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<TITLE>Predicates</TITLE>
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<H2>3.3 Predicates</H2>
<A NAME="idx51"><!></A>
<P>A <I>predicate</I> is simply a function object that returns a value convertible to a <SAMP>bool</SAMP>. Here is an example of a predicate, which takes an integer as argument and returns <SAMP>true</SAMP> if the number represents a leap year, and <SAMP>false</SAMP> otherwise:</P>

<UL><PRE>
// return true if year is leap year
struct isLeapYear {
  bool operator(unsigned int year) const {
    // Every fourth century is a leap year
    if (0 == year % 400) return true;
    // ...but other centuries aren't.
    if (0 == year % 100) return false;
    // Otherwise, every fourth year is
    if (0 == year % 4) return true;
    // ...but all other years are not.
    return false;
  }
};
</PRE></UL>
<P>A predicate is used as an argument, for example, in the generic algorithm named <SAMP>std::find_if()</SAMP>. This algorithm returns the first value that satisfies the predicate, returning the end-of-range value if no such element is found. Using this algorithm, the following locates the first leap year in a list of years:</P>

<UL><PRE>
std::list&lt;int&gt;::iterator firstLeap = 
   std::find_if(aList.begin(), aList.end(), isLeapYear);
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