RAII in Ruby (Or, How to Manage Resources in Ruby)
-
03-07-2019 - |
Question
I know it's by design that you can't control what happens when an object is destroyed. I am also aware of defining some class method as a finalizer.
However is the ruby idiom for C++'s RAII (Resources are initialized in constructor, closed in destructor)? How do people manage resources used inside objects even when errors or exceptions happen?
Using ensure works:
f = File.open("testfile")
begin
# .. process
rescue
# .. handle error
ensure
f.close unless f.nil?
end
but users of the class have to remember to do the whole begin-rescue-ensure chacha everytime the open method needs to be called.
So for example, I'll have the following class:
class SomeResource
def initialize(connection_string)
@resource_handle = ...some mojo here...
end
def do_something()
begin
@resource_handle.do_that()
...
rescue
...
ensure
end
def close
@resource_handle.close
end
end
The resource_handle won't be closed if the exception is cause by some other class and the script exits.
Or is the problem more of I'm still doing this too C++-like?
Solution
So that users don't "have to remember to do the whole begin-rescue-ensure chacha" combine rescue
/ensure
with yield
.
class SomeResource
...
def SomeResource.use(*resource_args)
# create resource
resource = SomeResource.new(*resource_args) # pass args direct to constructor
# export it
yield resource
rescue
# known error processing
...
ensure
# close up when done even if unhandled exception thrown from block
resource.close
end
...
end
Client code can use it as follows:
SomeResource.use(connection_string) do | resource |
resource.do_something
... # whatever else
end
# after this point resource has been .close()d
In fact this is how File.open
operates - making the first answer confusing at best (well it was to my work colleagues).
File.open("testfile") do |f|
# .. process - may include throwing exceptions
end
# f is guaranteed closed after this point even if exceptions are
# thrown during processing
OTHER TIPS
How about yield
ing a resource to a block? Example:
File.open("testfile") do |f|
begin
# .. process
rescue
# .. handle error
end
end
Or is the problem more of I'm still doing this too C++-like?
Yes it is since in C++ resource deallocation happens implicitly for everything on the stack. Stack unwound = resource destroyed = destructors called and from there things can be released. Since Ruby has no destructors there is no "do that when everything else is done with" place since grabage collection can be delayed several cycles from where you are. You do have finalizers but they are called "in limbo" (not everything is available to them) and they get called on GC.
Therefore if you are holding a handle to some resource that better be released you need to release it explicitly. Indeed the correct idiom to handle this kind of situation is
def with_shmoo
handle = allocate_shmoo
yield(handle)
ensure
handle.close
end
See http://www.rubycentral.com/pickaxe/tut_exceptions.html
In Ruby, you would use an ensure
statement:
f = File.open("testfile")
begin
# .. process
rescue
# .. handle error
ensure
f.close unless f.nil?
end
This will be familiar to users of Python, Java, or C# in that it works like try / catch / finally.