clang-tools  16.0.0git
Context.h
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1 //===--- Context.h - Mechanism for passing implicit data --------*- C++-*-===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // Context for storing and retrieving implicit data. Useful for passing implicit
10 // parameters on a per-request basis.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_SUPPORT_CONTEXT_H_
15 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_SUPPORT_CONTEXT_H_
16 
17 #include "llvm/ADT/STLExtras.h"
18 #include "llvm/Support/Compiler.h"
19 #include <memory>
20 #include <type_traits>
21 
22 namespace clang {
23 namespace clangd {
24 
25 /// Values in a Context are indexed by typed keys.
26 /// Key<T> serves two purposes:
27 /// - it provides a lookup key for the context (each Key is unique),
28 /// - it makes lookup type-safe: a Key<T> can only map to a T (or nothing).
29 ///
30 /// Example:
31 /// Key<int> RequestID;
32 /// Key<int> Version;
33 ///
34 /// Context Ctx = Context::empty().derive(RequestID, 10).derive(Version, 3);
35 /// assert(*Ctx.get(RequestID) == 10);
36 /// assert(*Ctx.get(Version) == 3);
37 ///
38 /// Keys are typically used across multiple functions, so most of the time you
39 /// would want to make them static class members or global variables.
40 template <class Type> class Key {
41 public:
42  static_assert(!std::is_reference<Type>::value,
43  "Reference arguments to Key<> are not allowed");
44 
45  constexpr Key() = default;
46 
47  Key(Key const &) = delete;
48  Key &operator=(Key const &) = delete;
49  Key(Key &&) = delete;
50  Key &operator=(Key &&) = delete;
51 };
52 
53 /// A context is an immutable container for per-request data that must be
54 /// propagated through layers that don't care about it. An example is a request
55 /// ID that we may want to use when logging.
56 ///
57 /// Conceptually, a context is a heterogeneous map<Key<T>, T>. Each key has
58 /// an associated value type, which allows the map to be typesafe.
59 ///
60 /// There is an "ambient" context for each thread, Context::current().
61 /// Most functions should read from this, and use WithContextValue or
62 /// WithContext to extend or replace the context within a block scope.
63 /// Only code dealing with threads and extension points should need to use
64 /// other Context objects.
65 ///
66 /// You can't add data to an existing context, instead you create a new
67 /// immutable context derived from it with extra data added. When you retrieve
68 /// data, the context will walk up the parent chain until the key is found.
69 class Context {
70 public:
71  /// Returns an empty root context that contains no data.
72  static Context empty();
73  /// Returns the context for the current thread, creating it if needed.
74  static const Context &current();
75  // Sets the current() context to Replacement, and returns the old context.
76  // Prefer to use WithContext or WithContextValue to do this safely.
77  static Context swapCurrent(Context Replacement);
78 
79 private:
80  struct Data;
81  Context(std::shared_ptr<const Data> DataPtr);
82 
83 public:
84  /// Same as Context::empty(), please use Context::empty() instead.
85  Context() = default;
86 
87  /// Copy operations for this class are deleted, use an explicit clone() method
88  /// when you need a copy of the context instead.
89  Context(Context const &) = delete;
90  Context &operator=(const Context &) = delete;
91 
92  Context(Context &&) = default;
93  Context &operator=(Context &&) = default;
94 
95  /// Get data stored for a typed \p Key. If values are not found
96  /// \returns Pointer to the data associated with \p Key. If no data is
97  /// specified for \p Key, return null.
98  template <class Type> const Type *get(const Key<Type> &Key) const {
99  for (const Data *DataPtr = this->DataPtr.get(); DataPtr != nullptr;
100  DataPtr = DataPtr->Parent.get()) {
101  if (DataPtr->KeyPtr == &Key)
102  return static_cast<const Type *>(DataPtr->Value->getValuePtr());
103  }
104  return nullptr;
105  }
106 
107  /// A helper to get a reference to a \p Key that must exist in the map.
108  /// Must not be called for keys that are not in the map.
109  template <class Type> const Type &getExisting(const Key<Type> &Key) const {
110  auto Val = get(Key);
111  assert(Val && "Key does not exist");
112  return *Val;
113  }
114 
115  /// Derives a child context
116  /// It is safe to move or destroy a parent context after calling derive().
117  /// The child will keep its parent alive, and its data remains accessible.
118  template <class Type>
119  Context derive(const Key<Type> &Key, std::decay_t<Type> Value) const & {
120  return Context(std::make_shared<Data>(
121  Data{/*Parent=*/DataPtr, &Key,
122  std::make_unique<TypedAnyStorage<std::decay_t<Type>>>(
123  std::move(Value))}));
124  }
125 
126  template <class Type>
128  std::decay_t<Type> Value) && /* takes ownership */ {
129  return Context(std::make_shared<Data>(
130  Data{/*Parent=*/std::move(DataPtr), &Key,
131  std::make_unique<TypedAnyStorage<std::decay_t<Type>>>(
132  std::move(Value))}));
133  }
134 
135  /// Derives a child context, using an anonymous key.
136  /// Intended for objects stored only for their destructor's side-effect.
137  template <class Type> Context derive(Type &&Value) const & {
138  static Key<std::decay_t<Type>> Private;
139  return derive(Private, std::forward<Type>(Value));
140  }
141 
142  template <class Type> Context derive(Type &&Value) && {
143  static Key<std::decay_t<Type>> Private;
144  return std::move(*this).derive(Private, std::forward<Type>(Value));
145  }
146 
147  /// Clone this context object.
148  Context clone() const;
149 
150 private:
151  class AnyStorage {
152  public:
153  virtual ~AnyStorage() = default;
154  virtual void *getValuePtr() = 0;
155  };
156 
157  template <class T> class TypedAnyStorage : public Context::AnyStorage {
158  static_assert(std::is_same<typename std::decay<T>::type, T>::value,
159  "Argument to TypedAnyStorage must be decayed");
160 
161  public:
162  TypedAnyStorage(T &&Value) : Value(std::move(Value)) {}
163 
164  void *getValuePtr() override { return &Value; }
165 
166  private:
167  T Value;
168  };
169 
170  struct Data {
171  // We need to make sure Parent outlives the Value, so the order of members
172  // is important. We do that to allow classes stored in Context's child
173  // layers to store references to the data in the parent layers.
174  std::shared_ptr<const Data> Parent;
175  const void *KeyPtr;
176  std::unique_ptr<AnyStorage> Value;
177  };
178 
179  std::shared_ptr<const Data> DataPtr;
180 };
181 
182 /// WithContext replaces Context::current() with a provided scope.
183 /// When the WithContext is destroyed, the original scope is restored.
184 /// For extending the current context with new value, prefer WithContextValue.
185 class [[nodiscard]] WithContext {
186 public:
187  WithContext(Context C) : Restore(Context::swapCurrent(std::move(C))) {}
188  ~WithContext() { Context::swapCurrent(std::move(Restore)); }
189  WithContext(const WithContext &) = delete;
190  WithContext &operator=(const WithContext &) = delete;
191  WithContext(WithContext &&) = delete;
192  WithContext &operator=(WithContext &&) = delete;
193 
194 private:
195  Context Restore;
196 };
197 
198 /// WithContextValue extends Context::current() with a single value.
199 /// When the WithContextValue is destroyed, the original scope is restored.
200 class [[nodiscard]] WithContextValue {
201 public:
202  template <typename T>
203  WithContextValue(const Key<T> &K, typename std::decay<T>::type V)
204  : Restore(Context::current().derive(K, std::move(V))) {}
205 
206  // Anonymous values can be used for the destructor side-effect.
207  template <typename T>
209  : Restore(Context::current().derive(std::forward<T>(V))) {}
210 
211 private:
212  WithContext Restore;
213 };
214 
215 } // namespace clangd
216 } // namespace clang
217 
218 #endif
clang::clangd::WithContextValue::WithContextValue
WithContextValue(const Key< T > &K, typename std::decay< T >::type V)
Definition: Context.h:203
clang::clangd::Context::derive
Context derive(Type &&Value) const &
Derives a child context, using an anonymous key.
Definition: Context.h:137
Type
NodeType Type
Definition: HTMLGenerator.cpp:73
clang::clangd::Context::current
static const Context & current()
Returns the context for the current thread, creating it if needed.
Definition: Context.cpp:27
clang::clangd::Context::clone
Context clone() const
Clone this context object.
Definition: Context.cpp:20
clang::clangd::Context::get
const Type * get(const Key< Type > &Key) const
Get data stored for a typed Key.
Definition: Context.h:98
clang::clangd::WithContext::~WithContext
~WithContext()
Definition: Context.h:188
clang::clangd::Key::operator=
Key & operator=(Key const &)=delete
clang::clangd::Context::derive
Context derive(const Key< Type > &Key, std::decay_t< Type > Value) const &
Derives a child context It is safe to move or destroy a parent context after calling derive().
Definition: Context.h:119
clang::clangd::WithContextValue
WithContextValue extends Context::current() with a single value.
Definition: Context.h:200
clang::clangd::Context::Context
Context()=default
Same as Context::empty(), please use Context::empty() instead.
clang::clangd::Key
Values in a Context are indexed by typed keys.
Definition: Context.h:40
clang::clangd::Context::swapCurrent
static Context swapCurrent(Context Replacement)
Definition: Context.cpp:29
clang::clangd::WithContextValue::WithContextValue
WithContextValue(T &&V)
Definition: Context.h:208
clang::clangd::WithContext
WithContext replaces Context::current() with a provided scope.
Definition: Context.h:185
clang::clangd::WithContext::WithContext
WithContext(Context C)
Definition: Context.h:187
Parent
const Node * Parent
Definition: ExtractFunction.cpp:157
clang::clangd::Context::operator=
Context & operator=(const Context &)=delete
C
const Criteria C
Definition: FunctionCognitiveComplexityCheck.cpp:93
clang::clangd::Context::empty
static Context empty()
Returns an empty root context that contains no data.
Definition: Context.cpp:15
clang
===– Representation.cpp - ClangDoc Representation --------—*- C++ -*-===//
Definition: ApplyReplacements.h:27
clang::clangd::Context::derive
Context derive(Type &&Value) &&
Definition: Context.h:142
clang::clangd::Context::derive
Context derive(const Key< Type > &Key, std::decay_t< Type > Value) &&
Definition: Context.h:127
clang::clangd::Key::Key
constexpr Key()=default
clang::clangd::Context::getExisting
const Type & getExisting(const Key< Type > &Key) const
A helper to get a reference to a Key that must exist in the map.
Definition: Context.h:109
clang::clangd::Context
A context is an immutable container for per-request data that must be propagated through layers that ...
Definition: Context.h:69
K
Kind K
Definition: Rename.cpp:436
Value
static constexpr bool Value
Definition: SuspiciousCallArgumentCheck.cpp:72