Unable to add Internal Library to project even though its .pro file exists.

[enjoysmath]

See this screen shot? There's nothing listed in the Library: combobox. I have a folder selected that is in the build tree on the left and it has a functor.pro file inside of it.

How can I add an internal library to an Qt Widgets App?

[JKSH]

Here, "Internal" means "within the same SUBDIRS project". This is not directly documented but is implied at https://doc.qt.io/qtcreator/creator-project-qmake-libraries.html#example-of-adding-internal-libraries

(See also https://doc.qt.io/qtcreator/creator-project-creating.html#adding-subprojects-to-projects)

You need a top-level SUBDIRS project that contains both your main banana_cats_cpp project and your functor project:

# Root.pro

TEMPLATE = subdirs

SUBDIRS += \
    /path/to/banana_cats_cpp \
    /path/to/functor

Close everything, then re-open the Root project. In the Projects pane, right-click banana_cats_cpp (which should now be nested under Root) and select Add Library.... functor should now be selectable as an Internal Library.

Personally, I prefer to just use External Libraries

[enjoysmath]

@JKSH interesting, how would you do an external library plugin?

[JKSH]

@enjoysmath said in Unable to add Internal Library to project even though its .pro file exists.:

interesting, how would you do an external library plugin?

Oh, a plugin?

Typically, an application searches for (optional) plugins at runtime; you don't need to add plugin libraries to your .pro file. See https://doc.qt.io/qt-5/qtwidgets-tools-echoplugin-example.html for a simple example.

Adding libraries to your project (either internal or external libraries) is typically done for required dependencies, not for plugins.

Let's take a step back: What's the purpose of these plugins? Do you want a plugin library, or do you just want a "regular" library?