Autodetect std::filesystem support

Author: wravery

CMakeLists.txt

@@ -45,7 +45,7 @@ endfunction()
 
 find_package(Threads MODULE REQUIRED)
 
-find_package(pegtl 3.0.0 QUIET CONFIG)
+find_package(pegtl 3.0.1 QUIET CONFIG)
 if(NOT pegtl_FOUND)
   # If a compatible version of PEGTL is not already installed, build and install it from the submodule directory.
   set(PEGTL_BUILD_TESTS OFF CACHE BOOL "Disable PEGTL tests")

PEGTL

@@ -0,0 +1,22 @@
+// This is a dummy program that just needs to compile and link to tell us if
+// the C++17 std::filesystem API is available. Use CMake's configure_file
+// command to replace the FILESYSTEM_HEADER and FILESYSTEM_NAMESPACE tokens
+// for each combination of headers and namespaces which we want to pass to the
+// CMake try_compile command.
+
+#include <@FILESYSTEM_HEADER@>
+
+int main()
+{
+    try
+    {
+        throw @FILESYSTEM_NAMESPACE@::filesystem_error("instantiate one to make sure it links",
+            std::make_error_code(std::errc::function_not_supported));
+    }
+    catch (const @FILESYSTEM_NAMESPACE@::filesystem_error& error)
+    {
+        return -1;
+    }
+
+    return !@FILESYSTEM_NAMESPACE@::temp_directory_path().is_absolute();
+}

cmake/test_filesystem.cpp.in

@@ -50,13 +50,55 @@ if(GRAPHQL_BUILD_SCHEMAGEN)
 
   set(BOOST_COMPONENTS program_options)
   set(BOOST_LIBRARIES Boost::program_options)
-  
-  if(NOT MSVC)
-    set(BOOST_COMPONENTS ${BOOST_COMPONENTS} filesystem)
-    set(BOOST_LIBRARIES ${BOOST_LIBRARIES} Boost::filesystem)
-    target_compile_options(schemagen PRIVATE -DUSE_BOOST_FILESYSTEM)
+
+  # Try compiling a test program with std::filesystem or one of its alternatives.
+  function(check_filesystem_impl FILESYSTEM_HEADER FILESYSTEM_NAMESPACE OPTIONAL_LIBS OUT_RESULT)
+    set(TEST_FILE "test_${OUT_RESULT}.cpp")
+    configure_file(${CMAKE_CURRENT_SOURCE_DIR}/../cmake/test_filesystem.cpp.in ${TEST_FILE} @ONLY)
+
+    try_compile(TEST_RESULT
+      ${CMAKE_CURRENT_BINARY_DIR}
+      ${CMAKE_CURRENT_BINARY_DIR}/${TEST_FILE}
+      CXX_STANDARD 17)
+
+    if(NOT TEST_RESULT)
+      # Retry with each of the optional libraries.
+      foreach(OPTIONAL_LIB IN LISTS OPTIONAL_LIBS)
+        try_compile(TEST_RESULT
+          ${CMAKE_CURRENT_BINARY_DIR}
+          ${CMAKE_CURRENT_BINARY_DIR}/${TEST_FILE}
+          LINK_LIBRARIES ${OPTIONAL_LIB}
+          CXX_STANDARD 17)
+
+        if(TEST_RESULT)
+          # Looks like the optional library was required, go ahead and add it to the link options.
+          target_link_libraries(schemagen PRIVATE ${OPTIONAL_LIB})
+          break()
+        endif()
+      endforeach(OPTIONAL_LIB)
+    endif()
+
+    set(${OUT_RESULT} ${TEST_RESULT} PARENT_SCOPE)
+  endfunction(check_filesystem_impl)
+
+  # Try compiling a minimal program with each header/namespace, in order of preference:
+  #   C++17: #include <filesystem> // std::filesystem
+  #   Experimental C++17: #include <experimental/filesystem> // std::experimental::filesystem
+  #   Boost.Filesystem: #include <boost/filesystem.hpp> // boost::filesystem
+  check_filesystem_impl("filesystem" "std::filesystem" "stdc++fs;c++fs" STD_FILESYTEM)
+  if(STD_FILESYTEM)
+    target_compile_definitions(schemagen PRIVATE USE_STD_FILESYSTEM)
+  else()
+    check_filesystem_impl("experimental/filesystem" "std::experimental::filesystem" "stdc++fs;c++fs" STD_EXPERIMENTAL_FILESYTEM)
+    if(STD_EXPERIMENTAL_FILESYTEM)
+      target_compile_definitions(schemagen PRIVATE USE_STD_EXPERIMENTAL_FILESYSTEM)
+    else()
+      set(BOOST_COMPONENTS ${BOOST_COMPONENTS} filesystem)
+      set(BOOST_LIBRARIES ${BOOST_LIBRARIES} Boost::filesystem)
+      target_compile_definitions(schemagen PRIVATE USE_BOOST_FILESYSTEM)
+    endif()
   endif()
-  
+
   find_package(Boost REQUIRED COMPONENTS ${BOOST_COMPONENTS})
   target_link_libraries(schemagen PRIVATE ${BOOST_LIBRARIES})
 

src/CMakeLists.txt

@@ -5,13 +5,24 @@
 
 #include <boost/program_options.hpp>
 
-#ifdef USE_BOOST_FILESYSTEM
-#include <boost/filesystem.hpp>
-namespace fs = boost::filesystem;
+// clang-format off
+#ifdef USE_STD_FILESYSTEM
+	#include <filesystem>
+	namespace fs = std::filesystem;
 #else
-#include <filesystem>
-namespace fs = std::filesystem;
+	#ifdef USE_STD_EXPERIMENTAL_FILESYSTEM
+		#include <experimental/filesystem>
+		namespace fs = std::experimental::filesystem;
+	#else
+		#ifdef USE_BOOST_FILESYSTEM
+			#include <boost/filesystem.hpp>
+			namespace fs = boost::filesystem;
+		#else
+			#error "No std::filesystem implementation defined"
+		#endif
+	#endif
 #endif
+// clang-format on
 
 #include <cctype>
 #include <fstream>