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 straight 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.
To create a function object, create a type and implement operator(), such as:
class Functor
{
public:
int operator()(int a, int b)
{
return a < b;
}
};
int main()
{
Functor f;
int a = 5;
int b = 7;
int ans = f(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 Functor type. This similarity between calling a function object and a function is how the term function object came about.
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 Traits=less<Key>,
class Allocator=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.
Another use of functional objects is in algorithms. For example, the remove_if
algorithm is declared as follows:
template <class ForwardIterator, class Predicate>
ForwardIterator remove_if(
ForwardIterator first,
ForwardIterator last,
Predicate 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 removed from the container being accessed by the iterators first
and last
. You can use any of the function objects declared in the <functional>
header for the argument pred
or you can create your own.