detecting need to link with stdfs

CMakeLists.txt

@@ -22,6 +22,8 @@ include(FetchContent)
 #Set default build type if none was specified
 include(cmake/helpers.cmake)
 default_build_type("Release")
+link_std_fs()
+message(STATUS fs_lib:${STD_FS_LIB})
 
 set(CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake" ${CMAKE_MODULE_PATH})
 
@@ -310,7 +312,15 @@ target_include_directories(aare_core PUBLIC
     "$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>"
 )
 
-target_link_libraries(aare_core PUBLIC fmt::fmt nlohmann_json::nlohmann_json PRIVATE aare_compiler_flags )
+target_link_libraries(
+    aare_core 
+    PUBLIC 
+    fmt::fmt 
+    nlohmann_json::nlohmann_json 
+    ${STD_FS_LIB} # from helpers.cmake
+    PRIVATE 
+    aare_compiler_flags 
+)
 
 set_target_properties(aare_core PROPERTIES
     ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}

cmake/helpers.cmake

@@ -4,3 +4,43 @@ if (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
   set(CMAKE_BUILD_TYPE ${val} CACHE STRING "Build type (default ${val})" FORCE)
 endif()
 endfunction()
+
+function(link_std_fs)
+# from pybind11
+# Check if we need to add -lstdc++fs or -lc++fs or nothing
+if(DEFINED CMAKE_CXX_STANDARD AND CMAKE_CXX_STANDARD LESS 17)
+  set(STD_FS_NO_LIB_NEEDED TRUE)
+elseif(MSVC)
+  set(STD_FS_NO_LIB_NEEDED TRUE)
+else()
+  file(
+    WRITE ${CMAKE_CURRENT_BINARY_DIR}/main.cpp
+    "#include <filesystem>\nint main(int argc, char ** argv) {\n  std::filesystem::path p(argv[0]);\n  return p.string().length();\n}"
+  )
+  try_compile(
+    STD_FS_NO_LIB_NEEDED ${CMAKE_CURRENT_BINARY_DIR}
+    SOURCES ${CMAKE_CURRENT_BINARY_DIR}/main.cpp
+    COMPILE_DEFINITIONS -std=c++17)
+  try_compile(
+    STD_FS_NEEDS_STDCXXFS ${CMAKE_CURRENT_BINARY_DIR}
+    SOURCES ${CMAKE_CURRENT_BINARY_DIR}/main.cpp
+    COMPILE_DEFINITIONS -std=c++17
+    LINK_LIBRARIES stdc++fs)
+  try_compile(
+    STD_FS_NEEDS_CXXFS ${CMAKE_CURRENT_BINARY_DIR}
+    SOURCES ${CMAKE_CURRENT_BINARY_DIR}/main.cpp
+    COMPILE_DEFINITIONS -std=c++17
+    LINK_LIBRARIES c++fs)
+endif()
+
+if(${STD_FS_NEEDS_STDCXXFS})
+  set(STD_FS_LIB stdc++fs)
+elseif(${STD_FS_NEEDS_CXXFS})
+  set(STD_FS_LIB c++fs)
+elseif(${STD_FS_NO_LIB_NEEDED})
+  set(STD_FS_LIB "")
+else()
+  message(WARNING "Unknown C++17 compiler - not passing -lstdc++fs")
+  set(STD_FS_LIB "")
+endif()
+endfunction()