Function Objects in the C++ Standard Library

A function object, or functor, is any type that implements operator(). This operator is referred to as the call operator or sometimes the application operator. The C++ Standard Library uses function objects primarily as sorting criteria for containers and in algorithms.

Function objects provide two main advantages over a regular function call. The first is that a function object can contain state. The second is that a function object is a type and therefore can be used as a template parameter.

Creating a Function Object

To create a function object, create a type and implement operator(), such as:

class LessThanFunctor
{
public:
    bool operator()(int a, int b)
    {
        return a < b;
    }
};

int main()
{
    LessThanFunctor less_than;
    int a = 5;
    int b = 7;
    bool ans = less_than(a, b);
}

The last line of the main function shows how you call the function object. This call looks like a call to a function, but it's actually calling operator() of the LessThanFunctor type. This similarity between calling a function object and a function is how the term function object came about.

Function Objects and Containers

The C++ Standard Library contains several function objects in the <functional> header file. One use of these function objects is as a sorting criterion for containers. For example, the set container is declared as follows:

template <class Key,
    class Compare = std::less<Key>,
    class Allocator = std::allocator<Key>>
class set;

The second template argument is the function object less. This function object returns true if the first parameter is less than the second parameter. Since some containers sort their elements, the container needs a way of comparing two elements. The comparison is done by using the function object. You can define your own sorting criteria for containers by creating a function object and specifying it in the template list for the container.

Function Objects and Algorithms

Another use of function objects is in algorithms. For example, the remove_if algorithm is declared as follows:

template <class ForwardIterator, class UnaryPredicate>
ForwardIterator remove_if(
    ForwardIterator first,
    ForwardIterator last,
    UnaryPredicate pred);

The last argument to remove_if is a function object that returns a boolean value (a predicate). If the result of the function object is true, then the element is shifted such that it's beyond the new end returned by remove_if. You can use any of the function objects declared in the <functional> header for the argument pred or you can create your own.

See also

C++ Standard Library Reference