A functor is a powerful C++ entity that everyone who wants to master C++ needs to know. A functor, which is short for “function object”, is a C++ class that acts like a function. Functors can be called using the familiar function call syntax, and can yield values and accept parameters just like regular functions.
To create a functor, we create a class (or a struct) that overloads the function operator()
. Note here the function is called operator()
, and it’s not the operator
function, i.e. ()
. We then create an instance of this class (or struct) to use the created functor.
Create and use functors
Let’s look at two examples of creating and using a functor. In the first example, a functor is created with a class
, and in the second example we use a struct
to create the functor.
Example: Create a functor with a Class
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Example: Create a functor with a Struct
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Functors access class data members
The key difference between a function and a functor is that a functor’s function call operator is a member function whereas a raw C++ function is a free function. This means that a functor can access the following information when being called:
 Its local variables
 Its parameters
 Global variables
 Class data members
If a functor’s operator()
member function requires access to data beyond what can be communicated by its parameters, we can store that information as a data member inside the functor class. Since operator()
is a member of the functor class, it can then access that data freely. The following example shows how a functor’s operator()
function access the class’s private member toAppend
.
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Functors are useful in STL algorithms
C++ STL algorithms use functors to increase the flexibility and efficiency. The most common uses for function objects are for generating data, for testing data, and for applying operations to data. here is an example of how STL for_each
uses functors.
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The reader of Stanford course CS106l explains functors in detail.