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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<int>::iterator firstLeap = |
| std::find_if(aList.begin(), aList.end(), isLeapYear); |
| </PRE></UL> |
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