Ruby Failure while calling an objects own private method?

November 6, 2021

The method puts is defined as private in the module Kernel. So nobody can execute an other objects puts – that’s clear. But why isn’t the following exmaple running although self and slf have the same ID? Aren’t they the same Object?

>> slf = self
>> slf.puts
(irb):206:in `<main>': private method `puts' called for main:Object (NoMethodError)
        from C:/Ruby30-x64/lib/ruby/gems/3.0.0/gems/irb-1.3.5/exe/irb:11:in `<top (required)>'
        from C:/Ruby30-x64/bin/irb:25:in `load'
        from C:/Ruby30-x64/bin/irb:25:in `<main>'
>> p self.object_id
320
>> p slf.object_id
320

>Solution :

Private methods in Ruby can only be invoke via a receiverless message send, i.e. with self as the implicit receiver, or with the literal pseudo-variable self as the explicit receiver.

IOW, the two ways that are allowed are

foo(args) and
self.foo(args).

In your example, you are sending a message where the receiver is the local variable slf, i.e. neither implicit, nor the literal pseudo-variable self, therefore, invoking a private method is not allowed.