1 files changed, 103 insertions, 0 deletions
diff --git a/absl/flags/internal/flag.h b/absl/flags/internal/flag.h
new file mode 100644
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+++ b/absl/flags/internal/flag.h
@@ -0,0 +1,103 @@
+//
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_FLAGS_INTERNAL_FLAG_H_
+#define ABSL_FLAGS_INTERNAL_FLAG_H_
+
+#include "absl/flags/internal/commandlineflag.h"
+#include "absl/flags/internal/registry.h"
+
+namespace absl {
+namespace flags_internal {
+
+// This is "unspecified" implementation of absl::Flag<T> type.
+template <typename T>
+class Flag {
+ public:
+  constexpr Flag(const char* name, const flags_internal::HelpGenFunc help_gen,
+                 const char* filename,
+                 const flags_internal::FlagMarshallingOpFn marshalling_op,
+                 const flags_internal::InitialValGenFunc initial_value_gen)
+      : internal(name, flags_internal::HelpText::FromFunctionPointer(help_gen),
+                 filename, &flags_internal::FlagOps<T>, marshalling_op,
+                 initial_value_gen,
+                 /*retired_arg=*/false, /*def_arg=*/nullptr,
+                 /*cur_arg=*/nullptr) {}
+
+  // Not copyable/assignable.
+  Flag(const Flag<T>&) = delete;
+  Flag<T>& operator=(const Flag<T>&) = delete;
+
+  absl::string_view Name() const { return internal.Name(); }
+  std::string Help() const { return internal.Help(); }
+  std::string Filename() const { return internal.Filename(); }
+
+  absl::flags_internal::CommandLineFlag internal;
+
+  void SetCallback(const flags_internal::FlagCallback mutation_callback) {
+    internal.SetCallback(mutation_callback);
+  }
+
+ private:
+  // TODO(rogeeff): add these validations once UnparseFlag invocation is fixed
+  // for built-in types and when we cleanup existing code from operating on
+  // forward declared types.
+  //  auto IsCopyConstructible(const T& v) -> decltype(T(v));
+  //  auto HasAbslParseFlag(absl::string_view in, T* dst, std::string* err)
+  //      -> decltype(AbslParseFlag(in, dst, GlobalStringADLGuard(err)));
+  //  auto HasAbslUnparseFlag(const T& v) -> decltype(AbslUnparseFlag(v));
+};
+
+// This class facilitates Flag object registration and tail expression-based
+// flag definition, for example:
+// ABSL_FLAG(int, foo, 42, "Foo help").OnUpdate(NotifyFooWatcher);
+template <typename T, bool do_register>
+class FlagRegistrar {
+ public:
+  explicit FlagRegistrar(Flag<T>* flag) : flag_(flag) {
+    if (do_register) flags_internal::RegisterCommandLineFlag(&flag_->internal);
+  }
+
+  FlagRegistrar& OnUpdate(flags_internal::FlagCallback cb) && {
+    flag_->SetCallback(cb);
+    return *this;
+  }
+
+  // Make the registrar "die" gracefully as a bool on a line where registration
+  // happens. Registrar objects are intended to live only as temporary.
+  operator bool() const { return true; }  // NOLINT
+
+ private:
+  Flag<T>* flag_;  // Flag being registered (not owned).
+};
+
+// This struct and corresponding overload to MakeDefaultValue are used to
+// facilitate usage of {} as default value in ABSL_FLAG macro.
+struct EmptyBraces {};
+
+template <typename T>
+T* MakeFromDefaultValue(T t) {
+  return new T(std::move(t));
+}
+
+template <typename T>
+T* MakeFromDefaultValue(EmptyBraces) {
+  return new T;
+}
+
+}  // namespace flags_internal
+}  // namespace absl
+
+#endif  // ABSL_FLAGS_INTERNAL_FLAG_H_