Re: Is pushing into reserved space invalidating iterators?

From:

Alberto Ganesh Barbati <AlbertoBarbati@libero.it>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 5 Mar 2008 16:32:52 CST

Message-ID:

<GlEzj.29107$FR.140529@twister1.libero.it>

MathGis ha scritto:

Hi,

Is the following code legal?

"Legal" is an ambiguous word. It's not ill-formed, but it invokes
undefined behaviour.

I'm asking since the iterator debugging facility of VC++2005's stl
gives an assertion failed at runtime.

That's good.

#include <vector>

int main(int argc, char** argv)
{
     std::vector<int> x;
     x.reserve(1);
     std::vector<int>::iterator i = x.end();
     x.push_back(1); // No reallocation
     if ( i != x.end()) // Debug Assertion failed: iterators
incompatible?
         return *i; // Debug Assertion failed: vector iterator not
dereferencable?
}

I had understood that iterators only become invalid when vector
operations cause reallocation.

You understood wrong. About vector::insert(), the standard states
explicitly in 23.2.4.3: "Causes reallocation if the new size is greater
than the old capacity. If no reallocation happens, all the iterators and
references before the insertion point remain valid." In this case end()
is *after* the insertion point, so it gets invalidated. If you had asked
about begin() then the answer would have been different: it won't be
invalidated because it's before the insertion point.

HTH,

Ganesh

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