Qt5 Interface

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Revision as of 00:53, 22 November 2019 by Dbannon (talk | contribs) (→‎libqt5pas)
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This article applies to Qt5 widgetset only.

See also: Multiplatform Programming Guide


This interface is based on Qt 5 (Qt 5.6.2 is tested). For documentation, fixes and download, go to Qt Project (Installers at download 5.6.2). Lazarus with Qt5 interface (qt5-lcl) can be used on Windows 32/64, Linux x32/x64/arm, MacOSX x64(Cocoa). Qt5 widgetset have been available in lazarus from 1.8 stable release. There's no support for 32bit Qt5 binary versions from Digia, so if you need 32bit Qt5 on linux you must build complete Qt5 on your own, including libQt5Pas.

Most current Linux distros have a suitable Qt5 in their standard repositories however, long term release versions have issues due to their age. Ubuntu 16.04 has QT5 that is too old and 18.04 ships a problematic version of libQt5Pas that needs to be replaced, see below.


There is Q5Pas1.dll binary available.

no 64-bit version yet.

The build is based on mingw, thus you want to use mingw Qt library (i.e. qt-opensource-windows-x86-mingw492-5.6.2.exe).

If you need to build cbindings project, you need mingw. You can use mingw from Qt package (qt-opensource-windows-x86-mingw492-5.6.2.exe) - it's an optional component of Qt package installation.

Running and Deployment

The project needs to be deployed with Q5Pas1.dll.

For Qt5 .dlls that should be deployed, see Qt5 guides: https://doc.qt.io/qt-5/windows-deployment.html.

For MingW based package the following set of DLLs would be required to run a single FORM project.

Those DLLs come with Qt5 package (can be found i.e. at C:\Qt\Qt5.6.2\5.6\mingw49_32\bin).


Builds made base on MSVC rather than MingW might have different set of (non Qt5) dlls required. Note that https://svn.freepascal.org/svn/lazarus/binaries/i386-win32/qt5/Qt5Pas1.dll is built against Qt5-5.6.2, but Qt5Pas1.dll can be used with any Qt5 > 5.6.2.



Image of Qt5 selected

Lazarus, "out of the box" on Linux uses and makes apps for GTK2. It is not marked as having a dependency on Qt5 so if you wish to make Qt5 apps or build a Qt5 version of Lazarus you may need to install Qt5 and will almost certainly need to install libQt5Pas-dev. If you wish to use a Qt5 version of Lazarus, build from source with a command like "make bigide LCL_PLATFORM=qt5 <enter>"

From within Lazarus, to choose to build a Qt5 app, Projects->ProjectOptions->AdditionsandOverrides and click Set "LCLWidgetType", and select Qt5 for the current Build Mode.


A library that bridges between Qt5 and your Lazarus application. Newer distros have working versions available in their repositories.

Using your distros repository -

  • Fedora, Mageia - sudo dnf install qt5pas<enter>
  • Ubuntu, Debian - sudo apt install libqt5pas1 <enter>

Note that some long term release distributions, ie Ubuntu 18.04 have an incompatible version of libqt5pas (even though it appears to have the same version number as later distros!). You will see an error message and a crash if your app uses TMemo. You should build a new version of the library, everything you need is in your Lazarus tree, bindings sources are located in lcl/interfaces/qt5/cbindings , to build it just follow README.txt. As you will probably have Qt4 installed as well, override it with (eg) qmake -qt=Qt5 <enter>. You should also probably edit Qt5Pas.pro to build a release version rather than debug,

CONFIG += release CONFIG -= debug

The resulting library is about 3.5Mbytes, the install process strips it down to 2.5Mbytes.

Note that once built you can use that library in all (?) Qt5 installs (5.6 or greater) however, the libraries shipped with Ubuntu are not so portable.

Unfortunately, you cannot rely on the version number Ubuntu has stamped on their libQt5Pas to determine if it needs to be replaced or not. If you ship your application to Linux users, its quite likely they are using one of the LTR Ubuntus and will strike this problem.

Why Qt5 instead of GTK ?

Good question ! Most Linux Lazarus applications are built using the default GTK2. The various Linux distributions are delivering less and less GTK2 apps, generally 'upgrading' to GTK3. However, the Lazarus GTK3 interface still needs some work and its not ready for production systems yet. Until late 2019, this has not mattered much, GTK2 works fine. But Ubuntu19.10 does not have GTK2 libraries installed by default and Xfce has announced its next release will be GTK2 free, apparently many distributions are looking to drop GTK2 from their default kits. While you can still install GTK2, its pretty big....

Qt5 is an obvious solution, its already installed on disros like Fedora 30 plus, its a relatively lightweight addition to Ubuntu, only 48Meg compared with GTK2's 445Meg. If you mark your application dependent on libqt5pas then it will be installed at the same time, bringing along the other qt5 files if necessary. Manually installing libqt5pas with your package manager (as long as it resolves dependencies!) will also work.

Clearly, Qt based distributions, ones using Plasma or KDE as a desktop will require only libqt5pas.

macOS (64-bit)

The following steps have been tested with macOS 10.13.6 (High Sierra), Xcode command line tools 10.0.0 (no Xcode required), Lazarus 1.8.4 and minumum Qt 5.6.2 (Qt 5.12.xx works without problems too).

After installing Qt 5.6.2 (see above), open up a terminal window and run these commands (paths may vary):

cd $LazarusDir/lcl/interfaces/qt5/cbindings

If qmake fails with the following error:

xcode-select: error: tool 'xcodebuild' requires Xcode, but active developer directory '/Library/Developer/CommandLineTools' is a command line tools instance

it does not mean that you have to install Xcode. See Qt without Xcode for a workaround and try running qmake again. (The Xcode dependency seems to be fixed in Qt 5.9.4 or maybe earlier.)

Proceed the build process with the following commands:

make install

It may be helpful to add symlinks to the Qt5Pas.framework and other *.framework files (so no modifications to the path are required):

cd /Library/Frameworks/
sudo ln -s $QtDir/5.6/clang_64/lib/*.framework .

Remember that Qt5 is 64bit, so in Lazarus under Tools - Options - Environment, you need to change Compiler Executable from /usr/local/bin/ppc386 to /usr/local/bin/ppcx64 . With ppc386, you would later get the following errors when compiling your Lazarus projects:

Error: linker: Undefined symbols for architecture i386:
Error: linker: "_QAbstractButton_click", referenced from:
ld: symbol(s) not found for architecture i386

Finally, create a new Lazarus project (or open an existing one), open Project - Project options - Compiler options - Additions and overrides, set LCLWidgetType:=qt5 . You should now be able to compile your project with the Qt5 widget set.

See [1] for an example project.

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