drogon/cmake_modules/FindFilesystem.cmake

# Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
# file Copyright.txt or https://cmake.org/licensing for details.

#[=======================================================================[.rst:

FindFilesystem
##############

This module supports the C++17 standard library's filesystem utilities. Use the
:imp-target:`std::filesystem` imported target to

Options
*******

The ``COMPONENTS`` argument to this module supports the following values:

.. find-component:: Experimental
    :name: fs.Experimental

    Allows the module to find the "experimental" Filesystem TS version of the
    Filesystem library. This is the library that should be used with the
    ``std::experimental::filesystem`` namespace.

.. find-component:: Final
    :name: fs.Final

    Finds the final C++17 standard version of the filesystem library.

If no components are provided, behaves as if the
:find-component:`fs.Final` component was specified.

If both :find-component:`fs.Experimental` and :find-component:`fs.Final` are
provided, first looks for ``Final``, and falls back to ``Experimental`` in case
of failure. If ``Final`` is found, :imp-target:`std::filesystem` and all
:ref:`variables <fs.variables>` will refer to the ``Final`` version.


Imported Targets
****************

.. imp-target:: std::filesystem

    The ``std::filesystem`` imported target is defined when any requested
    version of the C++ filesystem library has been found, whether it is
    *Experimental* or *Final*.

    If no version of the filesystem library is available, this target will not
    be defined.

    .. note::
        This target has ``cxx_std_17`` as an ``INTERFACE``
        :ref:`compile language standard feature <req-lang-standards>`. Linking
        to this target will automatically enable C++17 if no later standard
        version is already required on the linking target.


.. _fs.variables:

Variables
*********

.. variable:: CXX_FILESYSTEM_IS_EXPERIMENTAL

    Set to ``TRUE`` when the :find-component:`fs.Experimental` version of C++
    filesystem library was found, otherwise ``FALSE``.

.. variable:: CXX_FILESYSTEM_HAVE_FS

    Set to ``TRUE`` when a filesystem header was found.

.. variable:: CXX_FILESYSTEM_HEADER

    Set to either ``filesystem`` or ``experimental/filesystem`` depending on
    whether :find-component:`fs.Final` or :find-component:`fs.Experimental` was
    found.

.. variable:: CXX_FILESYSTEM_NAMESPACE

    Set to either ``std::filesystem`` or ``std::experimental::filesystem``
    depending on whether :find-component:`fs.Final` or
    :find-component:`fs.Experimental` was found.


Examples
********

Using `find_package(Filesystem)` with no component arguments:

.. code-block:: cmake

    find_package(Filesystem REQUIRED)

    add_executable(my-program main.cpp)
    target_link_libraries(my-program PRIVATE std::filesystem)


#]=======================================================================]


if(TARGET std::filesystem)
    # This module has already been processed. Don't do it again.
    return()
endif()

# Ignore filesystem check if version too low 
if(CMAKE_VERSION VERSION_LESS 3.10)
    set(CXX_FILESYSTEM_HAVE_FS FALSE CACHE BOOL "TRUE if we have the C++ filesystem headers")
    set(Filesystem_FOUND FALSE CACHE BOOL "TRUE if we can run a program using std::filesystem" FORCE)
    return()
endif()

cmake_minimum_required(VERSION 3.10)
include(CMakePushCheckState)
include(CheckIncludeFileCXX)

# If we're not cross-compiling, try to run test executables.
# Otherwise, assume that compile + link is a sufficient check.
if(CMAKE_CROSSCOMPILING)
    include(CheckCXXSourceCompiles)
    macro(_cmcm_check_cxx_source code var)
        check_cxx_source_compiles("${code}" ${var})
    endmacro()
else()
    include(CheckCXXSourceRuns)
    macro(_cmcm_check_cxx_source code var)
        check_cxx_source_runs("${code}" ${var})
    endmacro()
endif()

cmake_push_check_state()

set(CMAKE_REQUIRED_QUIET ${Filesystem_FIND_QUIETLY})

# All of our tests required C++17 or later
set(BACKUP_CXX_STANDARD "${CMAKE_CXX_STANDARD}")
set(CMAKE_CXX_STANDARD 17)

# Normalize and check the component list we were given
set(want_components ${Filesystem_FIND_COMPONENTS})
if(Filesystem_FIND_COMPONENTS STREQUAL "")
    set(want_components Final)
endif()

# Warn on any unrecognized components
set(extra_components ${want_components})
list(REMOVE_ITEM extra_components Final Experimental)
foreach(component IN LISTS extra_components)
    message(WARNING "Extraneous find_package component for Filesystem: ${component}")
endforeach()

# Detect which of Experimental and Final we should look for
set(find_experimental TRUE)
set(find_final TRUE)
if(NOT "Final" IN_LIST want_components)
    set(find_final FALSE)
endif()
if(NOT "Experimental" IN_LIST want_components)
    set(find_experimental FALSE)
endif()

if(find_final)
    check_include_file_cxx("filesystem" _CXX_FILESYSTEM_HAVE_HEADER)
    mark_as_advanced(_CXX_FILESYSTEM_HAVE_HEADER)
    if(_CXX_FILESYSTEM_HAVE_HEADER)
        # We found the non-experimental header. Don't bother looking for the
        # experimental one.
        set(find_experimental FALSE)
    endif()
else()
    set(_CXX_FILESYSTEM_HAVE_HEADER FALSE)
endif()

if(find_experimental)
    check_include_file_cxx("experimental/filesystem" _CXX_FILESYSTEM_HAVE_EXPERIMENTAL_HEADER)
    mark_as_advanced(_CXX_FILESYSTEM_HAVE_EXPERIMENTAL_HEADER)
else()
    set(_CXX_FILESYSTEM_HAVE_EXPERIMENTAL_HEADER FALSE)
endif()

if(_CXX_FILESYSTEM_HAVE_HEADER)
    set(_have_fs TRUE)
    set(_fs_header filesystem)
    set(_fs_namespace std::filesystem)
    set(_is_experimental FALSE)
elseif(_CXX_FILESYSTEM_HAVE_EXPERIMENTAL_HEADER)
    set(_have_fs TRUE)
    set(_fs_header experimental/filesystem)
    set(_fs_namespace std::experimental::filesystem)
    set(_is_experimental TRUE)
else()
    set(_have_fs FALSE)
endif()

set(CXX_FILESYSTEM_HAVE_FS ${_have_fs} CACHE BOOL "TRUE if we have the C++ filesystem headers")
set(CXX_FILESYSTEM_HEADER ${_fs_header} CACHE STRING "The header that should be included to obtain the filesystem APIs")
set(CXX_FILESYSTEM_NAMESPACE ${_fs_namespace} CACHE STRING "The C++ namespace that contains the filesystem APIs")
set(CXX_FILESYSTEM_IS_EXPERIMENTAL ${_is_experimental} CACHE BOOL "TRUE if the C++ filesystem library is the experimental version")

set(_found FALSE)

if(CXX_FILESYSTEM_HAVE_FS)
    # We have some filesystem library available. Do link checks
    string(CONFIGURE [[
        #include <cstdio>
        #include <@CXX_FILESYSTEM_HEADER@>

        int main() {
            auto cwd = @CXX_FILESYSTEM_NAMESPACE@::current_path();
            printf("%s", cwd.generic_string().c_str());
            return EXIT_SUCCESS;
        }
    ]] code @ONLY)

    # HACK: Needed to compile correctly on Yocto Linux
    if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR CMAKE_CXX_COMPILER_ID STREQUAL "Clang"
        OR CMAKE_CXX_COMPILER_ID STREQUAL "AppleClang")
        set(CMAKE_REQUIRED_FLAGS ${prev_req_flags} -std=c++17)
    endif ()
    # Check a simple filesystem program without any linker flags
    _cmcm_check_cxx_source("${code}" CXX_FILESYSTEM_NO_LINK_NEEDED)

    set(can_link ${CXX_FILESYSTEM_NO_LINK_NEEDED})

    if(NOT CXX_FILESYSTEM_NO_LINK_NEEDED)
        set(prev_libraries ${CMAKE_REQUIRED_LIBRARIES})
        # Add the libstdc++ flag
        set(CMAKE_REQUIRED_LIBRARIES ${prev_libraries} -lstdc++fs)
        _cmcm_check_cxx_source("${code}" CXX_FILESYSTEM_STDCPPFS_NEEDED)
        set(can_link ${CXX_FILESYSTEM_STDCPPFS_NEEDED})
        if(NOT CXX_FILESYSTEM_STDCPPFS_NEEDED)
            # Try the libc++ flag
            set(CMAKE_REQUIRED_LIBRARIES ${prev_libraries} -lc++fs)
            _cmcm_check_cxx_source("${code}" CXX_FILESYSTEM_CPPFS_NEEDED)
            set(can_link ${CXX_FILESYSTEM_CPPFS_NEEDED})
        endif()
    endif()

    if(can_link)
        add_library(std::filesystem INTERFACE IMPORTED)
        set_property(TARGET std::filesystem APPEND PROPERTY INTERFACE_COMPILE_FEATURES cxx_std_17)
        set(_found TRUE)

        if(CXX_FILESYSTEM_NO_LINK_NEEDED)
            # Nothing to add...
        elseif(CXX_FILESYSTEM_STDCPPFS_NEEDED)
            set_property(TARGET std::filesystem APPEND PROPERTY INTERFACE_LINK_LIBRARIES -lstdc++fs)
        elseif(CXX_FILESYSTEM_CPPFS_NEEDED)
            set_property(TARGET std::filesystem APPEND PROPERTY INTERFACE_LINK_LIBRARIES -lc++fs)
        endif()
    endif()
endif()

cmake_pop_check_state()

set(Filesystem_FOUND ${_found} CACHE BOOL "TRUE if we can run a program using std::filesystem" FORCE)

if(Filesystem_FIND_REQUIRED AND NOT Filesystem_FOUND)
    message(FATAL_ERROR "Cannot run simple program using std::filesystem")
endif()

set(CMAKE_CXX_STANDARD "${BACKUP_CXX_STANDARD}")