Re: Map Iterator for Round Robin selection along with Map Updates

From:

Sam <sam@email-scan.com>

Newsgroups:

comp.lang.c++

Date:

Fri, 25 Oct 2013 05:58:30 -0500

Message-ID:

<cone.1382698706.59734.8082.1000@monster.email-scan.com>

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Vikram Karandikar writes:

Hi all,

I have class something like this

class Y
{
...
};

class X
{
public:
  map <uint32_t, Y *> yList;
  map <uint32_t, Y *>::iterator yListIter;

  X ()
  {
    yListIter = yList.begin ();
  }

  add (Y *obj)
  {
    yList [obj->key] = obj;
  }
}

Program

X x;
map <uint32_t, Y *>::iterator i1;

x.add (new Y (1));
i1 = x.yListIter;
x.add (new Y(2));

My question is can i safely assume that i1 is always valid if there are only
new things getting added to the map?

Well, yes, however your constructor calls begin() on an empty list, which
returns the ending iterator value, the end() value, and puts it into
yListIter.

After adding the first element to the list, your yListIter will still point
to the end() value, not the first value of the list.

std::map::begin() does not return an iterator value that will always
dereference to the first element in the map, or end(). It returns an
iterator value that dereferences to the first element in the map, or end(),
at the time that begin() was invoked. If another element gets added to the
map with the lowest key value of any existing element in the map, and thus
it becomes the new first element in the map, dereferencing what begin()
returned previously will not give you the current first element in the map.

So, although your code, above is well-formed, it's not going to do what you
appear to want it to do.

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