/src/abseil-cpp/absl/flags/commandlineflag.h
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1 | | // |
2 | | // Copyright 2020 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: commandlineflag.h |
18 | | // ----------------------------------------------------------------------------- |
19 | | // |
20 | | // This header file defines the `CommandLineFlag`, which acts as a type-erased |
21 | | // handle for accessing metadata about the Abseil Flag in question. |
22 | | // |
23 | | // Because an actual Abseil flag is of an unspecified type, you should not |
24 | | // manipulate or interact directly with objects of that type. Instead, use the |
25 | | // CommandLineFlag type as an intermediary. |
26 | | #ifndef ABSL_FLAGS_COMMANDLINEFLAG_H_ |
27 | | #define ABSL_FLAGS_COMMANDLINEFLAG_H_ |
28 | | |
29 | | #include <memory> |
30 | | #include <string> |
31 | | |
32 | | #include "absl/base/config.h" |
33 | | #include "absl/base/internal/fast_type_id.h" |
34 | | #include "absl/flags/internal/commandlineflag.h" |
35 | | #include "absl/strings/string_view.h" |
36 | | #include "absl/types/optional.h" |
37 | | |
38 | | namespace absl { |
39 | | ABSL_NAMESPACE_BEGIN |
40 | | namespace flags_internal { |
41 | | class PrivateHandleAccessor; |
42 | | } // namespace flags_internal |
43 | | |
44 | | // CommandLineFlag |
45 | | // |
46 | | // This type acts as a type-erased handle for an instance of an Abseil Flag and |
47 | | // holds reflection information pertaining to that flag. Use CommandLineFlag to |
48 | | // access a flag's name, location, help string etc. |
49 | | // |
50 | | // To obtain an absl::CommandLineFlag, invoke `absl::FindCommandLineFlag()` |
51 | | // passing it the flag name string. |
52 | | // |
53 | | // Example: |
54 | | // |
55 | | // // Obtain reflection handle for a flag named "flagname". |
56 | | // const absl::CommandLineFlag* my_flag_data = |
57 | | // absl::FindCommandLineFlag("flagname"); |
58 | | // |
59 | | // // Now you can get flag info from that reflection handle. |
60 | | // std::string flag_location = my_flag_data->Filename(); |
61 | | // ... |
62 | | |
63 | | // These are only used as constexpr global objects. |
64 | | // They do not use a virtual destructor to simplify their implementation. |
65 | | // They are not destroyed except at program exit, so leaks do not matter. |
66 | | #if defined(__GNUC__) && !defined(__clang__) |
67 | | #pragma GCC diagnostic push |
68 | | #pragma GCC diagnostic ignored "-Wnon-virtual-dtor" |
69 | | #endif |
70 | | class CommandLineFlag { |
71 | | public: |
72 | 0 | constexpr CommandLineFlag() = default; |
73 | | |
74 | | // Not copyable/assignable. |
75 | | CommandLineFlag(const CommandLineFlag&) = delete; |
76 | | CommandLineFlag& operator=(const CommandLineFlag&) = delete; |
77 | | |
78 | | // absl::CommandLineFlag::IsOfType() |
79 | | // |
80 | | // Return true iff flag has type T. |
81 | | template <typename T> |
82 | | inline bool IsOfType() const { |
83 | | return TypeId() == base_internal::FastTypeId<T>(); |
84 | | } |
85 | | |
86 | | // absl::CommandLineFlag::TryGet() |
87 | | // |
88 | | // Attempts to retrieve the flag value. Returns value on success, |
89 | | // absl::nullopt otherwise. |
90 | | template <typename T> |
91 | | absl::optional<T> TryGet() const { |
92 | | if (IsRetired() || !IsOfType<T>()) { |
93 | | return absl::nullopt; |
94 | | } |
95 | | |
96 | | // Implementation notes: |
97 | | // |
98 | | // We are wrapping a union around the value of `T` to serve three purposes: |
99 | | // |
100 | | // 1. `U.value` has correct size and alignment for a value of type `T` |
101 | | // 2. The `U.value` constructor is not invoked since U's constructor does |
102 | | // not do it explicitly. |
103 | | // 3. The `U.value` destructor is invoked since U's destructor does it |
104 | | // explicitly. This makes `U` a kind of RAII wrapper around non default |
105 | | // constructible value of T, which is destructed when we leave the |
106 | | // scope. We do need to destroy U.value, which is constructed by |
107 | | // CommandLineFlag::Read even though we left it in a moved-from state |
108 | | // after std::move. |
109 | | // |
110 | | // All of this serves to avoid requiring `T` being default constructible. |
111 | | union U { |
112 | | T value; |
113 | | U() {} |
114 | | ~U() { value.~T(); } |
115 | | }; |
116 | | U u; |
117 | | |
118 | | Read(&u.value); |
119 | | // allow retired flags to be "read", so we can report invalid access. |
120 | | if (IsRetired()) { |
121 | | return absl::nullopt; |
122 | | } |
123 | | return std::move(u.value); |
124 | | } |
125 | | |
126 | | // absl::CommandLineFlag::Name() |
127 | | // |
128 | | // Returns name of this flag. |
129 | | virtual absl::string_view Name() const = 0; |
130 | | |
131 | | // absl::CommandLineFlag::Filename() |
132 | | // |
133 | | // Returns name of the file where this flag is defined. |
134 | | virtual std::string Filename() const = 0; |
135 | | |
136 | | // absl::CommandLineFlag::Help() |
137 | | // |
138 | | // Returns help message associated with this flag. |
139 | | virtual std::string Help() const = 0; |
140 | | |
141 | | // absl::CommandLineFlag::IsRetired() |
142 | | // |
143 | | // Returns true iff this object corresponds to retired flag. |
144 | | virtual bool IsRetired() const; |
145 | | |
146 | | // absl::CommandLineFlag::DefaultValue() |
147 | | // |
148 | | // Returns the default value for this flag. |
149 | | virtual std::string DefaultValue() const = 0; |
150 | | |
151 | | // absl::CommandLineFlag::CurrentValue() |
152 | | // |
153 | | // Returns the current value for this flag. |
154 | | virtual std::string CurrentValue() const = 0; |
155 | | |
156 | | // absl::CommandLineFlag::ParseFrom() |
157 | | // |
158 | | // Sets the value of the flag based on specified string `value`. If the flag |
159 | | // was successfully set to new value, it returns true. Otherwise, sets `error` |
160 | | // to indicate the error, leaves the flag unchanged, and returns false. |
161 | | bool ParseFrom(absl::string_view value, std::string* error); |
162 | | |
163 | | protected: |
164 | | ~CommandLineFlag() = default; |
165 | | |
166 | | private: |
167 | | friend class flags_internal::PrivateHandleAccessor; |
168 | | |
169 | | // Sets the value of the flag based on specified string `value`. If the flag |
170 | | // was successfully set to new value, it returns true. Otherwise, sets `error` |
171 | | // to indicate the error, leaves the flag unchanged, and returns false. There |
172 | | // are three ways to set the flag's value: |
173 | | // * Update the current flag value |
174 | | // * Update the flag's default value |
175 | | // * Update the current flag value if it was never set before |
176 | | // The mode is selected based on `set_mode` parameter. |
177 | | virtual bool ParseFrom(absl::string_view value, |
178 | | flags_internal::FlagSettingMode set_mode, |
179 | | flags_internal::ValueSource source, |
180 | | std::string& error) = 0; |
181 | | |
182 | | // Returns id of the flag's value type. |
183 | | virtual flags_internal::FlagFastTypeId TypeId() const = 0; |
184 | | |
185 | | // Interface to save flag to some persistent state. Returns current flag state |
186 | | // or nullptr if flag does not support saving and restoring a state. |
187 | | virtual std::unique_ptr<flags_internal::FlagStateInterface> SaveState() = 0; |
188 | | |
189 | | // Copy-construct a new value of the flag's type in a memory referenced by |
190 | | // the dst based on the current flag's value. |
191 | | virtual void Read(void* dst) const = 0; |
192 | | |
193 | | // To be deleted. Used to return true if flag's current value originated from |
194 | | // command line. |
195 | | virtual bool IsSpecifiedOnCommandLine() const = 0; |
196 | | |
197 | | // Validates supplied value using validator or parseflag routine |
198 | | virtual bool ValidateInputValue(absl::string_view value) const = 0; |
199 | | |
200 | | // Checks that flags default value can be converted to string and back to the |
201 | | // flag's value type. |
202 | | virtual void CheckDefaultValueParsingRoundtrip() const = 0; |
203 | | }; |
204 | | #if defined(__GNUC__) && !defined(__clang__) |
205 | | #pragma GCC diagnostic pop |
206 | | #endif |
207 | | |
208 | | ABSL_NAMESPACE_END |
209 | | } // namespace absl |
210 | | |
211 | | #endif // ABSL_FLAGS_COMMANDLINEFLAG_H_ |