Understanding Ruby symbols

Symbols are one of the most misunderstood Ruby features. Let’s try to explain it as lay as possible.

The way I finally catched it is thinking about symbols as constants, whose value is the same as their name. The same as the integer 5 has the name 5 and the value 5, the symbol :thing has the name thing and the value “thing”. That’s why we can open an irb and do this:

irb(main):008:0> puts :myname
myname
=> nil
irb(main):009:0> :myname
=> :myname

Remember, symbols are not strings, therefore this is not possible:

irb(main):010:0> puts :myname + :othername
NoMethodError: undefined method `+' for :myname:Symbol
        from (irb):10

Instead we can get the value of the symbol, and convert it to a String to get this:

irb(main):011:0> puts :myname.to_s + :othername.to_s

mynameothername

=> nil

Very cute. So what’s the point?Symbols are mainly used to save memory. They have only one occurrence within a Ruby program, while strings with the same name are stored in different positions of memory every time. Check this:

irb(main):012:0> "string".object_id
=> 24125620
irb(main):013:0> "string".object_id
=> 24122170
irb(main):014:0> :string.object_id
=> 68258
irb(main):015:0* :string.object_id
=> 68258

And don’t forget that symbols are objects, just like everything in Ruby. They have a name and a string value, as we said, but an integer value to be recognized by the program as well, a value that is always constant. That brings many performance benefits (while comparing symbols, for example). They are like strings with a value that must not be changed.

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