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# CMakePackageConfigHelpers

Helpers functions for creating config files that can be included by other projects to find and use a package.

Adds the `configure_package_config_file()` and `write_basic_package_version_file()` commands.

## Generating a Package Configuration File

`configure_package_config_file`

  Create a config file for a project:

      configure_package_config_file(<input> <output>
        INSTALL_DESTINATION <path>
        [PATH_VARS <var1> <var2> ... <varN>]
        [NO_SET_AND_CHECK_MACRO]
        [NO_CHECK_REQUIRED_COMPONENTS_MACRO]
        [INSTALL_PREFIX <path>]
        )

`configure_package_config_file()` should be used instead of the plain `configure_file()` command when creating the `<Name>Config.cmake` or `<Name>-config.cmake` file for installing a project or library. It helps making the resulting package relocatable by avoiding hardcoded paths in the installed `Config.cmake` file.

In a `FooConfig.cmake` file there may be code like this to make the install destinations know to the using project:

    set(FOO_INCLUDE_DIR   "@CMAKE_INSTALL_FULL_INCLUDEDIR@" )
    set(FOO_DATA_DIR   "@CMAKE_INSTALL_PREFIX@/@RELATIVE_DATA_INSTALL_DIR@" )
    set(FOO_ICONS_DIR   "@CMAKE_INSTALL_PREFIX@/share/icons" )
    ...logic to determine installedPrefix from the own location...
    set(FOO_CONFIG_DIR  "${installedPrefix}/@CONFIG_INSTALL_DIR@" )

All 4 options shown above are not sufficient, since the first 3 hardcode the absolute directory locations, and the 4th case works only if the logic to determine the `installedPrefix` is correct, and if `CONFIG_INSTALL_DIR` contains a relative path, which in general cannot be guaranteed. This has the effect that the resulting `FooConfig.cmake` file would work poorly under Windows and OSX, where users are used to choose the install location of a binary package at install time, independent from how `CMAKE_INSTALL_PREFIX` was set at build/cmake time.

Using `configure_package_config_file` helps. If used correctly, it makes the resulting `FooConfig.cmake` file relocatable. Usage:

1. write a `FooConfig.cmake.in` file as you are used to
2. insert a line containing only the string `@PACKAGE_INIT@`
3. instead of `set(FOO_DIR "@SOME_INSTALL_DIR@")`, use `set(FOO_DIR "@PACKAGE_SOME_INSTALL_DIR@")` (this must be after the `@PACKAGE_INIT@` line)
4. instead of using the normal `configure_file()`, use `configure_package_config_file()`

The `<input>` and `<output>` arguments are the input and output file, the same way as in `configure_file()`.

The `<path>` given to `INSTALL_DESTINATION` must be the destination where the `FooConfig.cmake` file will be installed to. This path can either be absolute, or relative to the `INSTALL_PREFIX` path.

The variables `<var1>` to `<varN>` given as `PATH_VARS` are the variables which contain install destinations. For each of them the macro will create a helper variable `PACKAGE_<var...>`. These helper variables must be used in the `FooConfig.cmake.in` file for setting the installed location. They are calculated by `configure_package_config_file` so that they are always relative to the installed location of the package. This works both for relative and also for absolute locations. For absolute locations it works only if the absolute location is a subdirectory of `INSTALL_PREFIX`.

If the `INSTALL_PREFIX` argument is passed, this is used as base path to calculate all the relative paths. The `<path>` argument must be an absolute path. If this argument is not passed, the `CMAKE_INSTALL_PREFIX` variable will be used instead. The default value is good when generating a FooConfig.cmake file to use your package from the install tree. When generating a FooConfig.cmake file to use your package from the build tree this option should be used.

By default `configure_package_config_file` also generates two helper macros, `set_and_check()` and `check_required_components()` into the `FooConfig.cmake` file.

`set_and_check()` should be used instead of the normal `set()` command for setting directories and file locations. Additionally to setting the variable it also checks that the referenced file or directory actually exists and fails with a `FATAL_ERROR` otherwise. This makes sure that the created `FooConfig.cmake` file does not contain wrong references. When using the `NO_SET_AND_CHECK_MACRO`, this macro is not generated into the `FooConfig.cmake` file.

`check_required_components(<package_name>)` should be called at the end of the `FooConfig.cmake` file if the package supports components. This macro checks whether all requested, non-optional components have been found, and if this is not the case, sets the `Foo_FOUND` variable to `FALSE`, so that the package is considered to be not found. It does that by testing the `Foo_<Component>_FOUND` variables for all requested required components. When using the `NO_CHECK_REQUIRED_COMPONENTS_MACRO` option, this macro is not generated into the `FooConfig.cmake` file.

For an example see below the documentation for `write_basic_package_version_file()`.

## Generating a Package Version File

`write_basic_package_version_file`

  Create a version file for a project:

      write_basic_package_version_file(<filename>
        [VERSION <major.minor.patch>]
        COMPATIBILITY <AnyNewerVersion|SameMajorVersion|ExactVersion> )

Writes a file for use as `<package>ConfigVersion.cmake` file to `<filename>`. See the documentation of `find_package()` for details on this.

`<filename>` is the output filename, it should be in the build tree. `<major.minor.patch>` is the version number of the project to be installed.

If no `VERSION` is given, the `PROJECT_VERSION` variable is used. If this hasn’t been set, it errors out.

The `COMPATIBILITY` mode `AnyNewerVersion` means that the installed package version will be considered compatible if it is newer or exactly the same as the requested version. This mode should be used for packages which are fully backward compatible, also across major versions. If `SameMajorVersion` is used instead, then the behaviour differs from `AnyNewerVersion` in that the major version number must be the same as requested, e.g. version 2.0 will not be considered compatible if 1.0 is requested. This mode should be used for packages which guarantee backward compatibility within the same major version. If `ExactVersion` is used, then the package is only considered compatible if the requested version matches exactly its own version number (not considering the tweak version). For example, version 1.2.3 of a package is only considered compatible to requested version 1.2.3. This mode is for packages without compatibility guarantees. If your project has more elaborated version matching rules, you will need to write your own custom `ConfigVersion.cmake` file instead of using this macro.

Internally, this macro executes `configure_file()` to create the resulting version file. Depending on the `COMPATIBLITY`, either the file `BasicConfigVersion-SameMajorVersion.cmake.in` or `BasicConfigVersion-AnyNewerVersion.cmake.in` is used. Please note that these two files are internal to CMake and you should not call `configure_file()` on them yourself, but they can be used as starting point to create more sophisticted custom `ConfigVersion.cmake` files.

## Example Generating Package Files

Example using both `configure_package_config_file()` and `write_basic_package_version_file()`:

`CMakeLists.txt`:

    set(INCLUDE_INSTALL_DIR include/ ... CACHE )
    set(LIB_INSTALL_DIR lib/ ... CACHE )
    set(SYSCONFIG_INSTALL_DIR etc/foo/ ... CACHE )
    ...
    include(CMakePackageConfigHelpers)
    configure_package_config_file(FooConfig.cmake.in
      ${CMAKE_CURRENT_BINARY_DIR}/FooConfig.cmake
      INSTALL_DESTINATION ${LIB_INSTALL_DIR}/Foo/cmake
      PATH_VARS INCLUDE_INSTALL_DIR SYSCONFIG_INSTALL_DIR)
    write_basic_package_version_file(
      ${CMAKE_CURRENT_BINARY_DIR}/FooConfigVersion.cmake
      VERSION 1.2.3
      COMPATIBILITY SameMajorVersion )
    install(FILES ${CMAKE_CURRENT_BINARY_DIR}/FooConfig.cmake
                  ${CMAKE_CURRENT_BINARY_DIR}/FooConfigVersion.cmake
            DESTINATION ${LIB_INSTALL_DIR}/Foo/cmake )

`FooConfig.cmake.in`:

    set(FOO_VERSION x.y.z)
    ...
    @PACKAGE_INIT@
    ...
    set_and_check(FOO_INCLUDE_DIR "@PACKAGE_INCLUDE_INSTALL_DIR@")
    set_and_check(FOO_SYSCONFIG_DIR "@PACKAGE_SYSCONFIG_INSTALL_DIR@")

    check_required_components(Foo)

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