# File::open and File.open **URL:** \ **Category:** Ruby\ **Created:** [October 23, 2006, 10:17pm UTC](https://www.ruby-forum.com/t/file-open-and-file-open/73336 "2006-10-23T22:17:30Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![brad7a0](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/53a042/32.png) [@brad7a0](https://www.ruby-forum.com/u/brad7a0)\ **Post date:** [October 23, 2006, 10:17pm UTC](https://www.ruby-forum.com/t/file-open-and-file-open/73336/1 "2006-10-23T22:17:30Z")
What exactly is the difference between these? :: is a class method while . is an instance method??? If that is so, could someone clarify this with more detail? I use File.open, but File::open works too. However sometimes, this is not the case which adds to my confusion 🙂 For example, I can only do Digest::MD5.new() and not Digest.MD5.new() why is that? I don’t fully understand OO programming… perhaps this is my problem. Thank you for your time. ---
**Author:** ![brad7a0](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/53a042/32.png) [@brad7a0](https://www.ruby-forum.com/u/brad7a0)\ **Post date:** [October 23, 2006, 10:25pm UTC](https://www.ruby-forum.com/t/file-open-and-file-open/73336/2 "2006-10-23T22:25:55Z")
This is just Ruby syntax. It allows both :: and . as operators to class-level methods and variables. File::open and File.open are the exact same thing: a class method. Were it an instance method, you would have to do: file = File.new file.open aka, create an instance of the object, then call the function. For confusions sake, this is the same as: file = File::new file.open Jason ---
**Author:** ![brad7a0](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/53a042/32.png) [@brad7a0](https://www.ruby-forum.com/u/brad7a0)\ **Post date:** [October 23, 2006, 10:32pm UTC](https://www.ruby-forum.com/t/file-open-and-file-open/73336/3 "2006-10-23T22:32:07Z")
Brad T. wrote: > What exactly is the difference between these? :: is a class method while > is an instance method??? If that is so, could someone clarify this > with more detail? I use File.open, but File::open works too. The same method (a class method) is called. It is just a syntactic difference. So these are the same File.open(“foo”) … File::open(“foo”) … > However > sometimes, this is not the case which adds to my confusion 🙂 For > example, I can only do Digest::MD5.new() and not Digest.MD5.new() why is > that? Because the “.” cannot be used for constant lookup: irb(main):005:0\> File.NONBLOCK NoMethodError: undefined method `NONBLOCK’ for File:Class from (irb):5 from :0 irb(main):006:0\> File::NONBLOCK =\> 16384 “.” is reserved for method invocations. > I don’t fully understand OO programming… perhaps this is my problem. Nah, I think it is rather a minor ambiguity in the language that causes confusion. Usually people disambiguate it by using “.” for methods and “::” for constants only although they could use it for class methods as well. Kind regards ``` robert ``` ---
**Author:** ![brad7a0](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/53a042/32.png) [@brad7a0](https://www.ruby-forum.com/u/brad7a0)\ **Post date:** [October 23, 2006, 10:46pm UTC](https://www.ruby-forum.com/t/file-open-and-file-open/73336/4 "2006-10-23T22:46:49Z")
Robert K. schrieb: > ``` > from :0 > > ``` > > well. > I was bitten by it more than once > is there more to it … i mean ambiguity is usually not what one wants > to have in the language (at least not in the computer language 🙂 Regards, Daniel ---
**Author:** ![brad7a0](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/53a042/32.png) [@brad7a0](https://www.ruby-forum.com/u/brad7a0)\ **Post date:** [October 23, 2006, 10:36pm UTC](https://www.ruby-forum.com/t/file-open-and-file-open/73336/5 "2006-10-23T22:36:03Z")
Thank you Robert and Jason. I understand that much better now! ---
**Author:** ![brad7a0](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/53a042/32.png) [@brad7a0](https://www.ruby-forum.com/u/brad7a0)\ **Post date:** [October 24, 2006, 11:35pm UTC](https://www.ruby-forum.com/t/file-open-and-file-open/73336/6 "2006-10-24T23:35:13Z")
On 10/23/06, Jason R. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > This is just Ruby syntax. It allows both :: and . as operators to > class-level methods and variables. > > File::open and File.open are the exact same thing: a class method. Were it > an instance method, you would have to do: > > file = File.new > file.open And when talking about instance methods one sometimes uses classname#method name Since this is what ri understands when you ask for documentation: $ri to\_s More than one method matched your request. You can refine your search by asking for information on one of: ``` Method#to_s, Vector#to_s, Struct#to_s, Time#to_s, CGI::Cookie#to_s, Array#to_s, Matrix#to_s, MatchData#to_s, FalseClass#to_s, Pathname#to_s, Pathname#to_str, Rational#to_s, Hash#to_s, UnboundMethod#to_s, TrueClass#to_s, Module#to_s, Complex#to_s, Proc#to_s, Symbol#to_s, Symbol#to_sym, Exception#to_s, Exception#to_str, Bignum#to_s, Object#to_s, NilClass#to_s, Range#to_s, Date#to_s, NameError#to_s, Fixnum#to_s, Fixnum#to_sym, Float#to_s, String#to_s, String#to_str, String#to_sym, Regexp#to_s, Benchmark::Tms#to_s, URI::MailTo#to_s, URI::FTP#to_s, URI::Generic#to_s, Process::Status#to_s, Enumerable#to_set ``` ## $ri Object#to\_s ------------------------------------------------------------ Object#to\_s obj.to\_s =\> string ``` Returns a string representing _obj_. The default +to_s+ prints the object's class and an encoding of the object id. As a special case, the top-level object that is the initial execution context of Ruby programs returns ``main.'' ``` And, by the way, the open method in question is really a class method of IO rather than File, the File class inherits it. – Rick DeNatale My blog on Ruby [http://talklikeaduck.denhaven2.com/](http://talklikeaduck.denhaven2.com/) IPMS/USA Region 12 Coordinator [http://ipmsr12.denhaven2.com/](http://ipmsr12.denhaven2.com/) Visit the Project Mercury Wiki Site [http://www.mercuryspacecraft.com/](http://www.mercuryspacecraft.com/)