TITLE: sorting using STL

(Newsgroups: comp.lang.c++, 25 Jun 98)

CHUGH: Kevin P Chugh

> hi- i'm trying to use the STL set template.  i want to use my own classes
> and so i need to override the < operator.  but, i want to use a set
> of pointers, not a set of objects.  my question is this- how do i
> create a < operator for pointers to my objects.  i've tried this:
> 
> bool operator < (MyClass *c1,MyClass *c2)
> {
> return (c1->number<c2->number);
> }
> 
> but the compiler (borland 5.0) complains that this operator must
> be a member function or class.
> 
> so i tried writing it as a friend, but that didn't work either.
> 
> am i correct in assuming that if i want a less than operator for
> two pointers to a class type that i need a non-member operator- if
> this is true, how do i write one?


BECKER: Pete Becker <petebecker@acm.org>

Back up and ask the right question. The question is, how do I define my
own ordering operation for items in an STL set? And the answer is, write
a class for the set to use instead of the default less<T>, which uses
operator<. You do it like this:

struct lessThan : binary_function<MyClass*, MyClass*, bool>
{
public:
	bool operator()(const MyClass *m1, const MyClass *m2) const
	{ return /* whatever makes sense */; }
}

Then you create a set with this class as its comparator:

set<MyClass*, lessThan> items;

Now your set will be sorted in accordance with the rule that you wrote
in lessThan::operator(). Make sure that it defines a partial ordering,
though: don't write something that at one time says that o1 is less than
o2 and at another time says that o1 is greater than o2.