Add documentation on optional flags to the flags library overview.

PiperOrigin-RevId: 453766125
Change-Id: Id4d88ae20bdadc960a65bc1010eea746f1eba051

absl/flags/flag.h

@@ -67,6 +67,10 @@ ABSL_NAMESPACE_BEGIN
 //      ABSL_FLAG(int, count, 0, "Count of items to process");
 //
 // No public methods of `absl::Flag<T>` are part of the Abseil Flags API.
+//
+// For type support of Abseil Flags, see the marshalling.h header file, which
+// discusses supported standard types, optional flags, and additional Abseil
+// type support.
 #if !defined(_MSC_VER) || defined(__clang__)
 template <typename T>
 using Flag = flags_internal::Flag<T>;

absl/flags/marshalling.h

@@ -33,6 +33,7 @@
 // * `double`
 // * `std::string`
 // * `std::vector<std::string>`
+// * `std::optional<T>`
 // * `absl::LogSeverity` (provided natively for layering reasons)
 //
 // Note that support for integral types is implemented using overloads for
@@ -65,6 +66,42 @@
 // below.)
 //
 // -----------------------------------------------------------------------------
+// Optional Flags
+// -----------------------------------------------------------------------------
+//
+// The Abseil flags library supports flags of type `std::optional<T>` where
+// `T` is a type of one of the supported flags. We refer to this flag type as
+// an "optional flag." An optional flag is either "valueless", holding no value
+// of type `T` (indicating that the flag has not been set) or a value of type
+// `T`. The valueless state in C++ code is represented by a value of
+// `std::nullopt` for the optional flag.
+//
+// Using `std::nullopt` as an optional flag's default value allows you to check
+// whether such a flag was ever specified on the command line:
+//
+//   if (absl::GetFlag(FLAGS_foo).has_value()) {
+//     // flag was set on command line
+//   } else {
+//     // flag was not passed on command line
+//   }
+//
+// Using an optional flag in this manner avoids common workarounds for
+// indicating such an unset flag (such as using sentinal values to indicate this
+// state).
+//
+// An optional flag also allows a developer to pass a flag in an "unset"
+// valueless state on the command line, allowing the flag to later be set in
+// binary logic. An optional flag's valueless state is indicated by the special
+// notation of passing the value as an empty string through the syntax `--flag=`
+// or `--flag ""`.
+//
+//   $ binary_with_optional --flag_in_unset_state=
+//   $ binary_with_optional --flag_in_unset_state ""
+//
+// Note: as a result of the above syntax requirements, an optional flag cannot
+// be set to a `T` of any value which unparses to the empty string.
+//
+// -----------------------------------------------------------------------------
 // Adding Type Support for Abseil Flags
 // -----------------------------------------------------------------------------
 //