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Ruby Programming
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Freezing Objects

There are times when you've worked hard to make your object exactly right, and you'll be damned if you'll let anyone just change it. Perhaps you need to pass some kind of opaque object between two of your classes via some third-party object, and you want to make sure it arrives unmodified. Perhaps you want to use an object as a hash key, and need to make sure that no one modifies it while it's being used. Perhaps something is corrupting one of your objects, and you'd like Ruby to raise an exception as soon as the change occurs.

Ruby provides a very simple mechanism to help with this. Any object can be frozen by invoking Object#freeze . A frozen object may not be modified: you can't change its instance variables (directly or indirectly), you can't associate singleton methods with it, and, if it is a class or module, you can't add, delete, or modify its methods. Once frozen, an object stays frozen: there is no Object#thaw . You can test to see if an object is frozen using Object#frozen? .

What happens when you copy a frozen object? That depends on the method you use. If you call an object's clone method, the entire object state (including whether it is frozen) is copied to the new object. On the other hand, dup typically copies only the object's contents---the new copy will not inherit the frozen status.

str1 = "hello"
str1.freeze "hello"
str1.frozen? true
str2 = str1.clone
str2.frozen? true
str3 = str1.dup
str3.frozen? false

Although freezing objects may initially seem like a good idea, you might want to hold off doing it until you come across a real need. Freezing is one of those ideas that looks essential on paper but isn't used much in practice.


Ruby Programming
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