I keep hearing a lot about functors in C++. Can someone give me an overview as to what they are and in what cases they would be useful?
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Like has been repeated, functors are classes that can be treated as functions (overload operator ()).
They are most useful for situations in which you need to associate some data with repeated or delayed calls to a function.
For example, a linked-list of functors could be used to implement a basic low-overhead synchronous coroutine system, a task dispatcher, or interruptable file parsing. Examples:
Of course, these examples aren't that useful in themselves. They only show how functors can be useful, the functors themselves are very basic and inflexible and this makes them less useful than, for example, what boost provides.
For the newbies like me among us: after a little research I figured out what the code jalf posted did.
A functor is a class or struct object which can be "called" like a function. This is made possible by overloading the
() operator
. The() operator
(not sure what its called) can take any number of arguments. Other operators only take two i.e. the+ operator
can only take two values (one on each side of the operator) and return whatever value you have overloaded it for. You can fit any number of arguments inside a() operator
which is what gives it its flexibility.To create a functor first you create your class. Then you create a constructor to the class with a parameter of your choice of type and name. This is followed in the same statement by an initializer list (which uses a single colon operator, something I was also new to) which constructs the class member objects with the previously declared parameter to the constructor. Then the
() operator
is overloaded. Finally you declare the private objects of the class or struct you have created.My code (I found jalf's variable names confusing)
If any of this is inaccurate or just plain wrong feel free to correct me!
Here's an actual situation where I was forced to use a Functor to solve my problem:
I have a set of functions (say, 20 of them), and they are all identical, except each calls a different specific function in 3 specific spots.
This is incredible waste, and code duplication. Normally I would just pass in a function pointer, and just call that in the 3 spots. (So the code only needs to appear once, instead of twenty times.)
But then I realized, in each case, the specific function required a completely different parameter profile! Sometimes 2 parameters, sometimes 5 parameters, etc.
Another solution would be to have a base class, where the specific function is an overridden method in a derived class. But do I really want to build all of this INHERITANCE, just so I can pass a function pointer????
SOLUTION: So what I did was, I made a wrapper class (a "Functor") which is able to call any of the functions I needed called. I set it up in advance (with its parameters, etc) and then I pass it in instead of a function pointer. Now the called code can trigger the Functor, without knowing what is happening on the inside. It can even call it multiple times (I needed it to call 3 times.)
That's it -- a practical example where a Functor turned out to be the obvious and easy solution, which allowed me to reduce code duplication from 20 functions to 1.
Name "functor" has been traditionaly used in category theory long before C++ appeared on the scene. This has nothing to do with C++ concept of functor. It's better to use name function object instead of what we call "functor" in C++. This is how other programming languages call similar constructs.
Used instead of plain function:
Features:
Cons:
Used instead of function pointer:
Features:
Cons:
Used instead of virtual function:
Features:
Cons:
Except for used in callback, C++ functors can also help to provide a Matlab liking access style to a matrix class. There is a example.
Functor can also be used to simulate defining a local function within a function. Refer to the question and another.
But a local functor can not access outside auto variables. The lambda (C++11) function is a better solution.