Overloading Perl Objects

In this article Dave Cross looks at a very powerful (but underused) area of Perl.

This article was originally the lead article on perl.com in July 2003.

All object-oriented programming languages have a feature called overloading, but in most of them this term means something different from what it means in Perl. Take a look at this Java example:

In this example, we have three methods called Fraction. Java, like many languages, is very strict about the number and type of arguments that you can pass to a function. We therefore need three different methods to cover the three possibilities. In the first example, the method takes two integers (a numerator and a denominator) and it returns a Fraction object based on those numbers. In the second example, the method takes an existing Fraction object as an argument and returns a copy (or clone) of that object. The final method takes no arguments and returns a default Fraction object, maybe representing 1/1 or 0/1. When you call one of these methods, the Java Virtual Machine determines which of the three methods you wanted by looking at the number and type of the arguments.

In Perl, of course, we are far more flexible about what arguments we can pass to a method. Therefore the same method can be used to handle all of the three cases from the Java example. (We’ll see an example of this in a short while.) This means that in Perl we can save the term “overloading” for something far more interesting — operator overloading.

Number::Fraction — The Constructor

Imagine you have a Perl object that represents fractions (or, more accurately, rational numbers, but we’ll call them fractions as we’re not all maths geeks). In order to handle the same situations as the Java class we mentioned above, we need to be able to run code like this:

To do this, we would write a constructor method like this:

As promised, there’s just one method here and it does everything that the three Java methods did and more even, so it’s a good example of why we don’t need method overloading in Perl. Let’s look at the various parts in some detail.

The method starts out just like most Perl object constructors. It grabs the class which is passed in as the first argument and then declares a variable called $self which will contain the object.

This is where we start to work out just how the method was called. We look at @_ to see how many arguments we have been given. If we’ve got two arguments then we assume that they are the numerator and denominator of the fraction. Notice that there’s also another check to ensure that both arguments contain only digits. If this check fails, we return undef from the constructor.

If we’ve been given just one argument, then there are a couple of things we can do. First we see if the argument is a reference, and if it is, we check that it’s a reference to another Number::Fraction object (or a subclass). If it’s the right kind of object then we get the numerators and denominators (using the accessor functions) and use them to call the two argument forms of new. It the argument is the wrong type of reference then we complain bitterly to the user.

If the single argument isn’t a reference then we assume it’s a string of the form num/den, which we can split apart to get the numerator and denominator of the fraction. Once more we check for the correct format using a regex and return undef if the check fails.

If we are given no arguments, then we just create a default fraction which is 0/1.

At the end of the constructor we do more of the normal OO Perl stuff. We bless the object into the correct class and return the reference to our caller. Between these two actions we pause to call the normalise method, which converts the fraction to its simplest form. For example, it will convert 12/16 to 3/4.

Number::Fraction — Doing Calculations

Having now created fraction objects, we will want to start doing calculations with them. For that we’ll need methods that implement the various mathematical functions. Here’s the add method:

Once more we try to handle a number of different types of arguments. We can add the following things to our fraction object:

  • Another object of the same class (or a subclass).
  • A string in the format num/den.
  • An integer. This is converted to a fraction with a denominator of 1.

This then allows us to write code like this:

In my opinion, this code looks pretty horrible. It also has a nasty, subtle bug. Can you spot it? (Hint: What will be in $half after running this code?) To tidy up this code we can turn to operator overloading.

Number::Fraction — Operator Overloading

The module overload.pm is a standard part of the Perl distribution. It allows your objects to define how they will react to a number of Perl’s operators. For example, we can add code like this to Number::Fraction:

Whenever a Number::Fraction is used as one of the operands to the + operator, the add method will be called instead. Code like:

is converted to:

This is getting closer, but it still has a serious problem. The add method works on the $half object. In general, however, that’s not how an assignment should work. If you were working with ordinary scalars and had code like:

You would be very surprised if this altered the value of $bar. Our objects need to work in the same way. We need to change our add method so that it doesn’t alter $self but instead returns the new fraction.

In this example, I’ve only shown one of the sections, but I hope it’s clear how it would work. Notice that I’ve also renamed $self and $delta to $l and $r. I find this makes more sense as we are working with the left and right operands of the + operator.

Overloading Non-Commutative Operators

We can now happily handle code like:

Our object will do the right thing — $three_quarters will end up as a Number::Fraction object that contains the value 3/4. What will happen if we write code like this?

The overload modules handle this case as well. If your object is either operand of one of the overloaded operators, then your method will be called. You get passed an extra argument which indicates whether your object was the left or right operand of the operator. This argument is false if your object is the left operand and true if it is the right operand.

For commutative operators you probably don’t need to take any notice of this argument as, for example:

is the same as:

However, for non-commutative operators (like - and /) you will need to do something like this:

Overloadable Operators

Just about any Perl operator can be overloaded in this way. This is a partial list:

  • Arithmetic: +, +=, -, -=, *, *=, /, /=, %, %=, **, **=, <<, <<=, >>, >>=, x, x=, ., .=
  • Comparison: <, <=, >, =>, ==, !=, <=> lt, le, gt, ge, eq, ne, cmp
  • Increment/Decrement: ++, -- (both pre- and post- versions)

A full list is given in overload.

It’s a very long list, but thankfully you rarely have to supply an implementation for more than a few operators. Perl is quite happy to synthesize (or autogenerate) many of the missing operators. For example:

  • ++ can be derived from +
  • += can be derived from +
  • - (unary) can be derived from – (binary)
  • All numeric comparisons can be derived from <=>
  • All string comparisons can be derived from cmp

Two other special operators give finer control over this autogeneration of methods. nomethod defines a subroutine that is called when no other function is found and fallback controls how hard Perl tries to autogenerate a method. fallback can have one of three values:

undef - Attempt to autogenerate methods and die if a method can’t be autogenerated. This is the default.
0 - Never try to autogenerate methods.
1 - Attempt to autogenerate methods but fall back on Perl’s default behaviour for the the object if a method can’t be autogenerated.

Here’s an example of an object that will die gracefully when an unknown operator is called. Notice that the nomethod subroutine is passed the usual three arguments (left operand, right operand, and the swap flag) together with an extra argument containing the operator that was used.

Three special operators are provided to control type conversion. They define methods to be called if the object is used in string, numeric, and boolean contexts. These operators are denoted by q{""}, 0+, and bool. Here’s how we can use these in Number::Fraction:

Now, when we print a Number::Fraction object, it will be displayed in num/den format. When we use the object in a numeric context, Perl will automatically convert it to its numeric equivalent.

We can use these type-conversion and fallback operators to cut down the number of operators we need to define even further.

Now, whenever our object is used where Perl is expecting a number and we haven’t already defined an overloading method, Perl will try to use our object as a number, which will, in turn, trigger our to_num method. This means that we only need to define operators where their behaviour will differ from that of a normal number. In the case of Number::Fraction, we don’t need to define any numeric comparison operators since the numeric value of the object will give the correct behaviour. The same is true of the string comparison operators if we define to_string.

Overloading Constants

We’ve come a long way with our overloaded objects. Instead of nasty code like:

we can now write code like:

There are still, however, two places where we need to use the full name of the class — when we load the module and when we create a new fraction object. We can’t do much about the first of these, but we can remove the need for that ugly new call by overloading constants.

You can use overload::constant to control how Perl interprets constants in your program. overload::constant expects a hash where the keys identify various kinds of constants and the values are subroutines which handle the constants. The keys can be any of integer (for integers), float (for floating point numbers), binary (for binary, octal, and hex numbers), q (for strings), and qr (for the constant parts of regular expressions).

When a constant of the right type is found, Perl will call the associated subroutine, passing it the string representation of the constant and the way that Perl would interpret the constant by default. Subroutines associated with q or qr also get a third argument — either qq, q, s, or tr –which indicates how the string is being used in the program.

As an example, here is how we would set up constant handlers so that strings of the form num/den are always converted to the equivalent Number::Fraction object:

We’ve defined a hash, %_const_handlers, which only contains one entry as we are only interested in strings. The associated subroutine calls the new method in the current package (which will be Number::Fraction or a subclass) passing it the string as found in the program source. If this string can be used to create a valid Number::Fraction object, a reference to that object is returned.

If a valid object isn’t returned then the subroutine returns its second argument, which is Perl’s default interpretation of the constant. As a result, any strings in the program that can be interpreted as a fraction are converted to the correct Number::Fraction object and other strings are left unchanged.

The constant handler is loaded as part of our package’s import subroutine. Notice that it is only loaded if the import subroutine is passed the optional argument :constants. This is because this is a potentially big change to the way that a program’s source code is interpreted so we only want to turn it on if the user wants it. Number::Fraction can be used in this way by putting the following line in your program:

If you don’t want the scary constant-refining stuff you can just use:

Also note that we’ve defined an unimport subroutine which removes the constant handler. An unimport subroutine is called when a program calls no Number::Fraction — it’s the opposite of use. If you’re going to make major changes to the way that Perl parses a program then it’s only polite to undo your changes if the programmer asks you to.

Conclusion

We’ve finally managed to get rid of most of the ugly class names from our code. We can now write code like this:

I hope you can agree that this has the potential to make code far easier to read and understand.

Number::Fraction is available on the CPAN. Please feel free to take a closer look at how it is implemented. If you come up with any more interesting overloaded modules, I’d love to hear about them.

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