Question

While I use iterator like this ,

//include header files
using namespace std;

int main()
{
    map<int,int> intIntMap;
    map<int,int>::iterator pos;
    pos = intIntMap.begin();

    intIntMap[0] = 1;
    intIntMap[3] = 5;
    intIntMap[4] = 9;
    intIntMap[5] = 5;

    //遍历
    cout << (*pos).first << endl;

    while( pos != intIntMap.end() )
    {
        cout << pos->first << " <---> " << pos->second << endl;
        pos++;
    }

}

The output is 4;

But while I use iterator like this:

//include header file
using namespace std;

int main()
{
    map<int,int> intIntMap;
    map<int,int>::iterator pos;

    intIntMap[0] = 1;
    intIntMap[3] = 5;
    intIntMap[4] = 9;
    intIntMap[5] = 5;

    //遍历
    pos = intIntMap.begin();
    cout << (*pos).first << endl;

    while( pos != intIntMap.end() )
    {
        cout << pos->first << " <---> " << pos->second << endl;
        pos++;
    }

}

The output is what I want;

I want to know what is the difference between use of iterator, what happended to the first iterator when I insert new key-value pair ? Thanks!

addtion:
compile is use gcc 4.1.2 , in feel more confused,like this :

Answer 1

Since you called begin() when the container was empty, you got an iterator that was equal to end() (§23.1/7: “If the container is empty, then begin() == end()”).

Inserting items into the container didn’t change that, so you still have pos == intIntMap.end().

You then execute zero iterations of your loop, since pos==end(), and you’r executing the loop only as long as pos != end().

In the second example, you set pos() after you’ve inserted the data, so you get the first items in the collection, and iterate to the last.

Edit: As far as printing out the contents of the map goes, I’d probably do it more like this:

std::ostream &operator<<(std::ostream &os, std::pair<int, int> const &d) { 
    return os << d.first << " <---> " << d.second;
}

// ...

std::copy(intIntMap.begin(), intIntMap.end(), 
          std::ostream_iterator<std::pair<int, int> >(std::cout, "\n"));