# Qt

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.

## Packaging Libraries for Nixpkgs

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 `mkLibsForQt5`, which is used to build a set of libraries for every Qt 5 version. A special `callPackage` function is used in this scope to ensure that the entire dependency tree uses the same Qt 5 version. Import dependencies unqualified, i.e., `qtbase` not `qt5.qtbase`. Do not import a package set such as `qt5` or `libsForQt5`.

If a library does not support a particular version of Qt 5, it is best to mark it as broken by setting its `meta.broken` attribute. A package may be marked broken for certain versions by testing the `qtbase.version` attribute, which will always give the current Qt 5 version.

## Packaging Applications for Nixpkgs

Call your application expression using `libsForQt5.callPackage` instead of `callPackage`. Import dependencies unqualified, i.e., `qtbase` not `qt5.qtbase`. Do not import a package set such as `qt5` or `libsForQt5`.

Qt 5 maintains strict backward compatibility, so it is generally best to build an application package against the latest version using the `libsForQt5` 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. `libsForQt55` 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.

When testing applications in Nixpkgs, it is a common practice to build the package with `nix-build` 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 `nix-env` or run it inside `nix-shell`.