How does Ruby use require to load custom classes or modules from the outside?

For example, you write a class named SendMail and put it in a file named send_mail.rb,

If you attempt to reference this class in the same directory

Require "send_mail"

It's a waste of time, we need to add this paragraph

$: <"File. dirname (_ FILE __)"

This means that the current directory is temporarily added to the Environment path of the database. Now you can use the require statement to correctly introduce it. For another example, I want to put send_mail under the lib directory so that clearly see this

Various reference relationships can be written in this way

$: <"# {File. dirname (_ FILE _)}/lib"
Require "sent_mail"

Haha. It's simple. It's simple. If you don't understand it, you can't.