265 lines
10 KiB
C++
265 lines
10 KiB
C++
//
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// Copyright 2019 The Abseil Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//
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// -----------------------------------------------------------------------------
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// File: marshalling.h
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// -----------------------------------------------------------------------------
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//
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// This header file defines the API for extending Abseil flag support to
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// custom types, and defines the set of overloads for fundamental types.
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//
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// Out of the box, the Abseil flags library supports the following types:
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//
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// * `bool`
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// * `int16_t`
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// * `uint16_t`
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// * `int32_t`
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// * `uint32_t`
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// * `int64_t`
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// * `uint64_t`
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// * `float`
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// * `double`
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// * `std::string`
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// * `std::vector<std::string>`
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// * `absl::LogSeverity` (provided natively for layering reasons)
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//
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// Note that support for integral types is implemented using overloads for
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// variable-width fundamental types (`short`, `int`, `long`, etc.). However,
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// you should prefer the fixed-width integral types (`int32_t`, `uint64_t`,
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// etc.) we've noted above within flag definitions.
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//
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// In addition, several Abseil libraries provide their own custom support for
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// Abseil flags. Documentation for these formats is provided in the type's
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// `AbslParseFlag()` definition.
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//
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// The Abseil time library provides the following support for civil time values:
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//
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// * `absl::CivilSecond`
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// * `absl::CivilMinute`
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// * `absl::CivilHour`
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// * `absl::CivilDay`
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// * `absl::CivilMonth`
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// * `absl::CivilYear`
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//
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// and also provides support for the following absolute time values:
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//
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// * `absl::Duration`
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// * `absl::Time`
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//
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// Additional support for Abseil types will be noted here as it is added.
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//
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// You can also provide your own custom flags by adding overloads for
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// `AbslParseFlag()` and `AbslUnparseFlag()` to your type definitions. (See
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// below.)
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//
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// -----------------------------------------------------------------------------
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// Adding Type Support for Abseil Flags
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// -----------------------------------------------------------------------------
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//
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// To add support for your user-defined type, add overloads of `AbslParseFlag()`
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// and `AbslUnparseFlag()` as free (non-member) functions to your type. If `T`
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// is a class type, these functions can be friend function definitions. These
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// overloads must be added to the same namespace where the type is defined, so
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// that they can be discovered by Argument-Dependent Lookup (ADL).
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//
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// Example:
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//
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// namespace foo {
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//
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// enum OutputMode { kPlainText, kHtml };
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//
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// // AbslParseFlag converts from a string to OutputMode.
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// // Must be in same namespace as OutputMode.
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//
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// // Parses an OutputMode from the command line flag value `text`. Returns
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// // `true` and sets `*mode` on success; returns `false` and sets `*error`
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// // on failure.
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// bool AbslParseFlag(absl::string_view text,
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// OutputMode* mode,
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// std::string* error) {
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// if (text == "plaintext") {
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// *mode = kPlainText;
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// return true;
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// }
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// if (text == "html") {
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// *mode = kHtml;
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// return true;
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// }
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// *error = "unknown value for enumeration";
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// return false;
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// }
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//
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// // AbslUnparseFlag converts from an OutputMode to a string.
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// // Must be in same namespace as OutputMode.
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//
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// // Returns a textual flag value corresponding to the OutputMode `mode`.
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// std::string AbslUnparseFlag(OutputMode mode) {
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// switch (mode) {
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// case kPlainText: return "plaintext";
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// case kHtml: return "html";
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// }
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// return absl::StrCat(mode);
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// }
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//
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// Notice that neither `AbslParseFlag()` nor `AbslUnparseFlag()` are class
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// members, but free functions. `AbslParseFlag/AbslUnparseFlag()` overloads
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// for a type should only be declared in the same file and namespace as said
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// type. The proper `AbslParseFlag/AbslUnparseFlag()` implementations for a
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// given type will be discovered via Argument-Dependent Lookup (ADL).
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//
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// `AbslParseFlag()` may need, in turn, to parse simpler constituent types
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// using `absl::ParseFlag()`. For example, a custom struct `MyFlagType`
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// consisting of a `std::pair<int, std::string>` would add an `AbslParseFlag()`
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// overload for its `MyFlagType` like so:
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//
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// Example:
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//
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// namespace my_flag_type {
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//
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// struct MyFlagType {
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// std::pair<int, std::string> my_flag_data;
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// };
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//
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// bool AbslParseFlag(absl::string_view text, MyFlagType* flag,
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// std::string* err);
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//
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// std::string AbslUnparseFlag(const MyFlagType&);
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//
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// // Within the implementation, `AbslParseFlag()` will, in turn invoke
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// // `absl::ParseFlag()` on its constituent `int` and `std::string` types
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// // (which have built-in Abseil flag support).
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//
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// bool AbslParseFlag(absl::string_view text, MyFlagType* flag,
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// std::string* err) {
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// std::pair<absl::string_view, absl::string_view> tokens =
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// absl::StrSplit(text, ',');
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// if (!absl::ParseFlag(tokens.first, &flag->my_flag_data.first, err))
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// return false;
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// if (!absl::ParseFlag(tokens.second, &flag->my_flag_data.second, err))
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// return false;
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// return true;
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// }
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//
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// // Similarly, for unparsing, we can simply invoke `absl::UnparseFlag()` on
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// // the constituent types.
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// std::string AbslUnparseFlag(const MyFlagType& flag) {
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// return absl::StrCat(absl::UnparseFlag(flag.my_flag_data.first),
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// ",",
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// absl::UnparseFlag(flag.my_flag_data.second));
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// }
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#ifndef ABSL_FLAGS_MARSHALLING_H_
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#define ABSL_FLAGS_MARSHALLING_H_
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#include <string>
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#include <vector>
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#include "absl/base/config.h"
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#include "absl/strings/string_view.h"
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namespace absl {
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ABSL_NAMESPACE_BEGIN
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namespace flags_internal {
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// Overloads of `AbslParseFlag()` and `AbslUnparseFlag()` for fundamental types.
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bool AbslParseFlag(absl::string_view, bool*, std::string*);
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bool AbslParseFlag(absl::string_view, short*, std::string*); // NOLINT
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bool AbslParseFlag(absl::string_view, unsigned short*, std::string*); // NOLINT
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bool AbslParseFlag(absl::string_view, int*, std::string*); // NOLINT
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bool AbslParseFlag(absl::string_view, unsigned int*, std::string*); // NOLINT
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bool AbslParseFlag(absl::string_view, long*, std::string*); // NOLINT
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bool AbslParseFlag(absl::string_view, unsigned long*, std::string*); // NOLINT
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bool AbslParseFlag(absl::string_view, long long*, std::string*); // NOLINT
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bool AbslParseFlag(absl::string_view, unsigned long long*, // NOLINT
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std::string*);
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bool AbslParseFlag(absl::string_view, float*, std::string*);
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bool AbslParseFlag(absl::string_view, double*, std::string*);
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bool AbslParseFlag(absl::string_view, std::string*, std::string*);
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bool AbslParseFlag(absl::string_view, std::vector<std::string>*, std::string*);
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template <typename T>
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bool InvokeParseFlag(absl::string_view input, T* dst, std::string* err) {
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// Comment on next line provides a good compiler error message if T
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// does not have AbslParseFlag(absl::string_view, T*, std::string*).
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return AbslParseFlag(input, dst, err); // Is T missing AbslParseFlag?
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}
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// Strings and std:: containers do not have the same overload resolution
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// considerations as fundamental types. Naming these 'AbslUnparseFlag' means we
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// can avoid the need for additional specializations of Unparse (below).
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std::string AbslUnparseFlag(absl::string_view v);
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std::string AbslUnparseFlag(const std::vector<std::string>&);
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template <typename T>
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std::string Unparse(const T& v) {
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// Comment on next line provides a good compiler error message if T does not
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// have UnparseFlag.
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return AbslUnparseFlag(v); // Is T missing AbslUnparseFlag?
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}
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// Overloads for builtin types.
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std::string Unparse(bool v);
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std::string Unparse(short v); // NOLINT
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std::string Unparse(unsigned short v); // NOLINT
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std::string Unparse(int v); // NOLINT
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std::string Unparse(unsigned int v); // NOLINT
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std::string Unparse(long v); // NOLINT
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std::string Unparse(unsigned long v); // NOLINT
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std::string Unparse(long long v); // NOLINT
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std::string Unparse(unsigned long long v); // NOLINT
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std::string Unparse(float v);
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std::string Unparse(double v);
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} // namespace flags_internal
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// ParseFlag()
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//
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// Parses a string value into a flag value of type `T`. Do not add overloads of
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// this function for your type directly; instead, add an `AbslParseFlag()`
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// free function as documented above.
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//
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// Some implementations of `AbslParseFlag()` for types which consist of other,
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// constituent types which already have Abseil flag support, may need to call
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// `absl::ParseFlag()` on those consituent string values. (See above.)
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template <typename T>
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inline bool ParseFlag(absl::string_view input, T* dst, std::string* error) {
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return flags_internal::InvokeParseFlag(input, dst, error);
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}
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// UnparseFlag()
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//
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// Unparses a flag value of type `T` into a string value. Do not add overloads
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// of this function for your type directly; instead, add an `AbslUnparseFlag()`
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// free function as documented above.
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//
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// Some implementations of `AbslUnparseFlag()` for types which consist of other,
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// constituent types which already have Abseil flag support, may want to call
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// `absl::UnparseFlag()` on those constituent types. (See above.)
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template <typename T>
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inline std::string UnparseFlag(const T& v) {
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return flags_internal::Unparse(v);
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}
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// Overloads for `absl::LogSeverity` can't (easily) appear alongside that type's
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// definition because it is layered below flags. See proper documentation in
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// base/log_severity.h.
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enum class LogSeverity : int;
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bool AbslParseFlag(absl::string_view, absl::LogSeverity*, std::string*);
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std::string AbslUnparseFlag(absl::LogSeverity);
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ABSL_NAMESPACE_END
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} // namespace absl
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#endif // ABSL_FLAGS_MARSHALLING_H_
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