absl/flags/declare.h

//
//  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.
//
// -----------------------------------------------------------------------------
// File: declare.h
// -----------------------------------------------------------------------------
//
// This file defines the ABSL_DECLARE_FLAG macro, allowing you to declare an
// `absl::Flag` for use within a translation unit. You should place this
// declaration within the header file associated with the .cc file that defines
// and owns the `Flag`.

#ifndef ABSL_FLAGS_DECLARE_H_
#define ABSL_FLAGS_DECLARE_H_

#include "absl/base/config.h"

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace flags_internal {

// absl::Flag<T> represents a flag of type 'T' created by ABSL_FLAG.
template <typename T>
class Flag;

}  // namespace flags_internal

// Flag
//
// Forward declaration of the `absl::Flag` type for use in defining the macro.
template <typename T>
using Flag = flags_internal::Flag<T>;

ABSL_NAMESPACE_END
}  // namespace absl

// ABSL_DECLARE_FLAG()
//
// This macro is a convenience for declaring use of an `absl::Flag` within a
// translation unit. This macro should be used within a header file to
// declare usage of the flag within any .cc file including that header file.
//
// The ABSL_DECLARE_FLAG(type, name) macro expands to:
//
//   extern absl::Flag<type> FLAGS_name;
#define ABSL_DECLARE_FLAG(type, name) ABSL_DECLARE_FLAG_INTERNAL(type, name)

// Internal implementation of ABSL_DECLARE_FLAG to allow macro expansion of its
// arguments. Clients must use ABSL_DECLARE_FLAG instead.
//
// The non-MSVC implementation declares the flag twice. This is to allow
// applying attributes to the second declaration. However, this causes a
// compile error (C4273) in MSVC if a `__declspec` is prepended to the macro.
#if defined(_MSC_VER)
#define ABSL_DECLARE_FLAG_INTERNAL(type, name) \
  extern absl::Flag<type> FLAGS_##name
#else
#define ABSL_DECLARE_FLAG_INTERNAL(type, name)               \
  extern absl::Flag<type> FLAGS_##name;                      \
  namespace absl /* block flags in namespaces */ {}          \
  /* second redeclaration is to allow applying attributes */ \
  extern absl::Flag<type> FLAGS_##name
#endif  // _MSC_VER

#endif  // ABSL_FLAGS_DECLARE_H_