require vs. package-initialize?

Question

Ok, this is a noob question but I've digging into this enough that I think it's reasonable to ask :)

My question: In one's .emacs / init.el file, can one use require to load packages that were installed via the emacs package manager? If so, how?

I've seen a lot of code that says (require 'package-name), where 'package-name is something like 'powerline or 'rainbow-delimiters. The docs say that require basically tries to load the file, making sure not to load it twice. load works by looking in all the dirs in the load-path variable. This makes sense.

package-initialize goes through the list of installed packages and tries to run their auto-loads (and possibly defer further work) so that all the packages I've previously installed will work. This makes sense.

My confusion comes from my init.el: even though I've got packages installed (I can both see the packages in the filesystem and everything works great if I run (package-initialize)) using require doesn't work. Emacs complains that use-package isn't in load-path and I can confirm that if I don't call (package-initialize) that the load-path doesn't ilst ~/.emacs.d/elpa (or anything under it).

My question: In one's .emacs / init.el file, can one use require to load packages that were installed via the emacs package manager? If so, how? Do I need to manually add the directory to load-path? Should this normally work and I probably broke something with my personal init.el?

Any help would be appreciated - I'm clearly not getting something about how require works (and many other things too, I'm sure :) )

EDIT: My version: GNU Emacs 26.1 (build 1, x86_64-w64-mingw32) of 2018-05-30

Also, I'm playing around with use-package and am running into the same problem - if I add the directory containing use-package to load path without running (package-initialize) first then use-package fails because it wants to use the package bind-key. bind-key is installed but not listed in load-path so emacs barfs at that point.

Answer 1

You should be able to use require, yes, but only after running package-initialize (in Emacs≥27, this is done for you before the beginning of the .emacs file).

Note that for 99% of the packages require should not be needed (the autoloads should cause the package to be loaded as soon as you try to make use of it). E.g. if you have use-package installed as an ELPA package, then you should be able to use (use-package ...) without first doing (require 'use-package).

But again, that's only true after having run package-initialize.