Add file::size method (#145)

Unlike `std::filesystem::file_size`, does not require existing path for
the file

.clang-tidy

@@ -14,6 +14,7 @@ Checks: >
   -bugprone-easily-swappable-parameters,
   -cppcoreguidelines-avoid-do-while,
   -cppcoreguidelines-init-variables,
+  -cppcoreguidelines-pro-type-member-init,
   -cppcoreguidelines-pro-type-vararg,
   -misc-const-correctness,
   -misc-include-cleaner,

include/tmp/file

@@ -72,6 +72,16 @@ public:
                    std::string_view label = "",
                    std::string_view extension = "");
 
+  /// Returns the size of this file
+  /// @returns the size of this file, in bytes
+  /// @throws std::filesystem::filesystem_error if cannot get a file size
+  std::uintmax_t size() const;
+
+  /// Returns the size of this file
+  /// @param[out] ec Parameter for error reporting
+  /// @returns the size of this file, in bytes
+  std::uintmax_t size(std::error_code& ec) const noexcept;
+
   /// Reads the entire contents of this file
   /// @returns A string with this file contents
   /// @throws std::filesystem::filesystem_error if cannot read the file contents

src/file.cpp

@@ -19,6 +19,7 @@
 #include <Windows.h>
 #else
 #include <cerrno>
+#include <sys/stat.h>
 #include <unistd.h>
 #endif
 
@@ -131,6 +132,39 @@ file file::copy(const fs::path& path, std::string_view label,
   return tmpfile;
 }
 
+std::uintmax_t file::size() const {
+  std::error_code ec;
+  std::uintmax_t result = size(ec);
+
+  if (ec) {
+    throw fs::filesystem_error("Cannot get a temporary file size", path(), ec);
+  }
+
+  return result;
+}
+
+std::uintmax_t file::size(std::error_code& ec) const noexcept {
+#ifdef _WIN32
+  DWORD size_upper;
+  DWORD size_lower = GetFileSize(native_handle(), &size_upper);
+  if (size_upper == INVALID_FILE_SIZE) {
+    ec = std::error_code(GetLastError(), std::system_category());
+    return static_cast<std::uintmax_t>(-1);
+  }
+
+  std::uintmax_t size = size_upper;
+  return size << sizeof(DWORD) * CHAR_BIT | size_lower;
+#else
+  struct stat stat;
+  if (fstat(native_handle(), &stat) == -1) {
+    ec = std::error_code(errno, std::system_category());
+    return static_cast<std::uintmax_t>(-1);
+  }
+
+  return stat.st_size;
+#endif
+}
+
 std::string file::read() const {
   std::error_code ec;
   std::string content = read(ec);

tests/file.cpp

@@ -110,6 +110,18 @@ TEST(file, copy_directory) {
   EXPECT_THROW(file::copy(tmpdir), fs::filesystem_error);
 }
 
+/// Tests getting a file size
+TEST(file, size) {
+  file empty = file();
+  EXPECT_EQ(empty.size(), 0);
+
+  std::string_view content = "Hello, world!";
+
+  file nonempty = file();
+  nonempty.write(content);
+  EXPECT_EQ(nonempty.size(), content.size());
+}
+
 /// Tests binary file reading
 TEST(file, read_binary) {
   file tmpfile = file();