xref: /aosp_15_r20/external/cronet/third_party/abseil-cpp/absl/flags/declare.h (revision 6777b5387eb2ff775bb5750e3f5d96f37fb7352b)
1 //
2 //  Copyright 2019 The Abseil Authors.
3 //
4 // Licensed under the Apache License, Version 2.0 (the "License");
5 // you may not use this file except in compliance with the License.
6 // You may obtain a copy of the License at
7 //
8 //      https://www.apache.org/licenses/LICENSE-2.0
9 //
10 // Unless required by applicable law or agreed to in writing, software
11 // distributed under the License is distributed on an "AS IS" BASIS,
12 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 // See the License for the specific language governing permissions and
14 // limitations under the License.
15 //
16 // -----------------------------------------------------------------------------
17 // File: declare.h
18 // -----------------------------------------------------------------------------
19 //
20 // This file defines the ABSL_DECLARE_FLAG macro, allowing you to declare an
21 // `absl::Flag` for use within a translation unit. You should place this
22 // declaration within the header file associated with the .cc file that defines
23 // and owns the `Flag`.
24 
25 #ifndef ABSL_FLAGS_DECLARE_H_
26 #define ABSL_FLAGS_DECLARE_H_
27 
28 #include "absl/base/config.h"
29 
30 namespace absl {
31 ABSL_NAMESPACE_BEGIN
32 namespace flags_internal {
33 
34 // absl::Flag<T> represents a flag of type 'T' created by ABSL_FLAG.
35 template <typename T>
36 class Flag;
37 
38 }  // namespace flags_internal
39 
40 // Flag
41 //
42 // Forward declaration of the `absl::Flag` type for use in defining the macro.
43 template <typename T>
44 using Flag = flags_internal::Flag<T>;
45 
46 ABSL_NAMESPACE_END
47 }  // namespace absl
48 
49 // ABSL_DECLARE_FLAG()
50 //
51 // This macro is a convenience for declaring use of an `absl::Flag` within a
52 // translation unit. This macro should be used within a header file to
53 // declare usage of the flag within any .cc file including that header file.
54 //
55 // The ABSL_DECLARE_FLAG(type, name) macro expands to:
56 //
57 //   extern absl::Flag<type> FLAGS_name;
58 #define ABSL_DECLARE_FLAG(type, name) ABSL_DECLARE_FLAG_INTERNAL(type, name)
59 
60 // Internal implementation of ABSL_DECLARE_FLAG to allow macro expansion of its
61 // arguments. Clients must use ABSL_DECLARE_FLAG instead.
62 #define ABSL_DECLARE_FLAG_INTERNAL(type, name)               \
63   extern absl::Flag<type> FLAGS_##name;                      \
64   namespace absl /* block flags in namespaces */ {}          \
65   /* second redeclaration is to allow applying attributes */ \
66   extern absl::Flag<type> FLAGS_##name
67 
68 #endif  // ABSL_FLAGS_DECLARE_H_
69