Feature #20317: Removing the `allocate` method should cause `new` to fail

• Status: Open
• Assignee: -
• Target version: -

[tenderlovemaking (Aaron Patterson)]

When you remove the allocate method from a class the you can't allocate the class via the allocate method. However, you can allocate the class via the new method:

class Foo; end

Foo.singleton_class.undef_method(:allocate)

begin
  Foo.allocate # doesn't work, of course
rescue NoMethodError
end

begin
  Class.instance_method(:allocate).bind_call(Foo) # also doesn't work
rescue TypeError
end

Foo.new # works?

I think that when we remove the allocate method, the new method should also fail as there is no allocate method for new to call.

[jeremyevans0 (Jeremy Evans)]

The current behavior may be what you want if you want to ensure that new instances of the class have #initialize called on them. I suppose you could switch to making allocate private, but that still would allow for send(:allocate).

If we disallow .new if .allocate is undefined, should we also disallow #dup and #clone, both of which also need to allocate an object?

[tenderlovemaking (Aaron Patterson)]

jeremyevans0 (Jeremy Evans) wrote in #note-1:

The current behavior may be what you want if you want to ensure that new instances of the class have #initialize called on them. I suppose you could switch to making allocate private, but that still would allow for send(:allocate).

If we disallow .new if .allocate is undefined, should we also disallow #dup and #clone, both of which also need to allocate an object?

I think that makes sense. For example if you wanted to ensure a particular instance is a singleton:

class Foo; end

singleton = Foo.new

Foo.singleton_class.undef_method(:allocate)

# now there can only be one instance of `Foo` as new / allocate / dup / clone will raise

[Dan0042 (Daniel DeLorme)]

May I ask what is the use case for this?
The only reason I can think to undefine allocate is if you want to allow only a single instance of the class, and in that case include Singleton should do the job fine I think.