Begin Rescue and Ensure in Ruby

Question

I ve recently started programming in Ruby  and I am looking at exception handling   I was wondering if ensure was the Ruby equivalent of finally in C   Should I have   file   File open  myFile txt    w    begin   file  lt  lt     content   n  rescue    handle the error here ensure   file close unless file nil  end   or should I do this    store the file file   File open  myFile txt    w    begin   file  lt  lt     content   n    file close rescue    handle the error here ensure   file close unless file nil  end   Does ensure get called no matter what  even if an exception isn t raised

User · Answer

This is why we need ensure   def hoge   begin     raise   rescue       raise   raise again   ensure       puts  ensure    will be executed   end     puts  end of func    never be executed end

User · Answer

FYI  even if an exception is re-raised in the rescue section  the ensure block will be executed before the code execution continues to the next exception handler  For instance   begin   raise  Error    rescue   puts  test1    raise   Reraise exception ensure   puts  Ensure block  end

User · Answer

Yes  ensure ensures that the code is always evaluated  That s why it s called ensure  So  it is equivalent to Java s and C  s finally   The general flow of begin rescue else ensure end looks like this   begin     something which might raise an exception rescue SomeExceptionClass   gt  some variable     code that deals with some exception rescue SomeOtherException   gt  some other variable     code that deals with some other exception else     code that runs only if  no  exception was raised ensure     ensure that this code always runs  no matter what     does not change the final value of the block end   You can leave out rescue  ensure or else  You can also leave out the variables in which case you won t be able to inspect the exception in your exception handling code   Well  you can always use the global exception variable to access the last exception that was raised  but that s a little bit hacky   And you can leave out the exception class  in which case all exceptions that inherit from StandardError will be caught   Please note that this does not mean that all exceptions are caught  because there are exceptions which are instances of Exception but not StandardError  Mostly very severe exceptions that compromise the integrity of the program such as SystemStackError  NoMemoryError  SecurityError  NotImplementedError  LoadError  SyntaxError  ScriptError  Interrupt  SignalException or SystemExit    Some blocks form implicit exception blocks  For example  method definitions are implicitly also exception blocks  so instead of writing  def foo   begin             rescue             end end   you write just  def foo         rescue         end   or  def foo         ensure         end   The same applies to class definitions and module definitions   However  in the specific case you are asking about  there is actually a much better idiom  In general  when you work with some resource which you need to clean up at the end  you do that by passing a block to a method which does all the cleanup for you  It s similar to a using block in C   except that Ruby is actually powerful enough that you don t have to wait for the high priests of Microsoft to come down from the mountain and graciously change their compiler for you  In Ruby  you can just implement it yourself     This is what you want to do  File open  myFile txt    w   do  file    file puts content end    And this is how you might implement it  def File open filename  mode  r   perm nil  opt nil    yield filehandle   new filename  mode  perm  opt  ensure   filehandle amp  close end   And what do you know  this is already available in the core library as File open  But it is a general pattern that you can use in your own code as well  for implementing any kind of resource cleanup     la using in C   or transactions or whatever else you might think of   The only case where this doesn t work  if acquiring and releasing the resource are distributed over different parts of the program  But if it is localized  as in your example  then you can easily use these resource blocks     BTW  in modern C   using is actually superfluous  because you can implement Ruby-style resource blocks yourself   class File       static T open lt T gt  string filename  string mode  Func lt File  T gt  block                var handle   new File filename  mode           try                       return block handle                     finally                       handle Dispose                          Usage   File open  myFile txt    w    file    gt        file WriteLine contents

User · Answer

Yes  ensure is called in any circumstances  For more information see  Exceptions  Catch  and Throw  of the Programming Ruby book and search for  ensure

User · Answer

Yes  ensure like finally guarantees that the block will be executed  This is very useful for making sure that critical resources are protected e g  closing a file handle on error  or releasing a mutex

User · Answer

If you want to ensure a file is closed you should use the block form of File open   File open  myFile txt    w   do  file    begin     file  lt  lt     content   n    rescue    handle the error here   end end

User · Answer

Yes  ensure ENSURES it is run every time  so you don t need the file close in the begin block   By the way  a good way to test is to do   begin     Raise an error here   raise  Error    rescue    handle the error here ensure   p           inside ensure block  end   You can test to see if           inside ensure block  will be printed out when there is an exception  Then you can comment out the statement that raises the error and see if the ensure statement is executed by seeing if anything gets printed out

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