In this third blog post of the Model/View Drag and Drop series (part 1 and part 2), the idea is to implement dropping onto items, rather than in between items. QListWidget and QTableWidget have out of the box support for replacing the value of existing items when doing that, but there aren't many use cases for that. What is much more common is to associate a custom semantic to such a drop. For instance, the examples detailed below show email folders and their contents, and dropping an email onto another folder will move (or copy) the email into that folder.

Initial state, the email is in the inbox

Dragging the email onto the Customers folder

Dropping the email

The email is now in the customers folder

With Model/View separation

Example code can be found here for flat models and here for tree models.

Setting up the view on the drag side

☑ Call view->setDragDropMode(QAbstractItemView::DragOnly)
unless of course the same view should also support drops. In our example, only emails can be dragged, and only folders allow drops, so the drag and drop sides are distinct.

☑ Call view->setDragDropOverwriteMode(...)
true if moving should clear cells, false if moving should remove rows.
Note that the default is true for QTableView and false for QListView and QTreeView. In our example, we want to remove emails that have been moved elsewhere, so false is correct.

☑ Call view->setDefaultDropAction(Qt::MoveAction) so that the drag defaults to a move and not a copy, adjust as needed

Setting up the model on the drag side

To implement dragging items out of a model, you need to implement the following -- this is very similar to the section of the same name in the previous blog post, obviously:

☑ Reimplement flags() to add Qt::ItemIsDragEnabled in the case of a valid index

☑ Reimplement supportedDragActions() to return Qt::MoveAction | Qt::CopyAction or whichever you want to support (the default is CopyAction only).

☑ Reimplement mimeData() to serialize the complete data for the dragged items. If the views are always in the same process, you can get away with serializing only node pointers (if you have that) and application PID (to refuse dropping onto another process). See the previous part of this blog series for more details.

☑ Reimplement removeRows(), it will be called after a successful drop with MoveAction. An example implementation looks like this:

Setting up the view on the drop side

☑ Call view->setDragDropMode(QAbstractItemView::DropOnly) unless of course it supports dragging too. In our example, we can drop onto email folders but we cannot reorganize the folders, so DropOnly is correct.

Setting up the model on the drop side

To implement dropping items into a model's existing items, you need to do the following:

☑ Reimplement flags()
For a valid index (and only in that case), add Qt::ItemIsDropEnabled. As you can see, you can also restrict drops to column 0, which can be more sensible when using QTreeView (the user should drop onto the folder name, not onto the folder size).

☑ Reimplement supportedDropActions() to return Qt::MoveAction | Qt::CopyAction or whichever you want to support (the default is CopyAction only).

☑ Reimplement mimeTypes() - the list should include the MIME type used by the drag model.

☑ Reimplement dropMimeData()
to deserialize the data and handle the drop.
This could mean calling setData() to replace item contents, or anything else that should happen on a drop: in the email example, this is where we copy or move the email into the destination folder. Once you're done, return true, so that the drag side then deletes the dragged rows by calling removeRows() on its model.

Using item widgets

Example code:

• QListWidget
• QTableWidget
• QTreeWidget

On the "drag" side

☑ Call widget->setDragDropMode(QAbstractItemView::DragOnly) or DragDrop if it should support both

☑ Call widget->setDefaultDropAction(Qt::MoveAction) so that the drag defaults to a move and not a copy, adjust as needed

☑ Reimplement Widget::mimeData() to serialize the complete data for the dragged items. If the views are always in the same process, you can get away with serializing only item pointers and application PID (to refuse dropping onto another process). In our email folders example we also serialize the pointer to the source folder (where the emails come from) so that we can detect dropping onto the same folder (which should do nothing).

To serialize pointers in QDataStream, cast them to quintptr, see the example code for details.

On the "drop" side

☑ Call widget->setDragDropMode(QAbstractItemView::DropOnly) or DragDrop if it should support both

☑ Call widget->setDragDropOverwriteMode(true) for a minor improvement: no forbidden cursor when moving the drag between folders. Instead Qt only computes drop positions which are onto items, as we want here.

☑ Reimplement Widget::mimeTypes() and return the same name as the one used on the drag side's mimeData

☑ Reimplement Widget::dropMimeData() (note that the signature is different between QListWidget, QTableWidget and QTreeWidget) This is where you deserialize the data and handle the drop. In the email example, this is where we copy or move the email into the destination folder.

Make sure to do all of the following:

• any necessary behind the scenes work (in our case, moving the actual email)
• updating the UI (creating or deleting items as needed)

This is a case where proper model/view separation is actually much simpler.

Improvements to Qt

While writing and testing these code examples, I improved the following things in Qt, in addition to those listed in the previous blog posts:

• QTBUG-2553 QTreeView with setAutoExpandDelay() collapses items while dragging over it, fixed in Qt 6.8.1

Conclusion

I hope you enjoyed this blog post series and learned a few things.

In the previous blog, you learned all about moving items within a single view, to reorder them.

In part 2, we are still talking about moving items, and still about inserting them between existing items (never overwriting items) but this time the user can move items from one view to another. A typical use case is a list of available items on the left, and a list of selected items on the right (one concrete example would be to let the user customize which buttons should appear in a toolbar). This also often includes reordering items in the right-side list, the good news being that this comes for free (no extra code needed).

With Model/View separation

Example code for flat models and example code for tree models.

Setting up the view on the drag side

To allow dragging items out of the view, make sure to do the following:

☑ Call view->setDragDropMode(QAbstractItemView::DragOnly) (or DragDrop if it should support both).

☑ Call view->setDragDropOverwriteMode(false) so that QTableView calls removeRows when moving rows, rather than just clearing their cells

☑ Call view->setDefaultDropAction(Qt::MoveAction) so it's a move and not a copy

Setting up the model on the drag side

To implement dragging items out of a model, you need to implement the following:

More precisely, the check-list is the following:

☑ Reimplement flags() to add Qt::ItemIsDragEnabled in the case of a valid index

☑ Reimplement supportedDragActions() to return Qt::MoveAction

☑ Reimplement mimeData() to serialize the complete data for the dragged items. If the views are always in the same process, you can get away with serializing only node pointers (if you have that, e.g. for tree models) and application PID (to refuse dropping onto another process). Otherwise you can encode the actual data, like this:

s_mimeType is the name of the type of data (make up a name, it usually starts with application/x-)

☑ Reimplement removeRows(), it will be called after a successful drop. For instance, if your data is in a vector called m_data, the implementation would look like this:

Setting up the view on the drop side

☑ Call view->setDragDropMode(QAbstractItemView::DragDrop) (already done if both views should support dragging and dropping)

Setting up the model on the drop side

To implement dropping items into a model (between existing items), you need to implement the following:

☑ Reimplement supportedDropActions() to return Qt::MoveAction

☑ Reimplement flags()
For a valid index, make sure Qt::ItemIsDropEnabled is NOT set (except for tree models where we need to drop onto items in order to insert a first child).
For the invalid index, add Qt::ItemIsDropEnabled, to allow dropping between items.

☑ Reimplement mimeTypes() and return the name of the MIME type used by the mimeData() function on the drag side.

☑ Reimplement dropMimeData()
to deserialize the data and insert new rows.
In the special case of in-process tree models, clone the dragged nodes.
In both cases, once you're done, return true, so that the drag side then deletes the dragged rows by calling removeRows() on its model.

Using item widgets

Example code can be found following this link.

For all kinds of widgets

On the "drag" side:

☑ Call widget->setDragDropMode(QAbstractItemView::DragOnly) or DragDrop if it should support both

☑ Call widget->setDefaultDropAction(Qt::MoveAction) so the drag starts as a move right away

On the "drop" side:

☑ Call widget->setDragDropMode(QAbstractItemView::DropOnly) or DragDrop if it should support both

☑ Reimplement supportedDropActions() to return only Qt::MoveAction

Additional requirements for QTableWidget

When using QTableWidget, in addition to the common steps above you need to:

On the "drag" side:

☑ Call item->setFlags(item->flags() & ~Qt::ItemIsDropEnabled); for each item, to disable dropping onto items.

☑ Call widget->setDragDropOverwriteMode(false) so that after a move the rows are removed rather than cleared

On the "drop" side:

☑ Call widget->setDragDropOverwriteMode(false) so that it inserts rows instead of replacing cells (the default is false for the other views anyway)

☑ Another problem is that the items created by a drop will automatically get the Qt::ItemIsDropEnabled flag, which you don't want. To solve this, use widget->setItemPrototype() with an item that has the right flags (see the example).

Additional requirements for QTreeWidget

When using QTreeWidget, you cannot disable dropping onto items (which creates a child of the item).

You could call item->setFlags(item->flags() & ~Qt::ItemIsDropEnabled); on your own items, but when QTreeWidget creates new items upon a drop, you cannot prevent them from having the flag Qt::ItemIsDropEnabled set. The prototype solution used above for QTableWidget doesn't exist for QTreeWidget.

This means, if you want to let the user build and reorganize an actual tree, you can use QTreeWidget. But if you just want a flat multi-column list, then you should use QTreeView (see previous section on model/view separation).

Addendum: Move/copy items between views

If the user should be able to choose between copying and moving items, follow the previous section and make the following changes.

With Model/View separation

On the "drag" side:

☑ Call view->setDefaultDropAction(...) to choose whether the default should be move or copy. The user can press Shift to force a move, and Ctrl to force a copy.

☑ Reimplement supportedDragActions() in the model to return Qt::MoveAction | Qt::CopyAction

On the "drop" side:

☑ Reimplement supportedDropActions() in the model to return Qt::MoveAction | Qt::CopyAction

The good news is that there's nothing else to do.

Using item widgets

On the "drag" side:

☑ Call widget->setDefaultDropAction(...) to choose whether the default should be move or copy. The user can press Shift to force a move, and Ctrl to force a copy.

Until Qt 6.10 there was no setSupportedDragActions() method in the item widget classes (that was QTBUG-87465, I implemented it for 6.10). Fortunately the default behavior is to use what supportedDropActions() returns so if you just want move and copy in both, reimplementing supportedDropActions() is enough.

On the "drop" side:

☑ Reimplement supportedDropActions() in the item widget class to return Qt::MoveAction | Qt::CopyAction

The good news is that there's nothing else to do.

Improvements to Qt

While writing and testing these code examples, I improved the following things in Qt:

• QTBUG-1387 "Drag and drop multiple columns with item views. Dragging a row and dropping it in a column > 0 creates multiple rows.", fixed in 6.8.1
• QTBUG-36831 "Drop indicator painted as single pixel when not shown" fixed in 6.8.1
• QTBUG-87465 ItemWidgets: add supportedDragActions()/setSupportedDragActions(), implemented in 6.10

Conclusion

In the next blog post of this series, you will learn how to move (or copy) onto existing items, rather than between them.

This blog series is all about implementing drag-and-drop in the Qt model/view framework. In addition to complete code examples, you'll find checklists that you can go through to make sure that you did not forget anything in your own implementation, when something isn't working as expected.

At first, we are going to look at Drag and Drop within a single view, to change the order of the items. The view can be a list, a table or a tree, there are very little differences in what you have to do.

The main question, however, is whether you are using QListView/QTableView/QTreeView on top of a custom item model, or QListWidget/QTableWidget/QTreeWidget with items in them. Let's explore each one in turn.

With Model/View separation

The code being discussed here is extracted from the example. That example features a flat model, while this example features a tree model. The checklist is the same for these two cases.

Setting up the view

☑ Call view->setDragDropMode(QAbstractItemView::InternalMove) to enable the mode where only moving within the same view is allowed

☑ When using QTableView, call view->setDragDropOverwriteMode(false) so that it inserts rows instead of replacing cells (the default is false for the other views anyway)

Adding drag-n-drop support to the model

For a model being used in QListView or QTableView, all you need is something like this:

The checklist for the changes you need to make in your model is therefore the following:

☑ Reimplement flags()
For a valid index, add Qt::ItemIsDragEnabled and make sure Qt::ItemIsDropEnabled is NOT set (except for tree models where we need to drop onto items in order to insert a first child). \

☑ Reimplement mimeTypes() and make up a name for the mimetype (usually starting with application/x-)

☑ Reimplement supportedDragActions() to return Qt::MoveAction

☑ Reimplement supportedDropActions() to return Qt::MoveAction

☑ Reimplement moveRows()

Note that this approach is only valid when using QListView or, assuming Qt >= 6.8.0, QTableView - see the following sections for details.

In a model that encapsulates a QVector called m_data, the implementation of moveRows can look like this:

QTreeView does not call moveRows

QTreeView does not (yet?) call moveRows in the model, so you need to:

☑ Reimplement mimeData() to encode row numbers for flat models, and node pointers for tree models

☑ Reimplement dropMimeData() to implement the move and return false (meaning: all done)

Note that this means a move is in fact an insertion and a deletion, so the selection isn't automatically updated to point to the moved row(s).

QTableView in Qt < 6.8.0

I implemented moving of rows in QTableView itself for Qt 6.8.0, so that moving rows in a table view is simpler to implement (one method instead of two), more efficient, and so that selection is updated. If you're not yet using Qt >= 6.8.0 then you'll have to reimplement mimeData() and dropMimeData() in your model, as per the previous section.

This concludes the section on how to implement a reorderable view using a separate model class.

Using item widgets

The alternative to model/view separation is the use of the item widgets (QListWidget, QTableWidget or QTreeWidget) which you populate directly by creating items.

Here's what you need to do to allow users to reorder those items.

Example code can be found following this link.

Reorderable QListWidget

☑ Call listWidget->setDragDropMode(QAbstractItemView::InternalMove) to enable the mode where only moving within the same view is allowed

For a QListWidget, this is all you need. That was easy!

Reorderable QTableWidget

When using QTableWidget:

☑ Call tableWidget->setDragDropMode(QAbstractItemView::InternalMove)

☑ Call tableWidget->setDragDropOverwriteMode(false) so that it inserts rows instead of replacing cells

☑ Call item->setFlags(item->flags() & ~Qt::ItemIsDropEnabled); on each item, to disable dropping onto items

Note: Before Qt 6.8.0, QTableWidget did not really support moving rows. It would instead move data into cells (like Excel). The example code shows a workaround, but since it calls code that inserts a row and deletes the old one, header data is lost in the process. My changes in Qt 6.8.0 implement support for moving rows in QTableWidget's internal model, so it's all fixed there. If you really need this feature in older versions of Qt, consider switching to QTableView.

Reorderable QTreeWidget

When using QTreeWidget:

☑ Call tableWidget->setDragDropMode(QAbstractItemView::InternalMove)

☑ Call item->setFlags(item->flags() & ~Qt::ItemIsDropEnabled); on each item, to disable dropping onto items

Conclusion about reorderable item widgets

Of course, you'll also need to iterate over the items at the end to grab the new order, like the example code does. As usual, item widgets lead to less code to write, but the runtime performance is worse than when using model/view separation. So, only use item widgets when the number of items is small (and you don't need proxy models).

Improvements to Qt

While writing and testing these code examples, I improved the following things in Qt 6.8:

• QTBUG-13873 / QTBUG-101475 - QTableView: implement moving rows by drag-n-drop
• QTBUG-69807 - Implement QTableModel::moveRows
• QTBUG-130045 - QTableView: fix dropping between items when precisely on the cell border
• QTBUG-1656 - Implement full-row drop indicator when the selection behavior is SelectRows

Conclusion

I hope this checklist will be useful when you have to implement your own reordering of items in a model or an item-widget. Please post a comment if anything appears to be incorrect or missing.

In the next blog post of this series, you will learn how to move (or even copy) items from one view to another.

Mozilla is the maker of the famous Firefox web browser and the birthplace of the likes of Rust and Servo (read more about Embedding the Servo Web Engine in Qt).

Firefox is a huge, multi-platform, multi-language project with 21 million lines of code back in 2020, according to their own blog post. Navigating in projects like those is always a challenge, especially at the cross-language boundaries and in platform-specific code.

To improve working with the Firefox code-base, Mozilla hosts an online code browser tailored for Firefox called Searchfox. Searchfox analyzes C++, JavaScript, various IDLs (interface definition languages), and Rust source code and makes them all browsable from a single interface with full-text search, semantic search, code navigation, test coverage report, and git blame support. It's the combination of a number of projects working together, both internal to Mozilla (like their Clang plugin for C++ analysis) and external (such as rust-analyzer maintained by Ferrous Systems).

It takes a whole repository in and separately indexes C++, Rust, JavaScript and now Java and Kotlin source code. All those analyses are then merged together across platforms, before running a cross-reference step and building the final index used by the web front-end available at searchfox.org.

Mozilla asked KDAB to help them with adding Java and Kotlin support to Searchfox in prevision of the merge of Firefox for Android into the main mozilla-central repository and enhance their C++ support with macro expansions. Let's dive into the details of those tasks.

Java/Kotlin Support

Mozilla merged the Firefox for Android source code into the main mozilla-central repository that Searchfox indexes. To add support for that new Java and Kotlin code to Searchfox, we reused open-source tooling built by Sourcegraph around the SemanticDB and SCIP code indexing formats. (Many thanks to them!)

Sourcegraph's semanticdb-javac and semanticdb-kotlinc compiler plugins are integrated into Firefox's CI system to export SemanticDB artifacts. The Searchfox indexer fetches those SemanticDB files and turns them into a SCIP index, using scip-semanticdb. That SCIP index is then consumed by the existing Searchfox-internal scip-indexer tool.

In the process, a couple of upstream contributions were made to rust-analyzer (which also emits SCIP data) and scip-semanticdb.

A few examples of Searchfox at work:

• Searching for the Java class: org.mozilla.geckoview.GeckoSession$PromptDelegate$AutocompleteRequest shows the definition, a superclass, some uses in Java source code and some uses in Kotlin tests.
• Searching for the Java interface method: org.mozilla.geckoview.Autocomplete$StorageDelegate$onAddressFetch shows the definition, a couple of users, and a couple of implementers across Java and Kotlin code.
• Querying the callers of a method with up to 2 indirections: calls-to:'org::mozilla::geckoview::Autofill::Session::getDefaultDimensions' depth:2

If you want to dive into more details, see the feature request on Bugzilla, the implementation and further discussion on GitHub and the release announcement on the mozilla dev-platform mailing list.

Java/C++ Cross-language Support

GeckoView is an Android wrapper around Gecko, the Firefox web engine. It extensively uses cross-language calls between Java and C++.

Searchfox already had support for cross-language interfaces, thanks to its IDL support. We built on top of that to support direct cross-language calls between Java and C++.

First, we identified the different ways the C++ and Java code interact and call each other. There are three ways Java methods marked with the native keyword call into C++:

• Case A1: By default, the JVM will search for a matching C function to call based on its name. For instance, calling org.mozilla.gecko.mozglue.GeckoLoader.nativeRun from Java will call Java_org_mozilla_gecko_mozglue_GeckoLoader_nativeRun on the C++ side.
• Case A2: This behavior can be overridden at runtime by calling the JNIEnv::RegisterNatives function on the C++ side to point at another function.
• Case A3: GeckoView has a code generator that looks for Java items decorated with the @WrapForJNI and native annotations and generates a C++ class template meant to be used through the Curiously Recurring Template Pattern. This template provides an Init static member function that does the right JNIEnv::RegisterNatives calls to bind the Java methods to the implementing C++ class's member functions.

We also identified two ways the C++ code calls Java methods:

• Case B1: directly with JNIEnv::Call… functions.
• Case B2: GeckoView's code generator also looks for Java methods marked with @WrapForJNI (without the native keyword this time) and generates a C++ wrapper class and member functions with the right JNIEnv::Call… calls.

Only the C++ side has the complete view of the bindings; so that's where we decided to extract the information from, by extending Mozilla's existing Clang plugin.

First, we defined custom C++ annotations bound_as and binding_to that the clang plugin transforms into the right format for the cross-reference analysis. This means we can manually set the binding information:

(This example is, in fact, extracted from our test suite, jni.cpp vs Jni.java.)

Then, we wrote some heuristics that try and identify cases A1 (C functions named Java_…), A3 and B2 (C++ code generated from @WrapForJNI decorators) and automatically generate these annotations. Cases A2 and B1 (manually calling JNIEnv::RegisterNatives or JNIEnv::Call… functions) are rare enough in the Firefox code base and impossible to reliably recognize; so it was decided not to cover them at the time. Developers who wish to declare such bindings could manually annotate them.

After this point, we used Searchfox's existing analysis JSON format and mostly re-used what was already available from IDL support. When triggering the context menu for a binding wrapper or bound function, the definitions in both languages are made available, with “Go to” actions that jump over the generally irrelevant binding internals.

The search results also display both sides of the bridge, for instance:

• searching for the mozilla::widget::GeckoViewSupport::Open C++ member function links to its Java binding org.mozilla.geckoview.GeckoSession$Window.open.
• searching for the org.mozilla.geckoview.GeckoSession.getCompositorFromNative Java method links to its generated C++ binding mozilla::java::GeckoSession::GetCompositor.

If you want to dive into more details, see the feature request and detailed problem analysis on Bugzilla, the implementation and further discussion on GitHub, and the release announcement on the Mozilla dev-platform mailing list.

Displaying Interactive Macro Expansions

Aside from this Java/Kotlin-related work, we also added support for displaying and interacting with macro expansions. This was inspired by KDAB's own codebrowser.dev, but improves it to:

• Display all expansion variants, if they differ across platforms or by definition:

• Make macros fully indexed and interactive:

This work mainly happened in the Mozsearch Clang plugin to extract macro expansions during the pre-processing stage and index them with the rest of the top-level code.

Again, if you want more details, the feature request is available on Bugzilla and the implementation and further technical discussion is on GitHub.

Summary

Because of the many technologies it makes use of, from compiler plugins and code analyzers written in many languages, to a web front-end written using the usual HTML/CSS/JS, by way of custom tooling and scripts in Rust, Python and Bash, Searchfox is a small but complex and really interesting project to work on. KDAB successfully added Java/Kotlin code indexing, including analyzing their C++ bindings, and are starting to improve Searchfox's C++ support itself, first with fully-indexed macro expansions and next with improved templates support.

The goal

When building C++ code with CMake, it is very common to want to set some pre-processor defines in the CMake code.

For instance, we might want to set the project's version number in a single place, in CMake code like this:

This sets the CMake variable PROJECT_VERSION to 1.5, which we can then use to pass -DMYAPP_VERSION_STRING=1.5 to the C++ compiler. The about dialog of the application can then use this to show the application version number, like this:

Similarly, we might have a boolean CMake option like START_MAXIMIZED, which the user compiling the software can set to ON or OFF:

If it's ON, you would pass -DSTART_MAXIMIZED, otherwise nothing. The C++ code will then use #ifdef. (We'll see that there's a better way.)

The common (but suboptimal) solution

A solution that many people use for this is the CMake function add_definitions. It would look like this:

Technically, this works but there are a number of issues.

First, add_definitions is deprecated since CMake 3.12 and add_compile_definitions should be used instead, which allows to remove the leading -D.

More importantly, there's a major downside to this approach: changing the project version or the value of the boolean option will force CMake to rebuild every single .cpp file used in targets defined below these lines (including in subdirectories). This is because add_definitions and add_compile_definitions ask to pass -D to all cpp files, instead of only those that need it. CMake doesn't know which ones need it, so it has to rebuild everything. On large real-world projects, this could take something like one hour, which is a major waste of time.

A first improvement we can do is to at least set the defines to all files in a single target (executable or library) instead of "all targets defined from now on". This can be done like this:

We have narrowed the rebuilding effect a little bit, but are still rebuilding all cpp files in myapp, which could still take a long time.

The recommended solution

There is a proper way to do this, such that only the files that use these defines will be rebuilt; we simply have to ask CMake to generate a header with #define in it and include that header in the few cpp files that need it. Then, only those will be rebuilt when the generated header changes. This is very easy to do:

We have to write the input file, myapp_config.h.in, and CMake will generate the output file, myapp_config.h, after expanding the values of CMake variables. Our input file would look like this:

A good thing about generated headers is that you can read them if you want to make sure they contain the right settings. For instance, myapp_config.h in your build directory might look like this:

For larger use cases, we can even make this more modular by moving the version number to another input file, say myapp_version.h.in, so that upgrading the version doesn't rebuild the file with the showMaximized() code and changing the boolean option doesn't rebuild the about dialog.

If you try this and you hit a "file not found" error about the generated header, that's because the build directory (where headers get generated) is missing in the include path. You can solve this by adding set(CMAKE_INCLUDE_CURRENT_DIR TRUE) near the top of your CMakeLists.txt file. This is part of the CMake settings that I recommend should always be set; you can make it part of your new project template and never have to think about it again.

There's just one thing left to explain: what's this #cmakedefine01 thing?

If your C++ code uses #ifdef, you want to use #cmakedefine, which either sets or doesn't set the define. But there's a major downside of doing that -- if you forget to include myapp_config.h, you won't get a compile error; it will just always go to the #else code path.

We want a solution that gives an error if the #include is missing. The generated header should set the define to either 0 or 1 (but always set it), and the C++ code should use #if. Then, you get a warning if the define hasn't been set and, because people tend to ignore warnings, I recommend that you upgrade it to an error by adding the compiler flag -Werror=undef, with gcc or clang.  Let me know if you are aware of an equivalent flag for MSVC.

And these are all the pieces we need. Never use add_definitions or add_compile_definitions again for things that are only used by a handful of files. Use configure_file instead, and include the generated header. You'll save a lot of time compared to recompiling files unnecessarily.

I hope this tip was useful.

For more content on CMake, we curated a collection of resources about CMake with or without Qt. Check out the videos.

Early-Bird tickets are on sale for the KDAB Training Day 2025 until 2025-03-31 23:59

The KDAB Training Day 2025 will take place in Munich on May 8th, right after the Qt World Summit on May 6th-7th. Choose to buy a combo ticket here (for access to QtWS and Training Day) or here (for access to Training Day only).

Seats are limited, so don't wait too long if you want to participate in a specific course. Tickets include access to the selected training course, training material, lunch buffet, beverages, and coffee breaks. Note: The Training Day is held at Hotel NH Collection München Bavaria, located at the Munich Central Station (not the same location as Qt World Summit).

Get your ticket

Why should you attend the KDAB Training Day?

With over 20 years of experience and a rich store of well-structured, constantly updated training material, KDAB offers hands-on, practical programming training in Qt/QML, Modern C++, 3D/OpenGL, Debugging & Profiling, and lately Rust - both for beginners as well as experienced developers.

All courses provided at the Training Day include central parts of the regular 3- to 4-day courses available as scheduled or customized on-site training. Choosing a compact, learning-rich one-day course, lets you experience the quality and effectiveness of KDAB’s usual training offerings.

Courses available at the KDAB Training Day 2025

• QML Application Architecture
• QML/C++ integration
• Modern C++ Paradigms
• Integrating Rust into Qt applications
• Effective Modern QML
• Integrating Custom 3D Renderers with Qt Applications

QML Application Architecture

In this training, we do a step-by-step walkthrough of how to build a QML-based embedded application from the ground up and discuss some challenges that are typically met along the way.

An important part of that journey is an investigation of where to put the boundaries between what you do in C++ and what you do in QML. We also look at some of the tools and building blocks we have available in QML that can help us achieve well-performing, well-structured, and well-maintainable applications.

This course is for

(Qt) developers looking to improve their understanding of how to construct maintainable and efficient larger-scale QML applications.

Prerequisite

Some real-world experience working on QML applications as well as a basic understanding of Qt and C++.

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QML/C++ Integration

In this training, we start with a recap of fundamentals:

• How do we expose C++ API to QML?
• How do we make data available to QML?

Afterward, we explore several more advanced techniques, often widely deployed within Qt's QML modules, such as Qt Quick.

This will answer questions such as:

• How would I do a Loader like component?
• How would I do a Layout like component?

This course is for

Qt/QML developers who are familiar with the QML APIs of QtQuick and related modules and who have wondered how these are implemented and want to use similar techniques in their project-specific APIs.

Prerequisite

Some real-world experience working on QML applications as well as a basic understanding of Qt and C++.

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Modern C++ Paradigms

Modern C++ emphasizes safer, more efficient, and maintainable code through higher-level abstractions that reduce error-prone manual work.

This training will explore key paradigms shaping recent C++ evolution, starting with value semantics in class design, which enhances code safety, local reasoning, and thread safety. We will examine modern C++ tools for creating value-oriented types, including move semantics, smart pointers, and other library enablers.

Next, we will look at expressive, type and value-based error handling.

Finally, we'll cover range-based programming, which enables clean, declarative code and unlocks new patterns through lazy, composable transformations.

This course is for

C++ developers who wish to improve the quality of their code, in particular those who wish to write future-proof APIs.

Prerequisites

Prior professional experience in C++. Experience with the latest C++ standards (C++20/23/26) is a plus. We will use several examples inspired by Qt APIs, so Qt knowledge is also a plus (but this is not going to be a Qt training).

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Integrating Rust into Qt Applications

In this step-by-step course, we start with a Qt/C++ application and add Rust code to it piece by piece. To achieve this, we will cover:

• Use of Cargo (Rusts build system) with CMake
• Accessing Rust code from C++ with CXX (and vice-versa)
• Defining your own QObject types in Rust with CXX-Qt

We discuss when to use Rust compared to C++ to make the best of both languages and how to use them together effectively to make Qt applications safer and easier to maintain.

This course is for

Qt/C++ Developers with an interest in Rust who want to learn how to use Rust in their existing applications.

Prerequisites

Basic Qt/C++ knowledge, as well as basic Rust knowledge, is required. A working Qt installation with CMake and a working Rust installation is needed. We will provide material before the training day that participants should use to check their setup before the training.

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Effective Modern QML

In this training, we look into all the new developments in QML over the last few years and how they lead to more expressive, performant, and maintainable code.

This includes:
- The qt_add_qml_module CMake API
- Declarative type registration
- The different QML compilers
- New language and library features
- New developments in Qt Quick Controls
- Usage of tools like qmllint, QML Language Server, and qmlformat

The focus will be on gradually modernizing existing codebases with new tools and practices.

This course is for

Developers who learned QML back in the days of Qt 5 and want to catch up with recent developments in QML and modernize their knowledge as well as codebases.

Prerequities

Some real-world experience with QML and a desire to learn about modern best practices.

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Integrating Custom 3D Renderers with Qt Applications

Qt has long offered ways of using low-level 3d libraries such as OpenGL to do custom rendering. Whether at the Window, the Widget, or Quick Item level, the underlying rendering system can be accessed in ways that make it safe to integrate such 3rd party renderers. This remains true in the Qt 6 timeline, although the underlying rendering system has changed and OpenGL has been replaced by RHI.

In this course, we look at how the graphic stack is structured in Qt 6 and how third-party renderers can be integrated on the various platforms supported by Qt.

We then focus on the specific case of integrating Vulkan-based renderers. Vulkan is the successor to OpenGL; it's much more powerful but harder to learn. To facilitate the initial use of Vulkan, we introduce KDGpu, a library that encapsulates Vulkan while preserving the underlying concepts of pipeline objects, buffer handling, synchronization, etc.

This course is for

This course targets developers wanting to understand the recent state of the graphics stack in Qt, discover the fundamental principles of modern graphics API, and integrate their custom renderers in their applications.

Prerequisite

Prior knowledge of Qt will be required. A basic understanding of 3d graphics would be beneficial.

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Video from KDAB Training Day 2023 held in Berlin

We just released CXX-Qt version 0.7!

CXX-Qt is a set of Rust crates for creating bidirectional Rust ⇄ C++ bindings with Qt. It supports integrating Rust into C++ applications using CMake or building Rust applications with Cargo. CXX-Qt provides tools for implementing QObject subclasses in Rust that can be used from C++, QML, and JavaScript.

For 0.7, we have stabilized the cxx-qt bridge macro API and there have been many internal refactors to ensure that we have a consistent baseline to support going forward. We encourage developers to reach out if they find any unclear areas or missing features, to help us ensure a roadmap for them, as this may be the final time we can adapt the API. In the next releases, we're looking towards stabilizing the cxx-qt-build and getting the cxx-qt-lib APIs ready for 1.0.

Check out the new release through the usual channels:

• The CXX-Qt book
• The CXX-Qt documentation
• The CXX-Qt-lib documentation
• Our Github repository

Some of the most notable developer-facing changes:

Stabilized #[cxx_qt::bridge] macro

CXX-Qt 0.7 reaches a major milestone by stabilizing the bridge macro that is at the heart of CXX-Qt.
You can now depend on your CXX-Qt bridges to remain compatible with future CXX-Qt versions.
As we're still pre-1.0, we may still introduce very minor breaking changes to fix critical bugs in the edge-cases of the API, but the vast majority of bridges should remain compatible with future versions.

This stabilization is also explicitly limited to the bridge API itself. Breaking changes may still occur in e.g. cxx-qt-lib, cxx-qt-build, and cxx-qt-cmake. We plan to stabilize those crates in the next releases.

Naming Changes

The handling of names internally has been refactored to ensure consistency across all usages. During this process, implicit automatic case conversion has been removed, so cxx_name and rust_name are now used to specify differing Rust and C++ names. Since the automatic case conversion is useful, it can be explicitly enabled using per extern block attributes auto_cxx_name and auto_rust_name, while still complimenting CXX. For more details on how these attributes can be used, visit the attributes page in the CXX-Qt book.

cxx_file_stem Removal

In previous releases, the output filename of generated C++ files used the cxx_file_stem attribute of the CXX-Qt bridge. This has been changed to use the filename of the Rust source file including the directory structure.

Previously, the code below would generate a C++ header path of my_file.cxxqt.h. After the changes, the cxx_file_stem must be removed and the generated C++ header path changes to crate-name/src/my_bridge.cxxqt.h. This follows a similar pattern to CXX.

Build System Changes

The internals of the build system have changed so that dependencies are automatically detected and configured by cxx-qt-build, libraries can pass build information to cxx-qt-build, and a CXX-Qt CMake module is now available providing convenience wrappers around corrosion. This means that the cxx-qt-lib-headers crate has been removed and only cxx-qt-lib is required. With these changes, there is now no need for the -header crates that existed before. Previously, some features were enabled by default in cxx-qt-lib. Now these are all opt-in. We have provided full and qt_full as convenience to enable all features; however, we would recommend opting in to the specific features you need.

We hope to improve the API of cxx-qt-build in the next cycle to match the internal changes and become more modular.

Further Improvements

CXX-Qt can now be successfully built for WASM, with documented steps available in the book and CI builds for WASM to ensure continued support.

Locking generation on the C++ side for all methods has been removed, which simplifies generation and improves performance. Using queue from cxx_qt::CxxQtThread is still safe, as it provides locking, but it is up to the developer to avoid incorrect multi-threading in C++ code (as in the CXX crate). Note that Qt generally works well here, with the signal/slot mechanism working safely across threads.

As with most releases, there are more Qt types wrapped in cxx-qt-lib and various other changes are detailed in the CHANGELOG.

Make sure to subscribe to the KDAB YouTube channel, where we'll post more videos on CXX-Qt in the coming weeks.

Thanks to all of our contributors that helped us with this release:

• Ben Ford
• Laurent Montel
• Matt Aber
• knox (aka @knoxfighter)
• Be Wilson
• Joshua Goins
• Alessandro Ambrosano
• Alexander Kiselev
• Alois Wohlschlager
• Darshan Phaldesai
• Jacob Alexander
• Sander Vocke