Why do I get undefined method `mktmpdir' for Dir:Class in irb using Ruby 2.6.3?

Question

https://ruby-doc.org/stdlib-2.6.3/libdoc/tmpdir/rdoc/Dir.html contains a description about the mktmpdir method of the Dir class.

mktmpdir(prefix_suffix=nil, *rest)
::mktmpdir creates a temporary directory.

An app uses it with no error but why it is not possible to run it in console / irb?

I have tested with Dir.new ... as well, below my attempt with Dir.mktmpdir.

irb(main):010:0> Dir.mktmpdir {|dir| dir is ".../d..." }
Traceback (most recent call last):
        5: from C:/Ruby26/bin/irb.cmd:31:in `<main>'
        4: from C:/Ruby26/bin/irb.cmd:31:in `load'
        3: from C:/Ruby26/lib/ruby/gems/2.6.0/gems/irb-1.0.0/exe/irb:11:in `<top (required)>'
        2: from (irb):10
        1: from (irb):10:in `rescue in irb_binding'
NoMethodError (undefined method `mktmpdir' for Dir:Class)

Dir.methods is not showing this method but only mkdir.

What is missing respectively what is the reason for this behavior?

Answer 1

You're confusing the patches to the Dir class that tmpdir in the standard library adds with the plain unpatched Dir class in core. If you look at the tmpdir package's documentation:

https://ruby-doc.com/stdlib/libdoc/tmpdir/rdoc/Dir.html

then you'll find Dir.mktmpdir (note the stdlib/libdoc/tmpdir) in the path. But if we look at Dir in core:

https://ruby-doc.org/core/Dir.html

you'll find all the usual class methods (chdir, chroot, ...) but no mktmpdir.

If you want to use mktmpdir then you have to require 'tmpdir':

Dir.method(:mktmpdir)
# NameError (undefined method `mktmpdir' for class `#<Class:Dir>')

require 'tmpdir'
Dir.method(:mktmpdir)
# #<Method: Dir.mktmpdir>