possible inconsistency in std::vector while erasing an element [duplicate] - c++

Question

This question already has answers here:
How to delete an element from a vector while looping over it?
(6 answers)
Closed 9 years ago.
while debugging a vector, I see an inconsistency. Assume the following code which tries to remove an entry from a vector which has only one element
#include <iostream>
#include <vector>
std::vector<int> v;
void myremove(int);
int main()
{
v.push_back(10);
std::cout << "10 pushed back\n";
myremove(10);
std::cout << "done :)\n";
return 0;
}
void myremove( int a )
{
std::vector<int>::iterator it = v.begin();
int counter = 0;
for ( ; it != v.end(); it++ ) {
std::cout << "iterating for " << counter << " times and vector size is " << v.size() << "\n";
if ( a == (*it) ) {
v.erase(it);
std::cout << "removed " << a << "\n";
}
++counter;
}
}
This is what I see in the output:
$ g++ test.cpp
$ ./a.out | more
10 pushed back
iterating for 0 times and vector size is 1
removed 10
iterating for 1 times and vector size is 0
iterating for 2 times and vector size is 0
iterating for 3 times and vector size is 0
iterating for 4 times and vector size is 0
iterating for 5 times and vector size is 0
iterating for 6 times and vector size is 0
....
....
iterating for 33790 times and vector size is 0
Segmentation fault
What I understand is that when the element is removed the size will become 0, however the iterator moves one step and still it tries to reach the end but he doesn't know that he has already passed the end point.
Can someone explain more what is going on and how to avoid that?

Answer 1

After the call to erase() the iterator it is invalidated:
Iterators and references to the erased elements and to the elements between them and the end of the container are invalidated.
Set it to the return value of erase() instead and only increment if no removal occured:
while (it != v.end())
{
if ( a == (*it) )
{
it = v.erase(it);
std::cout << "removed " << a << "\n";
}
else
{
++it;
}
}
where the return value of erase() is:
iterator following the last removed element.
Instead of hand coding a loop to erase elements you could use std::remove_if() instead:
v.erase(std::remove_if(v.begin(),
v.end(),
[](const int i) { return i == 10; }),
v.end());

Answer 2

When you erase
v.erase(it);
your iterator is not valid anymore. You have to use the returned iterator from erase. Erase gives you an iterator pointing to the element that followed the element erased by the call. And you have to break the loop if you erased the last element before the loop increments it.
it = v.erase(it);
if(it == v.end())
break;
Suggestion: you can go ahead and change the for loop to a while loop. And increment the iterator explicitly (i.e. only when you have not erased anything. If you have erased the iterator is kind of incremented already).
Like this
while(it != v.end()) {
if ( a == (*it) )
it = v.erase(it);
else
++it;
}

Answer 3

Every insert and erase invalidates all iterators for the container. The methods return the ONLY valid iterator after inserting/erasing.

Answer 4

In the documentation of std::vector::erase :
Iterator validity
Iterators, pointers and references pointing to position (or first) and beyond are invalidated, with all iterators, pointers and references to elements before position (or first) are guaranteed to keep referring to the same elements they were referring to before the call.
You erasing an element in your loop (which depends on an iterator) is making everything berserk. that's pretty much it !

Answer 5

When you call erase function the iterator is no more valid iterator, and when you increment the invalid iterator you will get spurious results.

Answer 6

The mistake is that you expect, after doing an erase on an iterator, that this iterator will still be in a consistent state. This is not the case, and your code illustrate precisely the situation when this occurs.
The semantic of your function is to remove all the elements of the vector equal to a. You can achieve the same result by filtering the vector. See that question for this point:
How to make std::vector from other vector with specific filter?