The autoload Method in Ruby

Ruby has an in-built module autoload, which comes into action whenever a specific module or a class is accessed or called upon from the parent or calling class or module. Upon receiving a call, this module registers the corresponding file path to the called module. This is performed when the first call to the specified module is made. This is an instance of a lazy loading mechanism where this module is loaded only upon explicitly invoking and calling the module.

Syntax:

autoload(mod-ch,'file-name.rb')

Here, mod-ch is the module/class to be called and file-name.rb is the file path to be called.

The following examples illustrate the use of the autoload method:

Example 1:

File name: f1.rb

Ruby

# Parent module
module A

  # First argument loads the module
  # B from the file f2.rb specified
  # in second argument
  autoload(:B, './f2.rb')

  # Prints the string before the
  # module B is loaded
  puts "Module B has not been loaded"

  # Explicitly calling the module B
  B

  # Prints the string after the
  # module B is loaded
  puts "Module B has been loaded"

# Module ends
end

File name: f2.rb

Ruby

# Child module
module B

  # Prints the string when the module
  puts 'Module B has started loading'

end

Upon execution of file f1.rb, the following code produces this output:

Module B has not been loaded
Module B has started loading
Module B has been loaded

Example 2:

File name: p.rb

Ruby

# Parent module
module P

  # autoload method called
  autoload(:C, './c.rb')

  # Check if the module is
  # registered in the namespace
  p autoload?(:C)

  # Making explicit call to module C
  C

  # Now checking if module C is loaded
  p autoload?(:C)

end

File name: c.rb

Ruby

# Child module
module C

 puts 'In progress'

end

Upon execution of file p.rb, the following code produces this output:

'./c.rb'
In progress
nil

Explanation: In the above example, when the explicit call to the module is not made, the autoload method returns the file path of the file containing the module. Upon calling the module, the contents of the module are printed, which is a string “In progress in this case”. After the module is loaded, the second call to the autoload function, returns the value nil because the module C is already loaded.

Working of the autoload Module:

Internally, the autoload method associates a constant value with a filename that is loaded. The constant is then referenced by the filename of which the path is specified. The call to this method flags the created constant had an undefined value.

File name: p.rb

Ruby

module A

  # Loading module C
  autoload(:C, './c.rb')

  # Accessing the constants
  # of the module
  p constants

end

File name: c.rb

Ruby

# Child module
module C
end

Upon execution of file p.rb, the following code produces this output:

[:C]

Here, the constant is automatically created even when an explicit call to the module is not made. The constant is created with the same name as the module. 

Usage of the autoload method

• The autoload module ensures if the specified module or class has been loaded and exists in the namespace.
• Depending on the execution of the program flow, the module is invoked.
• It also verified if the child or called module is registered in the program execution flow.