Implement file::native_handle for C++17 (#172)

When opening `tmp::filebuf`, simply save the open handle, which can be
used to get a native handle

Author: bugdea1er

include/tmp/file

@@ -61,16 +61,12 @@ public:
   static file copy(const std::filesystem::path& path,
                    std::ios::openmode mode = std::ios::in | std::ios::out);
 
-#if __cpp_lib_fstream_native_handle >= 202306L
   /// Implementation-defined handle type to the temporary file
-  using native_handle_type = std::filebuf::native_handle_type;
+  using native_handle_type = filebuf::native_handle_type;
 
-  /// Returns an implementation-defined handle to this entry
+  /// Returns an implementation-defined handle to this file
   /// @returns The underlying implementation-defined handle
-  native_handle_type native_handle() const noexcept {
-    return rdbuf()->native_handle();
-  }
-#endif
+  native_handle_type native_handle() const noexcept;
 
   /// Returns pointer to the underlying raw file device object
   /// @returns A pointer to the underlying raw file device

include/tmp/filebuf

@@ -16,20 +16,40 @@ namespace tmp {
 /// opening a file fails
 class filebuf : public std::filebuf {
 public:
+  /// Implementation-defined handle type to the open file
+#if defined(_WIN32)
+  using open_handle_type = std::FILE*;
+#elif __has_include(<unistd.h>)
+  using open_handle_type = int;    // POSIX file descriptor
+#else
+#error "Target platform not supported"
+#endif
+
   /// Implementation-defined handle type to the file
 #if defined(_WIN32)
-  using native_handle_type = std::FILE*;
+  using native_handle_type = void*;    // HANDLE
 #elif __has_include(<unistd.h>)
   using native_handle_type = int;    // POSIX file descriptor
 #else
 #error "Target platform not supported"
 #endif
 
+  /// Creates a file buffer that is not associated with any file
+  filebuf() noexcept;
+
   /// Opens a file and configures it as the associated character sequence
   /// @param handle A handle to the open file
   /// @param mode   The file opening mode
   /// @returns `this` file buffer, or a null pointer on failure
-  filebuf* open(native_handle_type handle, std::ios::openmode mode);
+  filebuf* open(open_handle_type handle, std::ios::openmode mode);
+
+  /// Returns an implementation-defined handle to this file
+  /// @returns The underlying implementation-defined handle
+  native_handle_type native_handle() const noexcept;
+
+private:
+  /// Native handle to the open file
+  open_handle_type handle;
 };
 }    // namespace tmp
 

src/create.cpp

@@ -119,7 +119,7 @@ const wchar_t* make_mdstring(std::ios::openmode mode) noexcept {
 
 /// Closes the given handle, ignoring any errors
 /// @param[in] handle The handle to close
-void close(filebuf::native_handle_type handle) noexcept {
+void close(filebuf::open_handle_type handle) noexcept {
 #ifdef _WIN32
   fclose(handle);
 #else

src/file.cpp

@@ -35,6 +35,10 @@ file file::copy(const fs::path& path, std::ios::openmode mode) {
   return tmpfile;
 }
 
+file::native_handle_type file::native_handle() const noexcept {
+  return sb.native_handle();
+}
+
 std::filebuf* file::rdbuf() const noexcept {
   return const_cast<filebuf*>(std::addressof(sb));    // NOLINT(*-const-cast)
 }

src/filebuf.cpp

@@ -2,13 +2,40 @@
 
 #include <ios>
 
+#ifdef _WIN32
+#include <Windows.h>
+#include <io.h>
+#endif
+
 #if __has_include(<__config>)
 #include <__config>    // libc++ configuration
 #endif
 
 namespace tmp {
 
-filebuf* filebuf::open(native_handle_type handle, std::ios::openmode mode) {
+// Confirm that native_handle_type matches `TriviallyCopyable` named requirement
+static_assert(std::is_trivially_copyable_v<filebuf::native_handle_type>);
+
+#ifdef _WIN32
+// Confirm that `HANDLE` is as implemented in `entry`
+static_assert(std::is_same_v<HANDLE, filebuf::native_handle_type>);
+#endif
+
+filebuf::filebuf() noexcept
+    :
+#if defined(_LIBCPP_VERSION) || defined(__GLIBCXX__)
+      handle(-1)
+#elif defined(_MSC_VER)
+      handle(nullptr)
+#elif
+#error "Target C++ standard library is not supported"
+#endif
+{
+}
+
+filebuf* filebuf::open(open_handle_type handle, std::ios::openmode mode) {
+  this->handle = handle;
+
 #if defined(_LIBCPP_VERSION)
   return this->__open(handle, mode) != nullptr ? this : nullptr;
 #elif defined(__GLIBCXX__)
@@ -40,4 +67,17 @@ filebuf* filebuf::open(native_handle_type handle, std::ios::openmode mode) {
 #error "Target C++ standard library is not supported"
 #endif
 }
+
+filebuf::native_handle_type filebuf::native_handle() const noexcept {
+#ifdef _WIN32
+  intptr_t osfhandle = _get_osfhandle(_fileno(handle));
+  if (osfhandle == -1) {
+    return nullptr;
+  }
+
+  return reinterpret_cast<void*>(osfhandle);
+#else
+  return handle;
+#endif
+}
 }    // namespace tmp

tests/file.cpp

@@ -26,7 +26,6 @@ bool is_open(const file& file) {
   return file.rdbuf()->is_open();
 }
 
-#if __cpp_lib_fstream_native_handle >= 202306L
 /// Checks if the given file handle is valid
 /// @param handle handle to check
 /// @returns whether the handle is valid
@@ -38,7 +37,6 @@ bool is_open(file::native_handle_type handle) {
   return fcntl(handle, F_GETFD) != -1;
 #endif
 }
-#endif
 
 /// Tests file type traits and member types
 TEST(file, type_traits) {
@@ -61,10 +59,7 @@ TEST(file, create) {
   EXPECT_TRUE(fs::is_regular_file(tmpfile));
   EXPECT_TRUE(fs::equivalent(parent, fs::temp_directory_path()));
   EXPECT_TRUE(is_open(tmpfile));
-
-#if __cpp_lib_fstream_native_handle >= 202306L
   EXPECT_TRUE(is_open(tmpfile.native_handle()));
-#endif
 
   fs::perms permissions = fs::status(tmpfile).permissions();
 #ifdef _WIN32
@@ -199,22 +194,16 @@ TEST(file, move_to_non_existing_directory) {
 /// Tests that destructor removes a file
 TEST(file, destructor) {
   fs::path path;
-#if __cpp_lib_fstream_native_handle >= 202306L
   file::native_handle_type handle;
-#endif
 
   {
     file tmpfile = file();
     path = tmpfile;
-#if __cpp_lib_fstream_native_handle >= 202306L
     handle = tmpfile.native_handle();
-#endif
   }
 
   EXPECT_FALSE(fs::exists(path));
-#if __cpp_lib_fstream_native_handle >= 202306L
   EXPECT_FALSE(is_open(handle));
-#endif
 }
 
 /// Tests file move constructor