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9.9 — Overloading the parenthesis operator

All of the overloaded operators you have seen so far let you define the type of the operator’s parameters, but not the number of parameters (which is fixed based on the type of the operator). For example, operator== always takes two parameters, whereas operator! always takes one. The parenthesis operator (operator()) is a particularly interesting operator in that it allows you to vary both the type AND number of parameters it takes.

There are two things to keep in mind: first, the parenthesis operator must be implemented as a member function. Second, in non-object-oriented C++, the () operator is used to call functions. In the case of classes, operator() is just a normal operator that calls a function (named operator()) like any other overloaded operator.

An example

Let’s take a look at an example that lends itself to overloading this operator:

Matrices are a key component of linear algebra, and are often used to do geometric modeling and 3D computer graphics work. In this case, all you need to recognize is that the Matrix class is a 4 by 4 two-dimensional array of doubles.

In the lesson on overloading the subscript operator, you learned that we could overload operator[] to provide direct access to a private one-dimensional array. However, in this case, we want access to a private two-dimensional array. Because operator[] is limited to a single parameter, it is not sufficient to let us index a two-dimensional array.

However, because the () operator can take as many parameters as we want it to have, we can declare a version of operator() that takes two integer index parameters, and use it to access our two-dimensional array. Here is an example of this:

Now we can declare a Matrix and access its elements like this:

which produces the result:

4.5

Now, let’s overload the () operator again, this time in a way that takes no parameters at all:

And here’s our new example:

which produces the result:

0

Because the () operator is so flexible, it can be tempting to use it for many different purposes. However, this is strongly discouraged, since the () symbol does not really give any indication of what the operator is doing. In our example above, it would be better to have written the erase functionality as a function called clear() or erase(), as matrix.erase() is easier to understand than matrix() (which could do anything!).

Having fun with functors

Operator() is also commonly overloaded to implement functors (or function object), which are classes that operate like functions. The advantage of a functor over a normal function is that functors can store data in member variables (since they are classes).

Here’s a simple functor:

Note that using our Accumulator looks just like making a normal function call, but our Accumulator object is storing an accumulated value.

You may wonder why we couldn’t do the same thing with a normal function and a static local variable to preserve data between function calls. We could, but because functions only have one global instance, we’d be limited to one. With functors, we can instantiate as many separate functor objects as we need.

Conclusion

Operator() is sometimes overloaded with two parameters to index multidimensional arrays, or to retrieve a subset of a one dimensional array (with the two parameters defining the subset to return). Anything else is probably better written as a member function with a more descriptive name.

Operator() is also often overloaded to create functors. Although simple functors (such as the example above) are fairly easily understood, functors are typically used in more advanced programming topics, and deserve their own lesson.

Quiz time

1) Write a class that holds a string. Overload operator() to return the substring that starts at the index of the first parameter, and includes however many characters are in the second parameter.

Hint: You can use array indices to access individual chars within the std::string
Hint: You can use operator+= to append something to a string

The following code should run:

This should print

world

Show Solution

9.10 -- Overloading typecasts
Index
9.8 -- Overloading the subscript operator

25 comments to 9.9 — Overloading the parenthesis operator

  • sergk

    This concept called “function objects” or “functors”.
    Functors in it’s essence is C++ style callbacks.

    • sergk

      I must say that functor callbacks is not perfect solution (actually, there is no such thing at all).

      For caller implementation one have to use either known interface - a base class with virtual operator(), or use template.

      • Wikipedia has some more information about functors here, for anybody interested in learning more about them.

        As I understand it, the basic idea of functors is that instead of passing a function pointer to a function to do a specific job, you pass a class with an overloaded () operator to do the same job. The advantage here is that classes can store information about the state of things, whereas functions can not.

        Overloading () for doing functors is an advanced-level C++ concept, so don’t sweat it if you’re just learning the language. 🙂

  • bla

    Is there a way to make the overloading of the parenthesis operator work usefully, when dealing with dynamically allocated objects?
    I did the overloading very similar to the way described here and the only way (that I found) to access my object is:

    (*object)(x,y)

    which looks quite ugly…

    • Alex

      No, you have to dereference the object first, as you suggest -- and since operator() has higher precedence than operator*, you have to put parenthesis around the dereference.

  • sunil_s

    Hi..
    In above example cMatrix is a class object and you have not overloaded << operator. still "std::cout << cMatrix(1, 2)" statement is working fine without error. how is it possible??

    • dismas256

      that is possible because the parenthesis operator in that class returns a reference to a built-in datatype “double” which of course has its own overloaded stream extraction operator.

  • Whew! It’s getting more complicated

  • Janez

    I don’t really get what is the "advantage" of overloading () operator, besides the name is shorter than if you would do the same thing with a member function. For other operators it was more logical but I don’t get this one (I understand how it works, but not why would someone use it).

    • Colin

      @Janez, in this context its a rather non-idiomatic use. However, when you need to treat a function as a first-class object it can be supremely useful. See the example written up at http://www.boost.org/doc/libs/1_58_0/doc/html/variant.html for a deeper understanding.

  • tata

    Hi, could someone check if the following comments/interpretations are correct?

    In essence, my question is:

    Do overloaded parenthesis operators rely solely on the presence of a class name to differentiate themselves from a call to a (default) constructor?

    Thanks

  • Arash

    Hi.    
    Correct this line.
    Thanks.

  • Ola Sh

    Hi Alex,

    Well done, another great tutorial. In the first example, we used operator[][] to access elements in data (data[row][col]). I am wondering how the overload operator[][] is implemented in C++, since its definition does not exist according to the Visual Studio compiler. Also, is there a way we can overload or define this operator? Thanks for your help.

    • Alex

      There is no operator[][]. However, if the result of operator[] is the appropriate type, operator[] can be applied to the result. So when we have data[row][col], this is really evaluating as (data[row])[col] (two sequential executions of operator[]).

  • Ola Sh

    Thank you. I was able to overload the subscript operator[] to return a pointer so I could use sequential executions of operator[]. Thanks.

  • Darren

    I’ve read about functors before but either the explanation was garbled or my brain was switch off. The concise explanation above just makes sense. A functor is a class that has an overloaded operator(), which performs the role of a function call. Neat.

    About the matrix example when you do:

    this, by row major convention, means set the matrix element at row 2, column 3 to 5. With out that convention the code above could mean setting column 2, row 3 to 5. But I guess it should be clear from the function prototype and/or comments as to the convention used.

  • Peter

    Hi Alex,

    My solution to quiz is

    I was wondering whether using push_back was less efficient than using +?

    • Alex

      It’s hard to say without knowing how push_back and operator+= are implemented. Likely they are implemented using the exact same code, so they probably perform the same.

  • Anddo

    Hey Alex,

    I’d like clearing something please, about this

    "You may wonder why we couldn’t do the same thing with a normal function and a static local variable to preserve data between function calls. We could, but because functions only have one global instance, we’d be limited to one. With functors, we can instantiate as many separate functor objects as we need."

    "As I understand it, the basic idea of functors is that instead of passing a function pointer to a function to do a specific job, you pass a class with an overloaded () operator to do the same job. The advantage here is that classes can store information about the state of things, whereas functions can not."

    What you meant by "state of things" ? We can use the member functions to store (edit) the class data, can’t we ? An example like the above with added member function to achieve same thing.

    • Alex

      I meant that classes can have member variable that can store values, whereas pure functions can not. Because functors are classes, you can add other member functions if you want.

  • Ali

    Hey Alex

    Firstly, thank you so much for these tutorials. These have been an incredible asset to me so far and I’m sure they will act as my primary reference in the future.

    One thing that I noticed is that you have a ";" at the end of the Constructor of your Accumulator class in the snippet of code right above your conclusion. This causes an error.

    I was wondering if you had an idea as to why is this the case?

    All the best. Thank you again for this series of up to date tutorials 🙂

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