In modern C++ programming we can use many different ways to call functions. One of the features from C11 is functor, an early function object in C++. The functor function objects are callable objects. A functor or function object is any object for which the function call operator is defined. Let’s learn what a functor is and how we can use a functor in C++.
What is a functor in C++?
A functor is an object that can be used as a function or function pointer by using operator(), this term is not same as function term in programming. We can pass many arguments with them; thus, we don’t need to define many global variables, we can use these kinds of variables in the scope that we use.
Here is the syntax how we can use define a functor by using function call operator operator(),
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return_type operator()( parameters ) |
Is there a simple example of how to run a functor object with std::thread in C++?
We can create operators as shown below in our simple functor example. This simple rectangle example function object returns its area as below.
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class TRectangle { public: double Width=10, Height=10; double operator()() { return Width*Height; } }; |
in this example, we can use its initial values to calculate its area by using our functor like so:
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TRectangle r1; r1(); |
Is there a full example of how to run a functor object with std::thread in C++?
Here is a full example of how to run a functor object with std::thread in C++.
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#include <iostream> class TRectangle { public: double Width, Height; double operator()() { return Width*Height; } }; int main() { TRectangle r1; r1.Width = 20; r1.Height = 10; std::cout << "Area:" << r1() << std::endl; r1.Height = 20; std::cout << "Area:" << r1() << std::endl; system("pause"); return 0; } |
and the output will be as follows,
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Area:200 Area:400 |
The function objects are callable objects, and they are enhanced by the new standard library features in C++17 (std::invoke), C++20 (jthread), C++23 (invoke_r, move_only_function) , and in the next C++26 (copyable_function).
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