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If a class defines a finalize method, when an instance of that class is garbage-collected that method will be invoked:

class Foo
  def finalize
    # Invoked when Foo is garbage-collected
    # Use to release non-managed resources (ie. C libraries, structs)

Use this method to release resources allocated by external libraries that are not directly managed by Crystal garbage collector.

Examples of this can be found in IO::FileDescriptor#finalize or OpenSSL::Digest#finalize.


  • The finalize method will only be invoked once the object has been fully initialized via the initialize method. If an exception is raised inside the initialize method, finalize won't be invoked. If your class defines a finalize method, be sure to catch any exceptions that might be raised in the initialize methods and free resources.

  • Allocating any new object instances during garbage-collection might result in undefined behavior and most likely crashing your program.