Ruby – Accessing a Class’ instance variable from outside

class-variablesinstance-variablesrubyvariables

I understand (I think) the difference between class variables and instance variables of a class in Ruby.

I'm wondering how one can access the instance variables of a class from OUTSIDE that class.

From within (that is, in a class method instead of an instance method), it can be accessed directly, but from outside, is there a way to do MyClass.class.[@$#]variablename?

I don't have any specific reason for doing this, just learning Ruby and wondering if it is possible.

Best Answer

class MyClass

    @my_class_instance_var = "foo"

    class << self
        attr_accessor :my_class_instance_var
    end

end

puts MyClass::my_class_instance_var

The foregoing yields:

>> foo

I believe that Arkku demonstrated how to access class variables (@@), not class instance variables (@) from outside the class.

I drew the foregoing from this essay: Seeing Metaclasses Clearly

Related Solutions

Python – Are static class variables possible in Python

Variables declared inside the class definition, but not inside a method are class or static variables:

>>> class MyClass:
...     i = 3
...
>>> MyClass.i
3

As @millerdev points out, this creates a class-level i variable, but this is distinct from any instance-level i variable, so you could have

>>> m = MyClass()
>>> m.i = 4
>>> MyClass.i, m.i
>>> (3, 4)

This is different from C++ and Java, but not so different from C#, where a static member can't be accessed using a reference to an instance.

See what the Python tutorial has to say on the subject of classes and class objects.

@Steve Johnson has already answered regarding static methods, also documented under "Built-in Functions" in the Python Library Reference.

class C:
    @staticmethod
    def f(arg1, arg2, ...): ...

@beidy recommends classmethods over staticmethod, as the method then receives the class type as the first argument, but I'm still a little fuzzy on the advantages of this approach over staticmethod. If you are too, then it probably doesn't matter.

Bash – How to trim whitespace from a Bash variable

A simple answer is:

echo "   lol  " | xargs

Xargs will do the trimming for you. It's one command/program, no parameters, returns the trimmed string, easy as that!

Note: this doesn't remove all internal spaces so "foo bar" stays the same; it does NOT become "foobar". However, multiple spaces will be condensed to single spaces, so "foo bar" will become "foo bar". In addition it doesn't remove end of lines characters.

Best Answer

Related Solutions

Python – Are static class variables possible in Python

Bash – How to trim whitespace from a Bash variable

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