<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 to support packages that may not be compatible with the latest version. When packaging applications and libraries for Nixpkgs, it is important to ensure that compatible versions of Qt 5 are used throughout; this consideration motivates the tools described below.</para>
<para>Libraries that depend on Qt 5 should be built with each available version to avoid linking a dependent package against incompatible versions of Qt 5. (Although Qt 5 maintains backward ABI compatibility, linking against multiple versions at once is generally not possible; at best it will lead to runtime faults.) Packages that provide 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. The <varname>callPackage</varname> provided in this scope will ensure that only one Qt version will be used throughout the dependency tree. Dependencies should be imported unqualified, i.e. <literal>qtbase</literal> not <literal>qt5.qtbase</literal>, so that <varname>callPackage</varname> can do its work. <emphasis>Do not</emphasis> import a package set such as <literal>qt5</literal> or <literal>libsForQt5</literal> into your package; although it may work fine in the moment, it could well break at the next Qt update.</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>
<para>Applications generally do not need to be built with every Qt version because they do not provide any libraries for dependent packages to link against. The primary consideration is merely ensuring that the application itself and its dependencies are linked against only one version of Qt. To call your application expression, use <literal>libsForQt5.callPackage</literal> instead of <literal>callPackage</literal>. Dependencies should be imported 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> into your package; although it may work fine in the moment, it could well break at the next Qt update.</para>
<para>It is generally best to build an application package against 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.</para>
<para>The KDE Frameworks are a set of libraries for Qt 5 which form the basis of the Plasma desktop environment and the KDE Applications suite. Packaging a Frameworks-based package does not require any steps beyond those described above for general Qt-based packages.</para>
