TITLE: common mistake in checking for entries in STL map

(Source: Mumit Khan's STL Newbie Guide,
 http://www.xraylith.wisc.edu/~khan/software/stl/STL.newbie.html)


Checking for an item in a map

This is from a bug we found in our code a while back. 

    typedef Map<int, X*, less<int> > XMap;
    XMap xmap;
    //
    // populate xmap will what-not.
    //
    const X* xx = xmap[5];
    if (xx == 0) {                      // not in map.
        do_something();
    } else {
        do_something_else();
    }

looks pretty innocuous, but what really happens is that a new entry for
xmap[5] is created and gets stuffed with a NULL pointer which causes
amazing amount of headache 10,000 lines of code later. The right way of
course (and documented in the fine manual) is the following: 

    typedef Map<int, X*, less<int> > XMap;
    XMap xmap;
    //
    // populate xmap will what-not.
    //
    XMap::const_iterator it = xmap.find(5);
    if (it == xmap.end()) {             // not in map.
        do_something();
    } else {
        do_something_else();
    }