declare.h (2424B)
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_