Sorted containers, and algorithms that operate on sorted containers, require comparison predicates that define the sort order of the elements. These predicates are used to test if elements are equal, they do so by checking that neither element preceeds the other in the sort order.

Returning true from a comparison predicate for equivalent elements means the container will never detect that elements are equal, resulting in an invalid state.

   
   // Potential algorithm determining equivalent elements for sorted
   // containers.
   //
   bool areElementsEqual( T& a, T& b )
   {
      // pred is a comparison predicate that defines the sort order
      // of a & b.
      //
      if ( !pred( a, b ) && !pred( b, a ) )
      {
         // a and b are equivalent
      }
   }

Rule 17.15

Effective STL Item 21;