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Diffstat (limited to 'absl/flags/commandlineflag.h')
| -rw-r--r-- | absl/flags/commandlineflag.h | 200 |
1 files changed, 200 insertions, 0 deletions
diff --git a/absl/flags/commandlineflag.h b/absl/flags/commandlineflag.h new file mode 100644 index 00000000..f2fa0897 --- /dev/null +++ b/absl/flags/commandlineflag.h @@ -0,0 +1,200 @@ +// +// Copyright 2020 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: commandlineflag.h +// ----------------------------------------------------------------------------- +// +// This header file defines the `CommandLineFlag`, which acts as a type-erased +// handle for accessing metadata about the Abseil Flag in question. +// +// Because an actual Abseil flag is of an unspecified type, you should not +// manipulate or interact directly with objects of that type. Instead, use the +// CommandLineFlag type as an intermediary. +#ifndef ABSL_FLAGS_COMMANDLINEFLAG_H_ +#define ABSL_FLAGS_COMMANDLINEFLAG_H_ + +#include <memory> +#include <string> + +#include "absl/base/config.h" +#include "absl/base/internal/fast_type_id.h" +#include "absl/flags/internal/commandlineflag.h" +#include "absl/strings/string_view.h" +#include "absl/types/optional.h" + +namespace absl { +ABSL_NAMESPACE_BEGIN +namespace flags_internal { +class PrivateHandleAccessor; +} // namespace flags_internal + +// CommandLineFlag +// +// This type acts as a type-erased handle for an instance of an Abseil Flag and +// holds reflection information pertaining to that flag. Use CommandLineFlag to +// access a flag's name, location, help string etc. +// +// To obtain an absl::CommandLineFlag, invoke `absl::FindCommandLineFlag()` +// passing it the flag name string. +// +// Example: +// +// // Obtain reflection handle for a flag named "flagname". +// const absl::CommandLineFlag* my_flag_data = +// absl::FindCommandLineFlag("flagname"); +// +// // Now you can get flag info from that reflection handle. +// std::string flag_location = my_flag_data->Filename(); +// ... +class CommandLineFlag { + public: + constexpr CommandLineFlag() = default; + + // Not copyable/assignable. + CommandLineFlag(const CommandLineFlag&) = delete; + CommandLineFlag& operator=(const CommandLineFlag&) = delete; + + // absl::CommandLineFlag::IsOfType() + // + // Return true iff flag has type T. + template <typename T> + inline bool IsOfType() const { + return TypeId() == base_internal::FastTypeId<T>(); + } + + // absl::CommandLineFlag::TryGet() + // + // Attempts to retrieve the flag value. Returns value on success, + // absl::nullopt otherwise. + template <typename T> + absl::optional<T> TryGet() const { + if (IsRetired() || !IsOfType<T>()) { + return absl::nullopt; + } + + // Implementation notes: + // + // We are wrapping a union around the value of `T` to serve three purposes: + // + // 1. `U.value` has correct size and alignment for a value of type `T` + // 2. The `U.value` constructor is not invoked since U's constructor does + // not do it explicitly. + // 3. The `U.value` destructor is invoked since U's destructor does it + // explicitly. This makes `U` a kind of RAII wrapper around non default + // constructible value of T, which is destructed when we leave the + // scope. We do need to destroy U.value, which is constructed by + // CommandLineFlag::Read even though we left it in a moved-from state + // after std::move. + // + // All of this serves to avoid requiring `T` being default constructible. + union U { + T value; + U() {} + ~U() { value.~T(); } + }; + U u; + + Read(&u.value); + // allow retired flags to be "read", so we can report invalid access. + if (IsRetired()) { + return absl::nullopt; + } + return std::move(u.value); + } + + // absl::CommandLineFlag::Name() + // + // Returns name of this flag. + virtual absl::string_view Name() const = 0; + + // absl::CommandLineFlag::Filename() + // + // Returns name of the file where this flag is defined. + virtual std::string Filename() const = 0; + + // absl::CommandLineFlag::Help() + // + // Returns help message associated with this flag. + virtual std::string Help() const = 0; + + // absl::CommandLineFlag::IsRetired() + // + // Returns true iff this object corresponds to retired flag. + virtual bool IsRetired() const; + + // absl::CommandLineFlag::DefaultValue() + // + // Returns the default value for this flag. + virtual std::string DefaultValue() const = 0; + + // absl::CommandLineFlag::CurrentValue() + // + // Returns the current value for this flag. + virtual std::string CurrentValue() const = 0; + + // absl::CommandLineFlag::ParseFrom() + // + // Sets the value of the flag based on specified string `value`. If the flag + // was successfully set to new value, it returns true. Otherwise, sets `error` + // to indicate the error, leaves the flag unchanged, and returns false. + bool ParseFrom(absl::string_view value, std::string* error); + + protected: + ~CommandLineFlag() = default; + + private: + friend class flags_internal::PrivateHandleAccessor; + + // Sets the value of the flag based on specified string `value`. If the flag + // was successfully set to new value, it returns true. Otherwise, sets `error` + // to indicate the error, leaves the flag unchanged, and returns false. There + // are three ways to set the flag's value: + // * Update the current flag value + // * Update the flag's default value + // * Update the current flag value if it was never set before + // The mode is selected based on `set_mode` parameter. + virtual bool ParseFrom(absl::string_view value, + flags_internal::FlagSettingMode set_mode, + flags_internal::ValueSource source, + std::string& error) = 0; + + // Returns id of the flag's value type. + virtual flags_internal::FlagFastTypeId TypeId() const = 0; + + // Interface to save flag to some persistent state. Returns current flag state + // or nullptr if flag does not support saving and restoring a state. + virtual std::unique_ptr<flags_internal::FlagStateInterface> SaveState() = 0; + + // Copy-construct a new value of the flag's type in a memory referenced by + // the dst based on the current flag's value. + virtual void Read(void* dst) const = 0; + + // To be deleted. Used to return true if flag's current value originated from + // command line. + virtual bool IsSpecifiedOnCommandLine() const = 0; + + // Validates supplied value usign validator or parseflag routine + virtual bool ValidateInputValue(absl::string_view value) const = 0; + + // Checks that flags default value can be converted to string and back to the + // flag's value type. + virtual void CheckDefaultValueParsingRoundtrip() const = 0; +}; + +ABSL_NAMESPACE_END +} // namespace absl + +#endif // ABSL_FLAGS_COMMANDLINEFLAG_H_ |