As a Ruby or Rails developer, I think you probably have seen some code snippets like this:
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They are quite common in every Rails project, but what does the
:find_product mean? Are they some kind of variables or strings? Can we replace them with variables or strings?
It might be one of the most frequently asked questions while people learning Ruby or Rails, especially for those who came from other programming languages, and even experienced Rails developers may not be able to explain it very well.
This thing is called “Symbol”, which looks like a variable with a prefix colon.
The rule of naming a symbol is almost just like a normal variable, you can use English letters with numbers or underscore, like
:title32, or non-English character, such as
:おはよう, even space characters are also fine, but you have to quote them with single or double quotes if you use space as part of the name, like
What’s a Symbol
Symbol is a little hard to understand for Ruby/Rails beginner even have other programming languages background. Some people think it’s a variable, or just a name, but it’s not that simple. You can think the symbol is “an object with a name"：
:name symbol is an instance of Symbol class:
It can be used to represent something. For example, I might define some constants when I write iOS app with Objective-C:
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or using Enum:
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then I can use them just like this:
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I can do the same thing in Ruby, but I don’t have to define or declare anything in advance, because the symbol is “an object with a name”, I can use symbols to represent something directly:
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You might be curious about what the heck are
:complete? Actually they are just some objects which represent the state of the pending and complete Order. Just because symbol is “an object with a name”, symbol, as its name, can represent something with this object.
hmmmm… in this case, is that possible if I replace those symbols with strings? Sure you can.
What’s different between Symbol and Variable?
Variable is a name point to an object, like:
In the above syntax, it means the name
greeting is pointing to a string object
"Hello Ruby", the
greeting name can not live along if there’s no
"Hello Ruby" string object to be pointed at.
But Symbol can. Symbol is “an object with a name”, it can live along and can be used even doesn’t point to anything, just like the
:complete example above.
Actually you can NOT use symbol as a variable, that will cause a syntax error.
In fact, when you declare a new variable in Ruby, for example:
Ruby will also create a symbol named
:my_name in the background, let’s open
irb console and do some experiments:
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Not only defining a new variable will create new symbol, but also defining a new method or class.
What’s different between Symbol and String?
One of the most FAQ in my training courses, is “What’s the different between Symbol and String?”.
String is mutable, but Symbol isn’t.
Symbol is a little like string, symbol also has some methods like string, such as
downcase..etc. Sring is mutable, you can change the content of a string if you like. But symbol is not. Let’s try it in
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So, you can also think symbol as a kind of immutable string.
Symbol has better performance
In Ruby, when you create a new string, it will ask Ruby to allocate a new memory for it, check this out:
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object_id method will return the unique serial number in the Ruby world, it will vary with different computer or Ruby version.
In the Ruby world, the same object will has the same object id, and objects with the same object id means they are the same object.
And you can see the example above, even the same string object
"hello" has different object id and occupied some memory spaces, means they are 5 different objects in Ruby.
But let’s check out symbols:
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The result showed they have the same object id, means they’re the same object. When you use the
:hello symbol at the first time, Ruby will allocate a memory and create this symbol for you, when you try to access that symbol again, Ruby will retrive it from memory instead of generating a new one, so symbol will cause less memory usage.
Although symbol saves memory, but before Ruby 2.2, the memory can not be recycled automatically, you might have to restart the application to release those memory, so it might cause memory leak issue if you create lots of symbols. After Ruby 2.2, the Symbol GC(Garbage Collection) mechanism was introduced, those symbols who were dynamic generated by
intern methods can be recycled just like the other objects.
reference: Symbol GC
BTW, in the above example, if you “freeze” the string to make the string immutable, the object id will be the same.
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Comparsion of symbols is faster than strings
Let’s do a little benchmark and check the results:
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As you can see, comparing symbols is much faster than string, that’s because symbols are just comparing if they’re the same object(which have same object id). Let’s dig into some Ruby source files, symbol use
rb_obj_equal function to do comparison:
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and let’s check what happened when comparing strings:
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In string comparison, Ruby call
str_eql function, as you can see Ruby compares the letters one by one. So, the time complexity of string comparison is O(N), it will increase by the length of the string N, but symbol is constantly O(1) because it just compare if they are the same object.
String and Symbol are convertable
String and Symbol classes are both provide some methods to covert to each other:
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When should use symbol?
So, When should I use strings, when should use symbols?
Symbol as the key of Hash
:age are Symbols, for more information you can check this post。
Because the immutable of symbol and lookup and comparison performance is faster than string, it’s very suitable to be the key of Hash.
String has more powerful and useful methods than Symbol
Although you can cover symbol to string if you like, but after all symbol is not string, symbol doesn’t has methods as many as string, so, if you want to utilize those useful methods of String class, choose String.
And if you want to print out something on screen, choose String. Because symbols will be converted to strings implictly when you call some printing method such as
puts, or doing string interpolation, which cause some extra method calls and lose a little performance.
Use String or Symbol as parameters?
Check this example:
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It can still work if I replace
attr_accessor :name with
Some methods use strings as parameters, some use symbols, some can use both, then how can I know which one should I use? The answer is pretty simple, just READ THE FANTASIC MANUAL! Don’t know how to use? just look up the API manual.
Symbol is an easy concept but not so easy to understand, hope this post can help you know more about the symbol in Ruby. Feel free to leave your comment below if you have any question, or there’s something wrong about this post.