C++ STL: Function Objects (Functors)

Function objects, often called functors, are a powerful feature in C++ that allow you to treat objects as if they were functions. They are instances of classes that overload the function call operator (

code

operator()

). This enables them to be used in contexts where a function pointer or a callable entity is expected, offering more flexibility and statefulness than traditional function pointers.

What are Function Objects?

At their core, function objects are objects of classes that define the

code

operator()

member function. This operator allows an object of that class to be invoked like a regular function. Unlike plain functions, function objects can carry state, meaning they can store data between calls.

Function objects are objects that behave like functions by overloading `operator()`.

Imagine a regular function. Now, imagine a class that has a special method, operator(), which you can call just like a function. That's a function object! It's an object that can be 'called'.

A class defining operator() allows its instances to be called using the familiar function-call syntax object(arguments). This is incredibly useful when working with STL algorithms like std::sort, std::transform, or std::for_each, which often accept callable entities as arguments to customize their behavior.

Why Use Function Objects?

Function objects offer several advantages over traditional function pointers or lambdas (though lambdas often compile to function objects internally):

Feature	Function Objects	Function Pointers
Statefulness	Can store and maintain state between calls.	Cannot store state directly; require external variables.
Flexibility	Can be templated for generic programming; can have constructors and member functions.	Limited to a specific function signature.
Performance	Often allow for better compiler optimizations (inlining) due to known type and potential for state.	Can sometimes hinder inlining if not carefully managed.

Creating and Using Function Objects

Let's look at a simple example. Suppose we want to create a functor that multiplies a number by a specific factor.

#include <iostream>

// Define a class that overloads the function call operator
class Multiplier {
private:
    int factor;
public:
    // Constructor to initialize the factor
    Multiplier(int f) : factor(f) {}

    // The overloaded function call operator
    int operator()(int num) const {
        return num * factor;
    }
};

int main() {
    // Create an instance of the Multiplier functor
    Multiplier multiplyByTwo(2);

    // Use the functor like a function
    int result = multiplyByTwo(5); // Calls operator()(5)
    std::cout << "5 multiplied by 2 is: " << result << std::endl;

    return 0;
}

In this example, Multiplier is a function object. Its operator() takes an integer num and returns num multiplied by the factor stored within the object. We create an instance multiplyByTwo initialized with a factor of 2, and then call it as if it were a function: multiplyByTwo(5).

📚

Text-based content

Library pages focus on text content

Function Objects with STL Algorithms

Function objects are commonly used with STL algorithms to customize operations on collections of data. For instance,

code

std::transform

can apply a function object to each element of a range.

What is the primary advantage of function objects over simple function pointers when used with STL algorithms?

Function objects can maintain state between calls, allowing for more dynamic and context-aware operations.

Consider using a

code

Multiplier

functor with

code

std::transform

to double every element in a vector:

Loading diagram...

This demonstrates how function objects seamlessly integrate with the STL, providing a powerful and flexible way to define custom behavior for algorithms.

Modern C++ often uses lambdas, which are syntactic sugar for creating function objects. Understanding the underlying concept of function objects is crucial for grasping how lambdas work and for writing efficient, stateful callable entities.

Learning Resources

C++ Function Objects (Functors) - GeeksforGeeks(blog)

A comprehensive introduction to function objects in C++, covering their definition, usage, and benefits with clear examples.

C++ Functors Explained - cppreference.com(documentation)

The official C++ reference detailing the function call operator and its role in creating function objects.

C++ STL Algorithms - std::transform(documentation)

Documentation for the `std::transform` algorithm, which commonly uses function objects or lambdas to modify elements in a range.

Understanding C++ Functors - Scott Meyers(paper)

A presentation by Scott Meyers discussing the nuances and power of C++ functors and their relation to modern C++ features.

C++ Functors Tutorial - LearnCpp.com(tutorial)

A step-by-step tutorial explaining how to create and use function objects in C++ with practical code examples.

C++ Functors vs Lambdas - YouTube(video)

A video explanation comparing and contrasting C++ function objects (functors) with lambda expressions.

The Power of Functors in C++ - Cplusplus.com(blog)

An article exploring the practical applications and advantages of using function objects in C++ programming.

C++ Standard Template Library (STL) - Wikipedia(wikipedia)

An overview of the Standard Template Library, providing context for where function objects are frequently utilized.

Effective C++: Item 30: Favor const iterators over mutable iterators - InformIT(blog)

While not directly about functors, this article touches on iterator concepts often used with STL algorithms that employ functors.

C++ STL: Function Objects and Lambda Expressions - Udemy Blog(blog)

A blog post that delves into function objects and their relationship with lambda expressions in the context of the C++ STL.