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Symbol in Ruby

In the article, we’re going to explore the following topics:

  • what’s a symbol in Ruby?
  • the Symbol class
  • symbols behind the scene

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What’s a Symbol in Ruby?

A symbol is a unique instance of the Symbol class which is generally used for identifying a specific resource. A resource can be:

  • a method
  • a variable
  • a hash key
  • a state
  • etc..

A symbol is unique because only one instance of the Symbol class can be created for a specific symbol in a running program

Here, we can see that the :pending symbol is only created once as the two calls to :pending.object_id return the same object identifier.

Symbols are often compared to strings. But the main difference between them relies on the fact that a new String object is created for each called string — even if they’re identical

Now that we’re more familiar with symbols let’s have a look to the Symbol class and the API that it provides.

The Symbol class

This class is part of the Ruby’s Core Library (a.k.a the core-lib).

This class is not publicly instantiable

irb> Symbol.new
NoMethodError (undefined method `new' for Symbol:Class)

Otherwise, a Symbol object is implicitly instantiated when a new symbol is declared

irb> :dummy_symbol.class
=> Symbol

Let’s have a look to its ancestor chain.

irb> Symbol.ancestors
=> [Symbol, Comparable, Object, Kernel, BasicObject]

The Symbol class inherits from the default parent class Object.

Note that it includes the Comparable module.

This class shares the exact same ancestor chain as the String and the Numeric classes.

Feel free to read the Ruby Object Model article if you are unfamiliar with the Object class and the ancestor chain.

Feel free to read The Comparable module article if you are unfamiliar with the Comparable module.

This class also provides a set of interesting instance methods to compare, modify and match symbols.

Most of the methods to modify and match symbols use the Symbol#to_s method in order to work with the String representation of the symbol.

Symbols behind the scene

As symbols are, for each of them, a unique instance of the Symbol class, then Ruby has to keep track of each of them to be able to ensure uniqueness.

To do so, Ruby provides an internal table named global_symbols which is in charge of keeping track of all the symbols of your running program.

Note that symbols are only put into memory once. This makes them very efficient to use. But they stay in memory until the program exits. This is true for all the Ruby versions < 2.2.0. Otherwise, Symbols are Garbage Collected.

The Symbol.all_symbols method returns an array that represents the content of the global_symbols table at the moment of the method call

First, we can see that the global_symbols table is not empty.

In effect, at program setup, this table is filled in with all the methods, variables and classes included in the Ruby’s Core Library. These resources are inserted in the table as symbol.

For example, the Struct class is part of the Ruby’s Core Library.

Then, we can see that a symbol that represents a resource is added to the global_symbols table when we define:

  • a new symbol
  • a new variable
  • a new method
  • a new class/module

Then, we can see that no new symbol is added to the table when we re-open the Hash class. This is because the :Hash symbol is already included in the global_symbols table. This mechanism is the same for symbols, variables and methods.

Note that this table is used any time that you’ve to deal with symbols.

Note that the == and === operators are using an object-level comparison.

But for all the other comparison’s methods, it uses the string representation of the symbol for comparison.

feel free to check this C macro for further information

Voilà !

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