- Qt 6.4
- Qt Widgets
- Plug & Paint Extra Filters Example
- Qt 6.4.3 Reference Documentation

# Plug & Paint Extra Filters Example

A plugin providing the extra filters.

The Extra Filters example is a plugin for the Plug & Paint example. It provides a set of filters in addition to those provided by the Basic Tools plugin.

Since the approach is identical to Basic Tools, we won't review the code here. The only part of interest is the `.pro` file, since Extra Filters is a dynamic plugin (Basic Tools is linked statically into the Plug & Paint executable).

Here's the project file for building the Extra Filters plugin:

     TEMPLATE      = lib
     CONFIG       += plugin
     QT           += widgets
     INCLUDEPATH  += ../../app
     HEADERS       = extrafiltersplugin.h
     SOURCES       = extrafiltersplugin.cpp
     TARGET        = $$qtLibraryTarget(pnp_extrafilters)
     DESTDIR       = ../../plugins

The `.pro` file differs from typical `.pro` files in many respects. First, it starts with a `TEMPLATE` entry specifying `lib`. (The default template is `app`.) It also adds `plugin` to the `CONFIG` variable. This is necessary on some platforms to avoid generating symbolic links with version numbers in the file name, which is appropriate for most dynamic libraries but not for plugins.

The `INCLUDEPATH` variable sets the search paths for global headers (i.e., header files included using `#include <...>`). We add `../../app` to the list, so that we can include `<interfaces.h>`.

The `TARGET` variable specifies which name we want to give the target library. We use `pnp_` as the prefix to show that the plugin is designed to work with Plug & Paint. On Unix, `lib` is also prepended to that name. On all platforms, a platform-specific suffix is appended (e.g., `.dll` on Windows, `.so` on Linux).

The `DESTDIR` variable specifies where we want to install the plugin. We put it in Plug & Paint's `plugins` subdirectory, since that's where the application looks for dynamic plugins.

The `CONFIG()` code at the end is necessary for this example because the example is part of the Qt distribution and Qt can be configured to be built simultaneously in debug and in release modes. You don't need to for your own plugins.

Example project @ code.qt.io