处理向量C ++中的指针

Question

Having problem using the vector in C++. The following code is having this

* Error in `./a.out': double free or corruption (out): 0x0000000001e5d050 * === Backtrace: === . . .

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);
    }