Compilation Variants

To create different variants of the same feature, but avoid code duplications within plugins, you have multiple options:

• Define a needs clause in a contract and reuse another plugin as it is. This should be preferred for filter and validation tasks.
• Have common code together in a helper library (or core library), see the CMake function add_lib for creating such a library (in the folder libs). This should be used for rather common functionality that might be useful for many plugins or even applications.
• Have configuration for plugins (See elektraPluginGetConfig() and dynamically switch with if according to the configuration. This should be preferred when you want to (de)activate some features of a plugin at runtime.
• Or use compilation variants to compile the plugin code multiple times with different COMPILE_DEFINITIONS (that are Macro definitions). This should be preferred when different macro definitions lead to different plugins. It should especially be used when the resulting plugins have different dependencies: it is possible to have different LINK_LIBRARIES.

The advantage of compilation variants are:

• No runtime overhead
• Can be used during Bootstrapping (when no configuration is available)
• Different compilation variants can be built at once (no recompilation with different cmake flags required)
• Different compilation variants can have different dependencies

How to use it

To use compilation variants, add your plugin in the CMake Cache Variable PLUGINS multiple times. There has to be the base variant, called in the same name as the directory. Then there can be an arbitrary number of variants that additional have a name appended with underscore, e.g.:

    myplugin;myplugin_varianta;myplugin_variantb

In the CMakeLists.txt of your plugin, you need a loop over all PLUGINS and create a plugin per compilation variant:

    foreach (plugin ${PLUGINS})
            if (${plugin} MATCHES "myplugin_.*")
                    # somehow process the variant names and include
                    # or change sources and compile definitions
                    # based on that.
                    add_plugin(${plugin}
                    SOURCES      <your sources here..>
                    COMPILE_DEFINITIONS   <definitions here..>
                            ELEKTRA_VARIANT=${plugin without prefix}
            LINK_LIBRARIES <libraries for variantb>

or simply list all plugins one after the other:

    if (${plugin} MATCHES "myplugin_varianta")
            # somehow process the variant names and include
            # or change sources and compile definitions
            # based on that.
            add_plugin(myplugin_varianta
            SOURCES      <your sources for varianta here..>
            COMPILE_DEFINITIONS   VARIANTA ELEKTRA_VARIANT=varianta
            LINK_LIBRARIES <libraries for varianta>
            )
    if (${plugin} MATCHES "myplugin_variantb")
            # somehow process the variant names and include
            # or change sources and compile definitions
            # based on that.
            add_plugin(myplugin_variantb
            SOURCES      <your sources for variantb here..>
            COMPILE_DEFINITIONS   VARIANTB  ELEKTRA_VARIANT=variantb
            LINK_LIBRARIES <libraries for variantb>

Note that every plugin (except the base plugin, if available) needs to have ELEKTRA_VARIANT differently set in COMPILE_DEFINITIONS, otherwise you will get a linker error that libelektra_<pluginname>_LTX_elektraPluginSymbol has multiple definitions.

Now every public function of the plugin conflicts with itself. To avoid that, you can use:

• static functions, but they are only visible within one file. This should be preferred, when possible.
• use helper libraries using add_lib to share code between compilation variants (only if code is also potentially useful for other plugins/applications)
• Get a unique name for every variant using the macro ELEKTRA_PLUGIN_FUNCTION(myplugin, open) where myplugin is the name of the plugin and the second argument is how the function should be called.
• Including a readme for every variant (with #ifdef for different text) using the macro #include ELEKTRA_README(myplugin)

As a summary, you can have many plugins build out of the same source. Using pluginname_variantnames many plugins will be compiled, each with other SOURCES or COMPILE_DEFINITIONS and even LINK_LIBRARIES: If you, e.g. just set the variants name as macro you can use

    #ifdef varianta
    #endif

within the code and can have two plugins: one (called myplugin_varianta) compiled included the #ifdef the other (base variant called myplugin) without.

Currently compilation variants is used in the resolver plugin.