Why doesn't C++ take care of iterators when insertion is done in a vector after capacity is reached?

Question

I wrote this small code just to see how an iterator actually gets invalidated and does not point to changed location of a vector after its capacity is reached.

Here the size of vector and capacity is both 5 initially . After that I inserted a few other elements in vector and does not re-initialize my iterator to point to myvector.begin(). This lead to a junk value of 49 in my output after maximum size of vector is : 1073741823 when printing elements of vector again.

My question is why C++ does not make point iterator again to a valid myvector.begin() after all elements are copied into new location?
This can also lead to some behavior which can be hard to debug. I know a safe way to work would be to always reinitialize the iterator just before using it.

  #include<iostream>
  #include<vector>
  #include<stdio.h>

  using namespace std;

  int main()
  {
    vector<int> myvector;
    vector<int>::iterator it;
    int myarray[]= {100,200,300,400};
    myvector.insert(it,500);
    it=myvector.begin();
    myvector.insert(it,myarray,myarray+4);
    it=myvector.begin();
    for(;it!=myvector.end();++it)
    cout <<*it<<endl;
    cout <<"size of vector is :" << myvector.size() <<"\n"; 
    cout <<"capacity of vector is : " << myvector.capacity()<<"\n";  
    cout <<"maximum size of vector is : " << myvector.max_size()<<"\n"; 
    myvector.push_back(600);
    for(;it!=myvector.end();++it)
    cout <<*it<<endl;
  }
  Output of program :-
  100
  200
  300
  400
  500
  size of vector is :5
  capacity of vector is : 5
  maximum size of vector is : 1073741823
  49
  100
  200
  300
  400
  500
  600

with std::vector anything which changes the size invalidates previous iterators — Flexo
@awoodland he appears to know that, I think he's asking why this is happening. — Luchian Grigore
Every STL container makes certain guarantees regarding validity of its iterators. It pays to understand when iterators are invalidated for a given container. You certainly don't want to always reset to a new begin() 'just in case'. See here for info re vector - sgi.com/tech/stl/Vector.html — Steve Townsend

Luchian Grigore Luchian Grigore · Accepted Answer · 2012-04-30T17:32:55

Because it's impractical, and probably impossible.

Is the vector supposed to keep a list of all its iterators and them modify all of them as soon as an invalide-triggering method is called?

Why doesn't C++ take care of iterators when insertion is done in a vector after capacity is reached?

4 Answers