Does 'require' in elisp/lisp prevent from reloading libraries?

Question

For C/C++, people use #ifdef .. #endif technique to prevent reloading libraries, and Objective-C uses import to do the same thing.

How about lisp/elisp? If (require 'cl) is used before, and (require 'cl) is seen somewhere, lisp is clever enough not to load it again? Or, is there any way to prevent this reloading libraries?

Answer 1

No, elisp won't load it again. From the docs (C-h f require):

"If feature FEATURE is not loaded, load it from FILENAME."

The same is true for Common Lisp:

"The require function tests whether a module is already present (using a case-sensitive comparison); if the module is not present, require proceeds to load the appropriate file or set of files."