I have some functions in another plugin that I'd like to use. For example, I'm using the shortcodes plugin, and it has a function shortcode_atts that I want to use in my plugin.
If the method you want to access is public and in another service file, you could access it via the service layer:
The method you are using above, the Scope Resolution Operator should work fine without declaring the \Craft namespace, as the plugins should already be loaded into the Craft namespace. It's used for accessing static and constant methods:
In cases where you may be loading a library that is not in the Craft namespace, the syntax you're using above would be needed, where you declare the full namespace: