How to make instance variables private in Ruby? - ruby

Question

Is there any way to make instance variables "private"(C++ or Java definition) in ruby? In other words I want following code to result in an error.
class Base
def initialize()
#x = 10
end
end
class Derived < Base
def x
#x = 20
end
end
d = Derived.new

Answer 1

Like most things in Ruby, instance variables aren't truly "private" and can be accessed by anyone with d.instance_variable_get :#x.
Unlike in Java/C++, though, instance variables in Ruby are always private. They are never part of the public API like methods are, since they can only be accessed with that verbose getter. So if there's any sanity in your API, you don't have to worry about someone abusing your instance variables, since they'll be using the methods instead. (Of course, if someone wants to go wild and access private methods or instance variables, there isn’t a way to stop them.)
The only concern is if someone accidentally overwrites an instance variable when they extend your class. That can be avoided by using unlikely names, perhaps calling it #base_x in your example.

Answer 2

Never use instance variables directly. Only ever use accessors. You can define the reader as public and the writer private by:
class Foo
attr_reader :bar
private
attr_writer :bar
end
However, keep in mind that private and protected do not mean what you think they mean. Public methods can be called against any receiver: named, self, or implicit (x.baz, self.baz, or baz). Protected methods may only be called with a receiver of self or implicitly (self.baz, baz). Private methods may only be called with an implicit receiver (baz).
Long story short, you're approaching the problem from a non-Ruby point of view. Always use accessors instead of instance variables. Use public/protected/private to document your intent, and assume consumers of your API are responsible adults.

Answer 3

It is possible (but inadvisable) to do exactly what you are asking.
There are two different elements of the desired behavior. The first is storing x in a read-only value, and the second is protecting the getter from being altered in subclasses.
Read-only value
It is possible in Ruby to store read-only values at initialization time. To do this, we use the closure behavior of Ruby blocks.
class Foo
def initialize (x)
define_singleton_method(:x) { x }
end
end
The initial value of x is now locked up inside the block we used to define the getter #x and can never be accessed except by calling foo.x, and it can never be altered.
foo = Foo.new(2)
foo.x # => 2
foo.instance_variable_get(:#x) # => nil
Note that it is not stored as the instance variable #x, yet it is still available via the getter we created using define_singleton_method.
Protecting the getter
In Ruby, almost any method of any class can be overwritten at runtime. There is a way to prevent this using the method_added hook.
class Foo
def self.method_added (name)
raise(NameError, "cannot change x getter") if name == :x
end
end
class Bar < Foo
def x
20
end
end
# => NameError: cannot change x getter
This is a very heavy-handed method of protecting the getter.
It requires that we add each protected getter to the method_added hook individually, and even then, you will need to add another level of method_added protection to Foo and its subclasses to prevent a coder from overwriting the method_added method itself.
Better to come to terms with the fact that code replacement at runtime is a fact of life when using Ruby.

Answer 4

Unlike methods having different levels of visibility, Ruby instance variables are always private (from outside of objects). However, inside objects instance variables are always accessible, either from parent, child class, or included modules.
Since there probably is no way to alter how Ruby access #x, I don't think you could have any control over it. Writing #x would just directly pick that instance variable, and since Ruby doesn't provide visibility control over variables, live with it I guess.
As #marcgg says, if you don't want derived classes to touch your instance variables, don't use it at all or find a clever way to hide it from seeing by derived classes.

Answer 5

It isn't possible to do what you want, because instance variables aren't defined by the class, but by the object.
If you use composition rather than inheritance, then you won't have to worry about overwriting instance variables.

Answer 6

If you want protection against accidental modification. I think attr_accessor can be a good fit.
class Data
attr_accessor :id
private :id
end
That will disable writing of id but would be readable. You can however use public attr_reader and private attr_writer syntax as well. Like so:
class Data
attr_reader :id
private
attr_writer :id
end

Answer 7

I know this is old, but I ran into a case where I didn't as much want to prevent access to #x, I did want to exclude it from any methods that use reflection for serialization. Specifically I use YAML::dump often for debug purposes, and in my case #x was of class Class, which YAML::dump refuses to dump.
In this case I had considered several options
Addressing this just for yaml by redefining "to_yaml_properties"
def to_yaml_properties
super-["#x"]
end
but this would have worked just for yaml and if other dumpers (to_xml ?) would not be happy
Addressing for all reflection users by redefining "instance_variables"
def instance_variables
super-["#x"]
end
Also, I found this in one of my searches, but have not tested it as the above seem simpler for my needs
So while these may not be exactly what the OP said he needed, if others find this posting while looking for the variable to be excluded from listing, rather than access - then these options may be of value.