Re: STL map question : directed to stl map expert(s)....

From:

Alberto Ganesh Barbati <AlbertoBarbati@libero.it>

Newsgroups:

comp.lang.c++.moderated

Date:

Sun, 23 Sep 2007 19:29:17 CST

Message-ID:

<a2DJi.119347$%k.265890@twister2.libero.it>

Keith H Duggar ha scritto:

The above-quoted comments get interesting if you have a std::map where the
key_type is a pointer. The default comparison for such a map is
less<key_type>, which for a pointer has default implementation a < b. Now

Although less<> is defined in terms of operator<, there is an additional
requriement for pointer types in 20.3.3/8: "For templates greater, less,
greater_equal, and less_equal, the specializations for any pointer type
yield a total order, even if the built-in operators <, >, <=, >= do not."

Then what about the following example:

    int * pa = new int() ;
    int * pb = new int() ;
    std::set<int *> pset ;
    pset.insert(pa) ;
    delete pa ;
    pset.insert(pb) ;

When pb is inserted into pset, a less<int *> comparison is performed
against the then invalid pa. On the hypothetical platform that traps
invalid pointers, how will it square this "use" of pa with 20.3.3/8?

Any use of an invalid pointer value results in undefined behaviour. The
fact that less<T*> is "better" than operator< by always providing a
total order, doesn't mean that less<T*> can be expected to produce a
result without "using" its arguments.

(Notice that "undefined behaviour" means that anything can happen,
including the possibility that the programs works flawlessly!)

Ganesh

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