Require, load and include --- what's the difference?

ruby-talk
1

Hello,

I'm browsing through some code, trying to familiarize myself with the
language (I have a C++ background, with only a #include), and I realize
that there are 3 calls that I'm trying to wrap my head around.

Here's my stab:

Require -- seems to include another file to gain access to other
classes, modules, and global data. This seems to be equivalent to a C++
include, with inclusion guards

Load -- seems to be the same as require, but will actually reload the
data every time it is called, where require will load once and cache the
file. This seems to be equivalent to C++ include, with out inclusion
guards.

Include -- seems to include a module name from a pre-determined path,
but not classes (unless encapsulated within a module). Would this be
similar to a C++ namespace being given the include directory at compile
time?

So a couple of things here, are my understandings of the 3 methods
close, if not, could someone please expand on them. The other is what's
the advantage of include vs require? I mean, why would I include a
module, rather than require it's filename?

I've read through the RDocs, but I still don't seem to understand.

I hope this post makes sense, let me know if I need to clarify.

Thanks

Matt

2

Hello,

I'm browsing through some code, trying to familiarize myself with the
language (I have a C++ background, with only a #include), and I realize
that there are 3 calls that I'm trying to wrap my head around.

Here's my stab:

Require -- seems to include another file to gain access to other
classes, modules, and global data. This seems to be equivalent to a C++
include, with inclusion guards

Right

Load -- seems to be the same as require, but will actually reload the
data every time it is called, where require will load once and cache the
file. This seems to be equivalent to C++ include, with out inclusion
guards.

Right

Include -- seems to include a module name from a pre-determined path,
but not classes (unless encapsulated within a module). Would this be
similar to a C++ namespace being given the include directory at compile
time?

include mixes in a module, so the methods defined on the module become
instance methods on the class.

module Foo
  def foo; "foo"; end
end

class Boring
  include Foo
end

Boring.new.foo => "foo"

So a couple of things here, are my understandings of the 3 methods
close, if not, could someone please expand on them. The other is what's
the advantage of include vs require? I mean, why would I include a
module, rather than require it's filename?

load and require are for loading a file into your code. include is
used to actually enhance an individual object (because a class itself
is an object).

Pat

···

On 12/17/06, matt <matt@kettlewell.net> wrote:

3

Hello,

Hi,

I'm browsing through some code, trying to familiarize myself with the
language (I have a C++ background, with only a #include), and I realize
that there are 3 calls that I'm trying to wrap my head around.

Here's my stab:

Require -- seems to include another file to gain access to other
classes, modules, and global data. This seems to be equivalent to a C++
include, with inclusion guards

Load -- seems to be the same as require, but will actually reload the
data every time it is called, where require will load once and cache the
file. This seems to be equivalent to C++ include, with out inclusion
guards.

Seems fine to me with the difference that #include is evaluated at
compile time whereas load, require works at run-time (there is no
compile-time in Ruby).

Include -- seems to include a module name from a pre-determined path,
but not classes (unless encapsulated within a module). Would this be
similar to a C++ namespace being given the include directory at compile
time?

You should see include as the ruby approach to multiple inheritance.
Include mix in a class the method defined in the given module. This
way you can share methods between different classes.

So a couple of things here, are my understandings of the 3 methods
close, if not, could someone please expand on them. The other is what's
the advantage of include vs require? I mean, why would I include a
module, rather than require it's filename?

require works on a file. It tells ruby to load a file if it is not
already loaded. It is closer to the source shell builtin than the
#include cpp directory from my point of view.
include adds the methods defined in a module into a class.

I hope this can help you.

Cheers,

···

On 12/17/06, matt <matt@kettlewell.net> wrote:

--
Nicolas Desprès

4

I think you've got it right.

require basically copies and pastes the referenced file into the source, but it only includes classes and modules, I don't think variables besides instance variables and class variables get imported.

include makes it so you don't have to use the scope resolution operator "::". It "unwraps" a module is how I like to think of it.

matt wrote:

···

Hello,

I'm browsing through some code, trying to familiarize myself with the
language (I have a C++ background, with only a #include), and I realize
that there are 3 calls that I'm trying to wrap my head around.

Here's my stab:

Require -- seems to include another file to gain access to other
classes, modules, and global data. This seems to be equivalent to a C++
include, with inclusion guards

Load -- seems to be the same as require, but will actually reload the
data every time it is called, where require will load once and cache the
file. This seems to be equivalent to C++ include, with out inclusion
guards.

Include -- seems to include a module name from a pre-determined path,
but not classes (unless encapsulated within a module). Would this be
similar to a C++ namespace being given the include directory at compile
time?

So a couple of things here, are my understandings of the 3 methods
close, if not, could someone please expand on them. The other is what's
the advantage of include vs require? I mean, why would I include a
module, rather than require it's filename?

I've read through the RDocs, but I still don't seem to understand.

I hope this post makes sense, let me know if I need to clarify.

Thanks

Matt

5

Require -- seems to include another file to gain access to other
classes, modules, and global data. This seems to be equivalent to a C++
include, with inclusion guards

Load -- seems to be the same as require, but will actually reload the
data every time it is called, where require will load once and cache the
file. This seems to be equivalent to C++ include, with out inclusion
guards.

Basically correct on these two.

Include -- seems to include a module name from a pre-determined path,
but not classes (unless encapsulated within a module). Would this be
similar to a C++ namespace being given the include directory at compile
time?

<snip>

what's
the advantage of include vs require? I mean, why would I include a
module, rather than require it's filename?

Include doesn't really have anything to do with files or paths. For
instance, the following code could all be in the same file:

## BEGIN ##
Woden:~ jwilger$ irb

module Foo
  def hello
    "defined in module Foo"
  end
end

=> nil

?> class Bar

  include Foo
end

=> Bar

?> x = Bar.new
=> #<Bar:0x12a1838>

x.hello

=> "defined in module Foo"
## END ##

When used this way, modules and include are Ruby's preferred way to
implement that which would be achieved via multiple inheritance in some
other languages.

···

On Dec 17, 9:19 am, matt <m...@kettlewell.net> wrote:

6

matt wrote:

Hello,

I'm browsing through some code, trying to familiarize myself with the
language (I have a C++ background, with only a #include), and I realize
that there are 3 calls that I'm trying to wrap my head around.

Here's my stab:

Require -- seems to include another file to gain access to other
classes, modules, and global data. This seems to be equivalent to a C++
include, with inclusion guards

Load -- seems to be the same as require, but will actually reload the
data every time it is called, where require will load once and cache the
file. This seems to be equivalent to C++ include, with out inclusion
guards.

  You got the point. There's a difference however, as you're including
directly the code and not the header as is done usually in C++.
Basically, you'll only ever need to use require, unless you need some
special kind of interaction with the (programmer-)user.

Include -- seems to include a module name from a pre-determined path,
but not classes (unless encapsulated within a module). Would this be
similar to a C++ namespace being given the include directory at compile
time?

  I don't really understand what you mean about the include directory.
Module serves two purposes: namespace separation and mixins. Mixins are
Ruby's version of multiple inheritance: whereas C++ authorize multiple
parents for classes, Ruby forbids this, but provides a mechanism for
classes that share common capabilities to share code as well. Let's make
this more explicit:

  Array is a descendant of Object, as are all Ruby classes. Hash is also
a descendant of Object. But both share some common capabilities, such as
they all provide a collection of Objects you might like to go through.
This is where modules come in: both include the Enumerable module, which
provides facilities to traverse collections of objects.

  In C++, the design would be different:

  class Object {};
  class Enumerable {};
  class Hash: public Object, Enumerable {};
  class Array: public Object, Enumerable {};

  In ruby,
  class Array
    include Enumerable
  end

  class Hash
    include Enumerable
  end

  There you go.

So a couple of things here, are my understandings of the 3 methods
close, if not, could someone please expand on them. The other is what's
the advantage of include vs require? I mean, why would I include a
module, rather than require it's filename?

  You need to require the filename first so that the module is
available. Then, you can include it in classes if you need their
functionality.

  Cheers,

  Vince

···

--
Vincent Fourmond, PhD student
http://vincent.fourmond.neuf.fr/