Question

I have a problem with assigning an unintialized to an initialized iterator. The following code excerpt produces an access violation when built with Visual Studio 2010. In previous versions of Visual Studio the code should work.

#include <list>

int main() {
    std::list<int> list;
    std::list<int>::iterator it = list.begin();
    std::list<int>::iterator jt;
    it = jt; // crashes in VS 2010
}

Wouldn’t this be considered valid C++?

I need this code to implement a “cursor” class that either points nowhere or to a specific element in a list. What else could I use as a value for an uninitialized iterator if I don’t have a reference to my container yet?

Answer 1

 it = jt; // crashes in VS 2010

This invokes undefined behaviour (UB). According to the C++ Standard ,jt is a singular iterator which is not associated with any container, and results of most expressions are undefined for singular iterator.

The section §24.1/5 from the C++ Standard (2003) reads (see the bold text specifically),

Just as a regular pointer to an array
guarantees that there is a pointer
value pointing past the last element
of the array, so for any iterator type
there is an iterator value that points
past the last element of a
corresponding container. These values
are called past-the-end values. Values
of an iterator i for which the
expression *i is defined are called
dereferenceable. The library never
assumes that past-the-end values are
dereferenceable. Iterators can also
have singular values that are not
associated with any container.
[Example: After the declaration of an
uninitialized pointer x (as with int*
x;), x must always be assumed to have
a singular value of a pointer.]
Results of most expressions are
undefined for singular values; the
only exception is an assignment of a
non-singular value to an iterator that
holds a singular value. In this case
the singular value is overwritten the
same way as any other value.
Dereferenceable values are always
nonsingular.

If MSVS2010 crashes this, it is one of infinite possibilities of UB, for UB means anything could happen; the Standard doesn’t prescribe any behavior.