HP Integrity NonStop J-Series - Manual (Page 122)

comp(x,z) (transitivity).
The truth of I.a implies that x must precede y within the collection class, while I.b says that y
must precede x. More interesting is I.c. If this statement is true, we say that x and y are
equivalent, and it doesn't matter in what order they occur within the collection class. This is the
notion of equality that prevails for the templates that take a comparator as a parameter. For
example, when the member function contains(T item) of an associative container-based template
tests to see if the collection class contains an element equivalent to item, it is really looking for
an element x in the collection class such that comp(x,item) and comp(item,x) are both false. It is
important to realize that the == operator is not used. Don't worry if at first it seems
counter-intuitive that so much negativity can give rise to equivalence_you are not alone! You'll
soon be comfortable with this flexible way of ensuring that everything has its proper place within
the collection class.
Comparators are generally quite simple in their implementation. Take for example:
class IncreasingLength {
public:
bool operator()(const RWCString& x, const RWCString& y)
{ return x.length() < y.length(); }
};
RWTValSet mySet;
Here mySet maintains a collection of strings, ordered from shortest to longest by the length of
those strings. You can verify that an instance of the comparator satisfies the given requirements
for total ordering. In the next example, mySet2 maintains a collection class of integers in
decreasing order:
class DecreasingInt {
public:
bool operator()(int x, int y)
{ return x > y; }
};
RWTValSet mySet2;
Although the sense of the comparison may seem backwards when you first look at it, the
comparator says that x should precede y within the collection class if x is greater than y; hence,
you have a decreasing sequence. Finally, let's look at a bad comparator:
// DON'T DO THIS:
class BadCompare {
public:
bool operator()(int x, int y)
{ return x <= y; } // OH-OH! Not a total ordering relation
};
RWSetVal mySet3; // ILLEGAL COMPARATOR!
To determine why it's bad, consider an instance badcomp of BadCompare. Note that when using
the value 7 for both x and y, none of the three statements I.a, I.b, or I.c is true, which violates the