Handling pointer in vector C++ - c++

Question

Having problem using the vector in C++.
The following code is having this Runtime Error :
* Error in `./a.out': double free or corruption (out): 0x0000000001e5d050 *
=== Backtrace: ===
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.
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I have written this code.
int main(){
vector<int> ve;
int n;
cin>>n;
for(int i=1; i<n+1; i++)ve.push_back(i);
int last, sec_last, ans;
while(!(ve.empty())){
ans = ve.back();
last = ve.back();
ve.pop_back();
sec_last = ve.back();
ve.pop_back();
ve.push_back(last + sec_last + last*sec_last);
}
cout<<"\nline 20\n";
cout<<ans<<"\n";
cout<<"\nline 22\n";
}
Please explain me what does this error mean, and how can I correct it...

Answer 1

"Calling back / pop_back on an empty container is undefined" -- Internet
While loop condition prevents you only once, until the vector ve is modified again by pop_back. In that case there is possibility that vector is empty and calling back on empty vector will likely result into a crash.
while(!(ve.empty())){
ans = ve.back();
last = ve.back();
ve.pop_back(); // <---- Vector could become empty
sec_last = ve.back(); // <---- If vector is indeed empty, then undefined behavior
ve.pop_back();
ve.push_back(last + sec_last + last*sec_last);
}