From: Joel VanderWerf <vjoel@...>
Date: 2002-02-21T09:49:22+09:00
Subject: Re: confused about '::' vs '.' in method calls

Raphael Clancy wrote:
> 
> I must admit I'm a bit confused... probably because I never realy
> understood the scope resoultion opperator when I learned C++, but I'm used
> to seeing '::' when you want to show the scope of a method... Foo::Bar()
> means Bar is a method in Foo called like so aFoo.Bar()... now in ruby, '::'
> and '.' seem to be interchangeable... that is I can use both aFoo::Bar()
> and aFoo.Bar() to call Bar on aFoo. In 'Ruby in a nutshell' Matz puts heavy
> emphasis on using '::' for class methods, but the pick-axe book tends to
> favor '.' Does it really matter which way you go? Are '::' and '.'
> functionaly the same? Sorry if this is old hat, but I had a hard time
> making google search the archive for '::' and '.' ;)
> 
> Thanks!
> 
> R.

From the pickaxe chapter The Ruby Language:

=====
When a receiver is explicitly specified in a method invocation, it may
be separated from the method name using either a period ``.'' or two
colons ``::''. The only difference between these two forms occurs if the
method name starts with an uppercase letter. In this case, Ruby will
assume that a receiver::Thing method call is actually an attempt to
access a constant called Thing in the receiver unless the method
invocation has a parameter list between parentheses. 

Foo.Bar()         #  method call
Foo.Bar           #  method call
Foo::Bar()        #  method call
Foo::Bar          #  constant access
=====

Something that didn't sink in for me right away is that the :: operator
is dynamic in the first argument even when used for constant access. For
example:

irb(main):001:0> module M
irb(main):002:1>   K = 2
irb(main):003:1> end
2
irb(main):004:0> x = M
M
irb(main):005:0> x::K
2

But using the form 'x.K' is an error, because I never defined a method
K.