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<TITLE>Predicates</TITLE>
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<H2>3.3 Predicates</H2>
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<A NAME="idx51"><!></A>
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<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>
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<UL><PRE>
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// return true if year is leap year
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struct isLeapYear {
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bool operator(unsigned int year) const {
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// Every fourth century is a leap year
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if (0 == year % 400) return true;
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// ...but other centuries aren't.
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if (0 == year % 100) return false;
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// Otherwise, every fourth year is
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if (0 == year % 4) return true;
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// ...but all other years are not.
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return false;
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}
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};
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</PRE></UL>
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<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>
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<UL><PRE>
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std::list<int>::iterator firstLeap =
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std::find_if(aList.begin(), aList.end(), isLeapYear);
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