Re: comparison operator for STL sets

From:

Pete Becker <pete@versatilecoding.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 07 May 2009 08:45:36 -0400

Message-ID:

<Qrqdne-lMIBtSp_XnZ2dnUVZ_j-dnZ2d@giganews.com>

srp113 wrote:

Hi Pete,
        Thanks a lot, that makes sense. Can you tell me in general
what are the steps to come up with a comparator operator like this.
Are there simple steps to follow to derive operator < from opeartor
==. For instance I am working on a more complex class that has
following attributes:
bool a;
string b;
struct {
  int d;
  int e;
  int f;
  }c;
and test for equality would be
operator==(const MyClass& lhs,const MyClass& rhs){
if ( lhs.a == true && rhs.a == true) {
   if( lhs.b == rhs.b && ((lhs.c.d == rhs.c.d)&&(lhs.c.e == rhs.c.e) &&
(lhs.c.f == rhs.c.f))
     return true;
}
else if(lhs.a == false && rhs.a == false && lhs.b == rhs.b)
  return true;
return false; // in all other cases
}
Coming up operator < in such a case seems bit involved... From my
point of view, its unique if operator == above returns false and I
want it to go into set.

But std::set cares about more than uniqueness; it cares about order,
too. In fact, order is most important; uniqueness falls out of that.
operator< imposes an order, and that order has to be consistent. So
decide which keys matter, and make decisions in order, based on the keys:

if (lhs.a < rhs.a)
    return true;
if (rhs.a < lhs.a)
    return false;
if (lhs.b < rhs.b)
    return true;
if (rhs.b < rhs.a)
    return false;
return true;

--
   Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of
"The Standard C++ Library Extensions: a Tutorial and Reference"
(www.petebecker.com/tr1book)