Some Predefined Function Objects

You might actually be wondering why we should even bother using function objects, since the examples so far have been fairly simple and non-informative. Well, you can use them with most of the containers we have already seen. In Container Summary, one of the constructors for set and map passed in a comparison object; list provides it's own sort method, which you could pass a comparrison object to tell it how to sort data. Well, we can use any of the comparison function objects given here; so, instead of ordering using less-than (the default ordering), we can instead use greater, for example:

       /* Create a set in which we sort elements using 'greater'
        * rather than 'less than': */
       std::set<int, greater<int>()> some_set;
     
       /* Now create a list: */
       std::list<int> some_list;
       /* ... add some elements to the list... */
     
       /* Sort elements from greatest to least: */
       some_list.sort(greater<int>());
     

The above example demonstrates (without going into too much detail) how the greater function object can be used to change the sorting criterion for a set, as well as sorting a list using greater. greater, along with a number of other predefined function objects, are detailed in Function Object Summary.

However, there are also a few other function objects that are worth metioning here, and are very useful. These are covered in the next section.