Is wordpress a library with a default GUI?

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First, I think that the WordPress-QT analogy is fragile at best, and will not lead to much useful understanding of WordPress, or to many useful comparisons between WordPress and QT. The former is a web-based content-management system application; the latter is an application/UI development framework. The output of WordPress is user-created content; the output of QT is an application.

Now, on to your specific points:

  1. WordPress is a Content Management System. It provides functions for creating, modifying, and managing content. It is not a “library”, but rather a fully-functional application.
  2. WordPress does not have a “sample front-end“; rather, WordPress includes a template system, that allows for considerable separation between the content-management and content-presentation layers.
  3. WordPress includes an administration panel that allows for user configuration of options and settings exposed by core, Plugins, and Themes.
  4. Definitely not correct. WordPress provides a Plugin API for extending core, and/or making changes outside of the default-exposed user-configuration options.

  5. WordPress does have a default Theme, among the few Themes packaged with core.

    Themes packaged with core are updated along with core, and repository-hosted Themes are updated whenever the developer submits an update to the Theme repository. Thus, any user changes to such Themes will be overwritten.

    Users may install custom Themes, that are entirely unaffected by core, Plugin, or Theme updates.

    Themes not under a user’s direct control with respect to updates should never be modified by the end user, unless the end user is comfortable forking a Theme, thereby making it a custom Theme, over which the user then controls updates.