31 '16 at 23:14

0

You could just put the correct items into a new vector.

answered Mar 31 '16 at 23:14

Michael Ebenstein

135
2
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Delete elements from a vector in C++11 while iterating over it

2 Answers2