Re: Vector iterator problem

From:

LR <lruss@superlink.net>

Newsgroups:

comp.lang.c++

Date:

Fri, 05 Aug 2011 05:43:20 -0400

Message-ID:

<4e3bb869$0$24744$cc2e38e6@news.uslec.net>

Nephi Immortal wrote:

I guess that Microsoft?s vector code is not good because iterator is
less flexible. If you want to use operator<= or operator>, then
assert is triggered.

If you try to access an element that isn't in the container, yes.

I suppose to use operator> if I don?t want to use reverse_iterator.
Why do vector documentation tell to use only operator!= or operator<?

Because end() doesn't point to an element in the container. Please see
below.

I wish Microsoft should rewrite their vector code.

MS seems to conform to the standard in this regard.

All iterators
must always use signed integer instead of unsigned integer.

I'm sorry, I'm not sure what you mean by this.

#include <vector>
using namespace std;

int main()
{
    vector< int > a;
    a.push_back( 1 );
    a.push_back( 2 );
    a.push_back( 3 );
    a.push_back( 4 );

    vector< int >::iterator B = a.begin();
    vector< int >::iterator E = a.end();
    vector< int >::iterator I;

    int _v;

    // OK
    for( I = B; I != E; ++I )
        _v = *I;

    // OK
    for( I = B; I < E; ++I )
        _v = *I;

    // ERROR
    for( I = B; I <= E; ++I )
        _v = *I;

As Victor Bazarov pointed out else thread, "This code has undefined
behaviour when I==E."

Since vector<>::end returns an iterator that points to the next element
after the last element in the vector.

The same is true of your loops below.

/* operator<= should call operator< automatically */

Why? It may be that someone would want operator<= instead of operator<.
For example,
        E--;
        for(I=B; I<=E; ++I)
            _v = *I;

     // ERROR
    for( I = E; I != B; --I )
        _v = *I;

    // ERROR
    for( I = E; I > B; --I )
        _v = *I;

    return 0;
}