Nullability of arguments and return values

[paul35621]

In the generated Java code of the Qt classes, the annotations @Nullable and @NonNull are used to indicate if an object may be a null pointer. Sometimes these are incorrect, for example QUndoGroup::addStack and has a @Nullable argument, but crashes if null is passed.

What determines if @Nullable or @NonNull is used? Is this an automatic process or manual? If I find an incorrect instance, can I change that in the Jambi source code and do a pull request or something?

[omix]

Thank you for your question.
Nullability annotations are primarily for Kotlin, as Kotlin supports nullable and non-nullable types.

• Everywhere the C++ Qt API has a T* argument, QtJambiGenerator creates a Java @Nullable T argument.
• For every const V& with a copyable value type V the Java API is @NonNull V. However, you can assign null to it in Java. Java null is converted in the default-constructed value V().
• A C++ T& argument is mapped to Java @StrictNonNull T. This annotation causes Kotlin compiler to give a compilation error in case you assign null. If you send null in Java, a NullPointerException will appear.
• There are methods in Qt accepting a T* pointer but don't allow a null pointer. In all these cases, the QtJambiGenerator process needs to be modified by explicitly defining an argument as NoNullPointer.

the latter needs to be done manually. So thank you for addressing this issue. In fact, QUndoGroup::addStack does not accept null and the QtJambi API needs a fix.

[paul35621]

Thanks, this makes things clear. It would be ideal if the Qt people would add something like /* NonNull */ to pointer arguments and return values that cannot be null. But that takes some convincing. Another example I encountered in my code is the QWidget::addAction methods, which never return null.

[paul35621]

I made a post on the Qt forum about this.