Qt Slot Execution Order

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Qt will indeed call directly the function pointer of the slot, and will not need moc introspection anymore. (It still needs it for the signal) (It still needs it for the signal) But what we can also do is connecting to any function or functor. In Qt v4.5 and earlier: No, the order is undefined as can be seen in the documentation here: If several slots are connected to one signal, the slots will be executed one after the other, in an arbitrary order, when the signal is emitted. Edit: From version 4.6 onwards this is no longer true. Now the slots will run in the order they are connected. Signals and Slots. In Qt, we have an alternative to the callback technique: We use signals and slots. A signal is emitted when a particular event occurs. Qt's widgets have many predefined signals, but we can always subclass widgets to add our own signals to them. A slot is a function that is called in response to a particular signal. The order of event execution differs between Linux and Windows for mousePressEvent, mouseReleaseEvent, and contextMenuEvent. Attached is a small qt project that will reproduce the issue. It is just a QPushButton that prints when it receives the above-mentioned events. Plain C code with signals and slots – the test state machine has the identical implementation as described above but uses signals and slots to integrate it into the application. Plain C code with QEvent – uses the plain C code approach but makes use of the Qt event queue for processing in and out events.

In a SELECT statement, the QUALIFY clause filters the results of window functions.

QUALIFY does with window functions what HAVING does with aggregate functions and GROUP BY clauses.

In the execution order of a query, QUALIFY is therefore evaluated after window functions are computed. Typically,a SELECT statement’s clauses are evaluated in the order shown below:

  1. From

  2. Where

  3. Group by

  4. Having

  5. Window

  6. QUALIFY

  7. Distinct

  8. Order by

  9. Limit

Syntax¶

Slot

The general form of a statement with QUALIFY is similar to the following(some variations in order are allowed, but are not shown):

column_list

This generally follows the rules for the projection clause of a SELECT statement.

data_source

The data source is usually a table, but can be another table-like data source, such as a view,UDTF, etc.

predicate

The predicate is an expression that filters the result after aggregates and window functions are computed.The predicate should look similar to a HAVING clause, but without thekeyword HAVING. In addition, the predicate can also contain window functions.

See the Examples section (in this topic) for predicate examples.

Usage Notes¶

  • The QUALIFY clause requires at least one window function to be specified in at least one of the following clausesof the SELECT statement:

    • The SELECT column list.

    • The filter predicate of the QUALIFY clause.

    Examples of each of these are shown in the Examples section below.

  • Expressions in the SELECT list, including window functions, can be referred to by the column alias defined in theSELECT list.

  • QUALIFY supports aggregates and subqueries in the predicate. For aggregates, the same rules as for the HAVING clauseapply.

  • The word QUALIFY is a reserved word.

  • The Snowflake syntax for QUALIFY is not part of the ANSI standard.

Examples¶

The QUALIFY clause simplifies queries that require filtering on the result of window functions. Without QUALIFY,filtering requires nesting. The example below uses the ROW_NUMBER() function to return only the first row in eachpartition.

Create and load a table:

This query uses nesting rather than QUALIFY:

This query uses QUALIFY:

You can also use QUALIFY to reference window functions that are in the SELECT column list:

You can see how the QUALIFY acts as a filter by removing the QUALIFY from the previous query and comparing theoutput:

The QUALIFY clause can also be combined with aggregates and can have subqueries in the predicate, e.g.:

Qt is a cross-platform C++ framework for creating GUI applications. Qt uses its own build system, qmake, and also supports building with CMake starting from the version Qt4.

A pure Qmake project can't be imported in CLion directly. However, when converted into CMake, it can be opened and managed as a regular CMake application. You can also create a CMake-based Qt project in CLion using the New Project wizard.

Another option to open a Qt project is to use a compilation database. You can generate compile_commands.json for your qmake project with the help of scan-build or via the Build Generate Compilation Database action in Qt Creator, then open this file as a project in CLion and add custom build targets and custom Run/Debug configurations for it.

CMake-based Qt projects

For CMake version 3.0 and newer, Qt ships the modules that let CMake find and use Qt4 and Qt5 libraries. Take the following steps to configure CMakeLists.txt for your Qt project.

CMakeLists.txt for a Qt project

  1. Add the find_package command to locate the required libraries and header files. For example:

    Then, use target_link_libraries to make your target dependent on these libraries and headers:

    target_link_libraries(helloworld Qt5::Widgets)
  2. For find_package to perform successfully, CMake should be instructed on where to find the Qt installation.

    One of the ways to do this is by setting the CMAKE_PREFIX_PATH variable. You can either pass it via -D in the CMake settings dialog or via the set command before find_package.

    For example, in the case of MinGW on Windows:

    set(CMAKE_PREFIX_PATH 'C:QtQt5.14.05.14.0mingw73_32')
  3. If your project uses MOC, UIC, or RCC, add the following lines to enable automatic invocation of the corresponding compilers:

    set(CMAKE_AUTOMOC ON) set(CMAKE_AUTOUIC ON) set(CMAKE_AUTORCC ON)
  4. List all the .ui and .qrc files in the add_executable() command along with your .cpp sources:

    add_executable( helloworld main.cpp mainwindow.cpp application.qrc)

Below you can find the full CMakeLists.txt script for a simple 'Hello, world' application:

cmake_minimum_required(VERSION 3.10)project(Qt-CMake-HelloWorld)set(CMAKE_CXX_STANDARD 17)set(CMAKE_INCLUDE_CURRENT_DIR ON)set(CMAKE_PREFIX_PATH 'C:QtQt5.14.05.14.0mingw73_32')find_package(Qt5Widgets REQUIRED)set(CMAKE_AUTOMOC ON)set(CMAKE_AUTOUIC ON)set(CMAKE_AUTORCC ON)add_executable(helloworld main.cpp mainwindow.cpp application.qrc)target_link_libraries(helloworld Qt5::Widgets)

Setting up a Qt project in CLion

Qt Signal Slot Execution Order

Toolchains on Windows

  1. When installing Qt on Windows, pick the distributive that matches the environment you are using in CLion, MinGW or MSVC.

    For MinGW, both the version and the bitness (32-bit MinGW or MinGW-w64) should match with your toolchain setup.

  2. In CLion, go to Settings(Ctrl+Alt+S), navigate to Build, Execution, Deployment Toolchain and select the toolchain that matches your Qt installation.

    If you have several Qt installations, make sure to select the same toolchain as the one you specified in CMAKE_PREFIX_PATH.

    As an example, in the case of MinGW:

For details on Windows toolchains, take a look at the MinGW or MSVC section of our Windows Tutorial.

CMake settings

  • Qt projects are handled as regular CMake projects in CLion, so you can configure CMake settings in Settings/Preferences Build, Execution, Deployment CMake as necessary.

    For example, you can create different CMake profiles with different build types and set up CMake generators of your choice.

    In this dialog, you can also specify CMake options, such as CMAKE_PREFIX_PATH instead of setting them in CMakeLists.txt:

Debugger renderers

You can use Qt type renderers in CLion, however, for now, you need to set them manually either using the gdbinit/lldbinit scripts or, in the case of MSVC, via native visualizers.

  • Windows MSVC

    CLion’s debugger for the MSVC toolchain can employ native visualizers if you have them in your project. Make sure to set the Enable NatVis renderers for LLDB option in Settings Build, Execution, Deployment Debugger Data Views C/C++.

    For example, try copying qt5.natvis under your project root - CLion will detect and use it automatically.

  • Windows MinGW / macOS / Linux

    For non-MSVC toolchains, a solution would be to configure the Qt renderers via .gdbinit/.lldbinit. These scripts are loaded on every invocation of GDB or LLDB, respectively. Try KDevelop formatters for GDB, KDevelop formatters for LLDB, or Lekensteyn's qt5printers (GDB).

Editing UI files in Qt Designer

By default, CLion opens .ui files in the editor. You can change this behavior in order to edit .ui files right in Qt Designer.

  1. Go to Settings / Preferences Editor File Types, select Qt UI Designer Form from the Recognized File Types list, and delete the associated file extension.

  2. Next time you click a .ui file for opening, set the Open in associated application checkbox, and the files of this type will always open in Qt Designer.

Creating a CMake-based Qt project

CLion provides two project templates for Qt: Qt Console Executable and Qt Widgets Executable.

Call File New Project and select the project type in the left-hand pane. Specify the location, language standard, and Qt version. You can also provide the path to be used in CMAKE_PREFIX_PATH. Click Create when ready.

CLion will generate a ready-to-go stub project with CMakeLists.txt filled in automatically, including the CMAKE_PREFIX_PATH variable:

Qt UI class templates

You can quickly create a Qt class with the corresponding .ui, .cpp, and .h files.

  1. In the Project view, select New from the context menu or press Alt+Insert. Choose Qt UI Class:

  2. In the dialog that opens, fill in the class name and configure the following settings:

    • Filename base - specify the name for the .ui/.cpp/.h files to be generated.

    • Parent class - select QWidget, QMainWindow, or QDialog as the parent class.

    • Add to targets - set this checkbox to automatically add the new files to the list of sources for the selected target(s).

  3. CLion will generate the .ui, .cpp, and .h files following the templates defined in Settings / Preferences Editor File and Code Templates: Qt Designer Form, Qt Class, and Qt Class Header, respectively. If required, you can adjust these templates for your needs.

Qt-specific code insight

CLion provides code completion for Qt signals and slots, filtering the suggestion list to show only the corresponding members:

Completion works for the SIGNAL() and SLOT() macros as well:

Also, CLion's auto-import supports Qt-style header files, offering the shortest possible #include:

Qt Slot Execution Order Diagram

Current limitations

Two umbrella tickets in CLion tracker: Qt project support (CPP-318) and Qt issues (CPP-1897).

  • Some of the CLion's code generation features may not work for Qt macros, such as Q_PROPERTY, Q_SIGNAL, Q_SLOT, and others.

  • Although QML is not supported in CLion out of the box (see the feature request), you can try the QML Support plugin.

Troubleshooting

If you get the Process finished with exit code -1073741515 (0xC0000135) error message when running on MinGW, the issue might relate to Qt deployment on Windows: dynamic libraries and Qt plugins must be found from the directory of the running binary. One of the possible workarounds is described below.

  1. Copy the .dll-s located in bin under the MinGW directory in the Qt installation to your project’s generation folder, which is cmake-build-debug or cmake-build-release by default.

    The libraries you will most likely need are libstdc++-6.dll, Qt5Widgets.dll, Qt5Gui.dll, and Qt5Core.dll, but your setup might require other libraries as well.

  2. In the settings of the configuration you use for running, set the QT_QPA_PLATFORM_PLUGIN_PATH environment variable to pluginsplatforms under the MinGW directory:

    Another option is to copy the contents of pluginsplatforms to cmake-build-debug(release)/platforms (make sure to create the platforms directory).

    Refer to the Qt Plugins documentation for more details.