THUAI6/CAPI/cpp/grpc/include/absl/flags/internal/registry.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.

#ifndef ABSL_FLAGS_INTERNAL_REGISTRY_H_
#define ABSL_FLAGS_INTERNAL_REGISTRY_H_

#include <functional>

#include "absl/base/config.h"
#include "absl/flags/commandlineflag.h"
#include "absl/flags/internal/commandlineflag.h"
#include "absl/strings/string_view.h"

// --------------------------------------------------------------------
// Global flags registry API.

namespace absl
{
    ABSL_NAMESPACE_BEGIN
    namespace flags_internal
    {

        // Executes specified visitor for each non-retired flag in the registry. While
        // callback are executed, the registry is locked and can't be changed.
        void ForEachFlag(std::function<void(CommandLineFlag&)> visitor);

        //-----------------------------------------------------------------------------

        bool RegisterCommandLineFlag(CommandLineFlag&, const char* filename);

        void FinalizeRegistry();

        //-----------------------------------------------------------------------------
        // Retired registrations:
        //
        // Retired flag registrations are treated specially. A 'retired' flag is
        // provided only for compatibility with automated invocations that still
        // name it.  A 'retired' flag:
        //   - is not bound to a C++ FLAGS_ reference.
        //   - has a type and a value, but that value is intentionally inaccessible.
        //   - does not appear in --help messages.
        //   - is fully supported by _all_ flag parsing routines.
        //   - consumes args normally, and complains about type mismatches in its
        //     argument.
        //   - emits a complaint but does not die (e.g. LOG(ERROR)) if it is
        //     accessed by name through the flags API for parsing or otherwise.
        //
        // The registrations for a flag happen in an unspecified order as the
        // initializers for the namespace-scope objects of a program are run.
        // Any number of weak registrations for a flag can weakly define the flag.
        // One non-weak registration will upgrade the flag from weak to non-weak.
        // Further weak registrations of a non-weak flag are ignored.
        //
        // This mechanism is designed to support moving dead flags into a
        // 'graveyard' library.  An example migration:
        //
        //   0: Remove references to this FLAGS_flagname in the C++ codebase.
        //   1: Register as 'retired' in old_lib.
        //   2: Make old_lib depend on graveyard.
        //   3: Add a redundant 'retired' registration to graveyard.
        //   4: Remove the old_lib 'retired' registration.
        //   5: Eventually delete the graveyard registration entirely.
        //

        // Retire flag with name "name" and type indicated by ops.
        void Retire(const char* name, FlagFastTypeId type_id, char* buf);

        constexpr size_t kRetiredFlagObjSize = 3 * sizeof(void*);
        constexpr size_t kRetiredFlagObjAlignment = alignof(void*);

        // Registered a retired flag with name 'flag_name' and type 'T'.
        template<typename T>
        class RetiredFlag
        {
        public:
            void Retire(const char* flag_name)
            {
                flags_internal::Retire(flag_name, base_internal::FastTypeId<T>(), buf_);
            }

        private:
            alignas(kRetiredFlagObjAlignment) char buf_[kRetiredFlagObjSize];
        };

    }  // namespace flags_internal
    ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_FLAGS_INTERNAL_REGISTRY_H_