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<section xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xml:id="sec-language-qt">
<title>Qt</title>
<para>
Qt is a comprehensive desktop and mobile application development toolkit for
C++. Legacy support is available for Qt 3 and Qt 4, but all current
development uses Qt 5. The Qt 5 packages in Nixpkgs are updated frequently to
take advantage of new features, but older versions are typically retained
until their support window ends. The most important consideration in
packaging Qt-based software is ensuring that each package and all its
dependencies use the same version of Qt 5; this consideration motivates most
of the tools described below.
</para>
<section xml:id="ssec-qt-libraries">
<title>Packaging Libraries for Nixpkgs</title>
<para>
Whenever possible, libraries that use Qt 5 should be built with each
available version. Packages providing libraries should be added to the
top-level function <varname>mkLibsForQt5</varname>, which is used to build a
set of libraries for every Qt 5 version. A special
<varname>callPackage</varname> function is used in this scope to ensure that
the entire dependency tree uses the same Qt 5 version. Import dependencies
unqualified, i.e., <literal>qtbase</literal> not
<literal>qt5.qtbase</literal>. <emphasis>Do not</emphasis> import a package
set such as <literal>qt5</literal> or <literal>libsForQt5</literal>.
</para>
<para>
If a library does not support a particular version of Qt 5, it is best to
mark it as broken by setting its <literal>meta.broken</literal> attribute. A
package may be marked broken for certain versions by testing the
<literal>qtbase.version</literal> attribute, which will always give the
current Qt 5 version.
</para>
</section>
<section xml:id="ssec-qt-applications">
<title>Packaging Applications for Nixpkgs</title>
<para>
Call your application expression using
<literal>libsForQt5.callPackage</literal> instead of
<literal>callPackage</literal>. Import dependencies unqualified, i.e.,
<literal>qtbase</literal> not <literal>qt5.qtbase</literal>. <emphasis>Do
not</emphasis> import a package set such as <literal>qt5</literal> or
<literal>libsForQt5</literal>.
</para>
<para>
Qt 5 maintains strict backward compatibility, so it is generally best to
build an application package against the latest version using the
<varname>libsForQt5</varname> library set. In case a package does not build
with the latest Qt version, it is possible to pick a set pinned to a
particular version, e.g. <varname>libsForQt55</varname> for Qt 5.5, if that
is the latest version the package supports. If a package must be pinned to
an older Qt version, be sure to file a bug upstream; because Qt is strictly
backwards-compatible, any incompatibility is by definition a bug in the
application.
</para>
<para>
When testing applications in Nixpkgs, it is a common practice to build the
package with <literal>nix-build</literal> and run it using the created
symbolic link. This will not work with Qt applications, however, because
they have many hard runtime requirements that can only be guaranteed if the
package is actually installed. To test a Qt application, install it with
<literal>nix-env</literal> or run it inside <literal>nix-shell</literal>.
</para>
</section>
</section>
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