3.3 Predicates

A predicate is simply a function object that returns a value convertible to a bool. Here is an example of a predicate, which takes an integer as argument and returns true if the number represents a leap year, and false otherwise:

// 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;
  }
};

A predicate is used as an argument, for example, in the generic algorithm named std::find_if(). 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:

std::list<int>::iterator firstLeap = 
   std::find_if(aList.begin(), aList.end(), isLeapYear);