clang-tools  12.0.0git
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  /// Constructor is defined to workaround a bug in MSVC's version of STL.
86  /// (arguments of std::future<> must be default-constructible in MSVC).
87  Context() = default;
88 
89  /// Copy operations for this class are deleted, use an explicit clone() method
90  /// when you need a copy of the context instead.
91  Context(Context const &) = delete;
92  Context &operator=(const Context &) = delete;
93 
94  Context(Context &&) = default;
95  Context &operator=(Context &&) = default;
96 
97  /// Get data stored for a typed \p Key. If values are not found
98  /// \returns Pointer to the data associated with \p Key. If no data is
99  /// specified for \p Key, return null.
100  template <class Type> const Type *get(const Key<Type> &Key) const {
101  for (const Data *DataPtr = this->DataPtr.get(); DataPtr != nullptr;
102  DataPtr = DataPtr->Parent.get()) {
103  if (DataPtr->KeyPtr == &Key)
104  return static_cast<const Type *>(DataPtr->Value->getValuePtr());
105  }
106  return nullptr;
107  }
108 
109  /// A helper to get a reference to a \p Key that must exist in the map.
110  /// Must not be called for keys that are not in the map.
111  template <class Type> const Type &getExisting(const Key<Type> &Key) const {
112  auto Val = get(Key);
113  assert(Val && "Key does not exist");
114  return *Val;
115  }
116 
117  /// Derives a child context
118  /// It is safe to move or destroy a parent context after calling derive().
119  /// The child will keep its parent alive, and its data remains accessible.
120  template <class Type>
122  typename std::decay<Type>::type Value) const & {
123  return Context(std::make_shared<Data>(
124  Data{/*Parent=*/DataPtr, &Key,
125  std::make_unique<TypedAnyStorage<typename std::decay<Type>::type>>(
126  std::move(Value))}));
127  }
128 
129  template <class Type>
130  Context
132  typename std::decay<Type>::type Value) && /* takes ownership */ {
133  return Context(std::make_shared<Data>(
134  Data{/*Parent=*/std::move(DataPtr), &Key,
135  std::make_unique<TypedAnyStorage<typename std::decay<Type>::type>>(
136  std::move(Value))}));
137  }
138 
139  /// Derives a child context, using an anonymous key.
140  /// Intended for objects stored only for their destructor's side-effect.
141  template <class Type> Context derive(Type &&Value) const & {
142  static Key<typename std::decay<Type>::type> Private;
143  return derive(Private, std::forward<Type>(Value));
144  }
145 
146  template <class Type> Context derive(Type &&Value) && {
147  static Key<typename std::decay<Type>::type> Private;
148  return std::move(*this).derive(Private, std::forward<Type>(Value));
149  }
150 
151  /// Clone this context object.
152  Context clone() const;
153 
154 private:
155  class AnyStorage {
156  public:
157  virtual ~AnyStorage() = default;
158  virtual void *getValuePtr() = 0;
159  };
160 
161  template <class T> class TypedAnyStorage : public Context::AnyStorage {
162  static_assert(std::is_same<typename std::decay<T>::type, T>::value,
163  "Argument to TypedAnyStorage must be decayed");
164 
165  public:
166  TypedAnyStorage(T &&Value) : Value(std::move(Value)) {}
167 
168  void *getValuePtr() override { return &Value; }
169 
170  private:
171  T Value;
172  };
173 
174  struct Data {
175  // We need to make sure Parent outlives the Value, so the order of members
176  // is important. We do that to allow classes stored in Context's child
177  // layers to store references to the data in the parent layers.
178  std::shared_ptr<const Data> Parent;
179  const void *KeyPtr;
180  std::unique_ptr<AnyStorage> Value;
181  };
182 
183  std::shared_ptr<const Data> DataPtr;
184 };
185 
186 /// WithContext replaces Context::current() with a provided scope.
187 /// When the WithContext is destroyed, the original scope is restored.
188 /// For extending the current context with new value, prefer WithContextValue.
189 class LLVM_NODISCARD WithContext {
190 public:
191  WithContext(Context C) : Restore(Context::swapCurrent(std::move(C))) {}
192  ~WithContext() { Context::swapCurrent(std::move(Restore)); }
193  WithContext(const WithContext &) = delete;
194  WithContext &operator=(const WithContext &) = delete;
195  WithContext(WithContext &&) = delete;
196  WithContext &operator=(WithContext &&) = delete;
197 
198 private:
199  Context Restore;
200 };
201 
202 /// WithContextValue extends Context::current() with a single value.
203 /// When the WithContextValue is destroyed, the original scope is restored.
204 class LLVM_NODISCARD WithContextValue {
205 public:
206  template <typename T>
207  WithContextValue(const Key<T> &K, typename std::decay<T>::type V)
208  : Restore(Context::current().derive(K, std::move(V))) {}
209 
210  // Anonymous values can be used for the destructor side-effect.
211  template <typename T>
213  : Restore(Context::current().derive(std::forward<T>(V))) {}
214 
215 private:
216  WithContext Restore;
217 };
218 
219 } // namespace clangd
220 } // namespace clang
221 
222 #endif
const Node * Parent
Key & operator=(Key const &)=delete
Values in a Context are indexed by typed keys.
Definition: Context.h:40
Context derive(Type &&Value) &&
Definition: Context.h:146
Context derive(const Key< Type > &Key, typename std::decay< Type >::type Value) &&
Definition: Context.h:131
static Context swapCurrent(Context Replacement)
Definition: Context.cpp:29
A context is an immutable container for per-request data that must be propagated through layers that ...
Definition: Context.h:69
WithContext replaces Context::current() with a provided scope.
Definition: Context.h:189
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:111
===– Representation.cpp - ClangDoc Representation --------—*- C++ -*-===//
Context derive(const Key< Type > &Key, typename std::decay< Type >::type Value) const &
Derives a child context It is safe to move or destroy a parent context after calling derive()...
Definition: Context.h:121
WithContextValue(const Key< T > &K, typename std::decay< T >::type V)
Definition: Context.h:207
WithContextValue extends Context::current() with a single value.
Definition: Context.h:204
constexpr Key()=default
NodeType Type
Context derive(Type &&Value) const &
Derives a child context, using an anonymous key.
Definition: Context.h:141