Why do two identical strings have the same objectid in Ruby?

As you know, in Ruby, two identical strings do not have the same object_id, while two are the same characters. For example:

irb(main):001:0> :george.object_id == :george.object_id
=> true
irb(main):002:0> "george".object_id == "george".object_id
=> false

      

However, my code below shows that two strings have the same value "one" with the same object_id.

class MyArray < Array
    def ==(x)
        comparison = Array.new()
        x.each_with_index{|item, i| comparison.push(item.object_id.equal?(self[i].object_id))}
        if comparison.include?(false) then
            false
        else
            true
        end
    end
end
class MyHash < Hash
    def ==(x)
         y = Hash[self.sort]
        puts y.class
        puts y
        x = Hash[x.sort]
        puts x.class
        puts x
puts "______"
        xkeys = MyArray.new(x.keys)
        puts xkeys.class
        puts xkeys.to_s
        puts xkeys.object_id

        puts xkeys[0].class
        puts xkeys[0]
        puts xkeys[0].object_id
puts "______"
        xvals = MyArray.new(x.values)
puts "______"
        selfkeys = MyArray.new(y.keys)
        puts selfkeys.class
        puts selfkeys.to_s
        puts selfkeys.object_id

        puts selfkeys[0].class
        puts selfkeys[0]
        puts selfkeys[0].object_id
puts "______"
        selfvals = MyArray.new(y.values)
puts xkeys.==(selfkeys)
puts xvals.==(selfvals)
    end
end


 a1 = MyHash[{"one" => 1, "two" => 2}]
 b1 = MyHash[{"one" => 1, "two" => 2}]
 puts a1.==(b1)

      

And get

Hash
{"one"=>1, "two"=>2}
Hash
{"one"=>1, "two"=>2}
______
MyArray
["one", "two"]
21638020
String
one
21641920
______
______
MyArray
["one", "two"]
21637580
String
one
21641920
______
true
true

      

As you can see from the result, 2 String objects have the same value "one" with the same object_id 21641920, whereas it is assumed to have a different ID. So can anyone give me hints or tell me how can I get different ids in this case? Regards.

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2 answers


When an object is String

used as a key in Hash

, the hash will duplicate and freeze the string internally and use that copy as its key.



Link: Hash#store

.

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Starting with ruby ​​2.2, strings used as keys in hash literals are frozen and deduplicated: the same string will be reused.

This is a performance optimization: by not allocating many copies of the same string facility, there are fewer objects to allocate and fewer to garbage collect.

Another way to see frozen string literals in action:



"foo".freeze.object_id == "foo".freeze.object_id

      

Will return true in ruby ​​versions> = 2.1

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