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Diffstat (limited to 'absl/flags/marshalling.h')
| -rw-r--r-- | absl/flags/marshalling.h | 255 |
1 files changed, 255 insertions, 0 deletions
diff --git a/absl/flags/marshalling.h b/absl/flags/marshalling.h new file mode 100644 index 00000000..5598d444 --- /dev/null +++ b/absl/flags/marshalling.h @@ -0,0 +1,255 @@ +// +// 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. +// +// ----------------------------------------------------------------------------- +// File: marshalling.h +// ----------------------------------------------------------------------------- +// +// This header file defines the API for extending Abseil flag support to +// custom types, and defines the set of overloads for fundamental types. +// +// Out of the box, the Abseil flags library supports the following types: +// +// * `bool` +// * `int16_t` +// * `uint16_t` +// * `int32_t` +// * `uint32_t` +// * `int64_t` +// * `uint64_t` +// * `float` +// * `double` +// * `std::string` +// * `std::vector<std::string>` +// +// Note that support for integral types is implemented using overloads for +// variable-width fundamental types (`short`, `int`, `long`, etc.). However, +// you should prefer the fixed-width integral types (`int32_t`, `uint64_t`, +// etc.) we've noted above within flag definitions. + +// +// In addition, several Abseil libraries provide their own custom support for +// Abseil flags. +// +// The Abseil time library provides the following support for civil time values: +// +// * `absl::CivilSecond` +// * `absl::CivilMinute` +// * `absl::CivilHour` +// * `absl::CivilDay` +// * `absl::CivilMonth` +// * `absl::CivilYear` +// +// and also provides support for the following absolute time values: +// +// * `absl::Duration` +// * `absl::Time` +// +// Additional support for Abseil types will be noted here as it is added. +// +// You can also provide your own custom flags by adding overloads for +// `AbslParseFlag()` and `AbslUnparseFlag()` to your type definitions. (See +// below.) +// +// ----------------------------------------------------------------------------- +// Adding Type Support for Abseil Flags +// ----------------------------------------------------------------------------- +// +// To add support for your user-defined type, add overloads of `AbslParseFlag()` +// and `AbslUnparseFlag()` as free (non-member) functions to your type. If `T` +// is a class type, these functions can be friend function definitions. These +// overloads must be added to the same namespace where the type is defined, so +// that they can be discovered by Argument-Dependent Lookup (ADL). +// +// Example: +// +// namespace foo { +// +// enum OutputMode { kPlainText, kHtml }; +// +// // AbslParseFlag converts from a string to OutputMode. +// // Must be in same namespace as OutputMode. +// +// // Parses an OutputMode from the command line flag value `text. Returns +// // `true` and sets `*mode` on success; returns `false` and sets `*error` +// // on failure. +// bool AbslParseFlag(absl::string_view text, +// OutputMode* mode, +// std::string* error) { +// if (text == "plaintext") { +// *mode = kPlainText; +// return true; +// } +// if (text == "html") { +// *mode = kHtml; +// return true; +// } +// *error = "unknown value for enumeration"; +// return false; +// } +// +// // AbslUnparseFlag converts from an OutputMode to a string. +// // Must be in same namespace as OutputMode. +// +// // Returns a textual flag value corresponding to the OutputMode `mode`. +// std::string AbslUnparseFlag(OutputMode mode) { +// switch (mode) { +// case kPlainText: return "plaintext"; +// case kHtml: return "html"; +// } +// return absl::StrCat(mode); +// } +// +// Notice that neither `AbslParseFlag()` nor `AbslUnparseFlag()` are class +// members, but free functions. `AbslParseFlag/AbslUnparseFlag()` overloads +// for a type should only be declared in the same file and namespace as said +// type. The proper `AbslParseFlag/AbslUnparseFlag()` implementations for a +// given type will be discovered via Argument-Dependent Lookup (ADL). +// +// `AbslParseFlag()` may need, in turn, to parse simpler constituent types +// using `absl::ParseFlag()`. For example, a custom struct `MyFlagType` +// consisting of a `std::pair<int, std::string>` would add an `AbslParseFlag()` +// overload for its `MyFlagType` like so: +// +// Example: +// +// namespace my_flag_type { +// +// struct MyFlagType { +// std::pair<int, std::string> my_flag_data; +// }; +// +// bool AbslParseFlag(absl::string_view text, MyFlagType* flag, +// std::string* err); +// +// std::string AbslUnparseFlag(const MyFlagType&); +// +// // Within the implementation, `AbslParseFlag()` will, in turn invoke +// // `absl::ParseFlag()` on its constituent `int` and `std::string` types +// // (which have built-in Abseil flag support. +// +// bool AbslParseFlag(absl::string_view text, MyFlagType* flag, +// std::string* err) { +// std::pair<absl::string_view, absl::string_view> tokens = +// absl::StrSplit(text, ','); +// if (!absl::ParseFlag(tokens.first, &flag->my_flag_data.first, err)) +// return false; +// if (!absl::ParseFlag(tokens.second, &flag->my_flag_data.second, err)) +// return false; +// return true; +// } +// +// // Similarly, for unparsing, we can simply invoke `absl::UnparseFlag()` on +// // the constituent types. +// std::string AbslUnparseFlag(const MyFlagType& flag) { +// return absl::StrCat(absl::UnparseFlag(flag.my_flag_data.first), +// ",", +// absl::UnparseFlag(flag.my_flag_data.second)); +// } +#ifndef ABSL_FLAGS_MARSHALLING_H_ +#define ABSL_FLAGS_MARSHALLING_H_ + +#include <string> +#include <vector> + +#include "absl/strings/string_view.h" + +namespace absl { +inline namespace lts_2019_08_08 { +namespace flags_internal { + +// Overloads of `AbslParseFlag()` and `AbslUnparseFlag()` for fundamental types. +bool AbslParseFlag(absl::string_view, bool*, std::string*); +bool AbslParseFlag(absl::string_view, short*, std::string*); // NOLINT +bool AbslParseFlag(absl::string_view, unsigned short*, std::string*); // NOLINT +bool AbslParseFlag(absl::string_view, int*, std::string*); // NOLINT +bool AbslParseFlag(absl::string_view, unsigned int*, std::string*); // NOLINT +bool AbslParseFlag(absl::string_view, long*, std::string*); // NOLINT +bool AbslParseFlag(absl::string_view, unsigned long*, std::string*); // NOLINT +bool AbslParseFlag(absl::string_view, long long*, std::string*); // NOLINT +bool AbslParseFlag(absl::string_view, unsigned long long*, + std::string*); // NOLINT +bool AbslParseFlag(absl::string_view, float*, std::string*); +bool AbslParseFlag(absl::string_view, double*, std::string*); +bool AbslParseFlag(absl::string_view, std::string*, std::string*); +bool AbslParseFlag(absl::string_view, std::vector<std::string>*, std::string*); + +template <typename T> +bool InvokeParseFlag(absl::string_view input, T* dst, std::string* err) { + // Comment on next line provides a good compiler error message if T + // does not have AbslParseFlag(absl::string_view, T*, std::string*). + return AbslParseFlag(input, dst, err); // Is T missing AbslParseFlag? +} + +// Strings and std:: containers do not have the same overload resolution +// considerations as fundamental types. Naming these 'AbslUnparseFlag' means we +// can avoid the need for additional specializations of Unparse (below). +std::string AbslUnparseFlag(absl::string_view v); +std::string AbslUnparseFlag(const std::vector<std::string>&); + +template <typename T> +std::string Unparse(const T& v) { + // Comment on next line provides a good compiler error message if T does not + // have UnparseFlag. + return AbslUnparseFlag(v); // Is T missing AbslUnparseFlag? +} + +// Overloads for builtin types. +std::string Unparse(bool v); +std::string Unparse(short v); // NOLINT +std::string Unparse(unsigned short v); // NOLINT +std::string Unparse(int v); // NOLINT +std::string Unparse(unsigned int v); // NOLINT +std::string Unparse(long v); // NOLINT +std::string Unparse(unsigned long v); // NOLINT +std::string Unparse(long long v); // NOLINT +std::string Unparse(unsigned long long v); // NOLINT +std::string Unparse(float v); +std::string Unparse(double v); + +} // namespace flags_internal + +// ParseFlag() +// +// Parses a string value into a flag value of type `T`. Do not add overloads of +// this function for your type directly; instead, add an `AbslParseFlag()` +// free function as documented above. +// +// Some implementations of `AbslParseFlag()` for types which consist of other, +// constituent types which already have Abseil flag support, may need to call +// `absl::ParseFlag()` on those consituent string values. (See above.) +template <typename T> +inline bool ParseFlag(absl::string_view input, T* dst, std::string* error) { + return flags_internal::InvokeParseFlag(input, dst, error); +} + +// UnparseFlag() +// +// Unparses a flag value of type `T` into a string value. Do not add overloads +// of this function for your type directly; instead, add an `AbslUnparseFlag()` +// free function as documented above. +// +// Some implementations of `AbslUnparseFlag()` for types which consist of other, +// constituent types which already have Abseil flag support, may want to call +// `absl::UnparseFlag()` on those constituent types. (See above.) +template <typename T> +inline std::string UnparseFlag(const T& v) { + return flags_internal::Unparse(v); +} + +} // inline namespace lts_2019_08_08 +} // namespace absl + +#endif // ABSL_FLAGS_MARSHALLING_H_ |