clang-tools  7.0.0
Context.h
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1 //===--- Context.h - Mechanism for passing implicit data --------*- C++-*-===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // Context for storing and retrieving implicit data. Useful for passing implicit
11 // parameters on a per-request basis.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_CONTEXT_H_
16 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_CONTEXT_H_
17 
18 #include "llvm/ADT/STLExtras.h"
19 #include "llvm/Support/Compiler.h"
20 #include <memory>
21 #include <type_traits>
22 
23 namespace clang {
24 namespace clangd {
25 
26 /// Values in a Context are indexed by typed keys.
27 /// Key<T> serves two purposes:
28 /// - it provides a lookup key for the context (each Key is unique),
29 /// - it makes lookup type-safe: a Key<T> can only map to a T (or nothing).
30 ///
31 /// Example:
32 /// Key<int> RequestID;
33 /// Key<int> Version;
34 ///
35 /// Context Ctx = Context::empty().derive(RequestID, 10).derive(Version, 3);
36 /// assert(*Ctx.get(RequestID) == 10);
37 /// assert(*Ctx.get(Version) == 3);
38 ///
39 /// Keys are typically used across multiple functions, so most of the time you
40 /// would want to make them static class members or global variables.
41 template <class Type> class Key {
42 public:
43  static_assert(!std::is_reference<Type>::value,
44  "Reference arguments to Key<> are not allowed");
45 
46  constexpr Key() = default;
47 
48  Key(Key const &) = delete;
49  Key &operator=(Key const &) = delete;
50  Key(Key &&) = delete;
51  Key &operator=(Key &&) = delete;
52 };
53 
54 /// A context is an immutable container for per-request data that must be
55 /// propagated through layers that don't care about it. An example is a request
56 /// ID that we may want to use when logging.
57 ///
58 /// Conceptually, a context is a heterogeneous map<Key<T>, T>. Each key has
59 /// an associated value type, which allows the map to be typesafe.
60 ///
61 /// There is an "ambient" context for each thread, Context::current().
62 /// Most functions should read from this, and use WithContextValue or
63 /// WithContext to extend or replace the context within a block scope.
64 /// Only code dealing with threads and extension points should need to use
65 /// other Context objects.
66 ///
67 /// You can't add data to an existing context, instead you create a new
68 /// immutable context derived from it with extra data added. When you retrieve
69 /// data, the context will walk up the parent chain until the key is found.
70 class Context {
71 public:
72  /// Returns an empty root context that contains no data.
73  static Context empty();
74  /// Returns the context for the current thread, creating it if needed.
75  static const Context &current();
76  // Sets the current() context to Replacement, and returns the old context.
77  // Prefer to use WithContext or WithContextValue to do this safely.
78  static Context swapCurrent(Context Replacement);
79 
80 private:
81  struct Data;
82  Context(std::shared_ptr<const Data> DataPtr);
83 
84 public:
85  /// Same as Context::empty(), please use Context::empty() instead.
86  /// Constructor is defined to workaround a bug in MSVC's version of STL.
87  /// (arguments of std::future<> must be default-construcitble in MSVC).
88  Context() = default;
89 
90  /// Copy operations for this class are deleted, use an explicit clone() method
91  /// when you need a copy of the context instead.
92  Context(Context const &) = delete;
93  Context &operator=(const Context &) = delete;
94 
95  Context(Context &&) = default;
96  Context &operator=(Context &&) = default;
97 
98  /// Get data stored for a typed \p Key. If values are not found
99  /// \returns Pointer to the data associated with \p Key. If no data is
100  /// specified for \p Key, return null.
101  template <class Type> const Type *get(const Key<Type> &Key) const {
102  for (const Data *DataPtr = this->DataPtr.get(); DataPtr != nullptr;
103  DataPtr = DataPtr->Parent.get()) {
104  if (DataPtr->KeyPtr == &Key)
105  return static_cast<const Type *>(DataPtr->Value->getValuePtr());
106  }
107  return nullptr;
108  }
109 
110  /// A helper to get a reference to a \p Key that must exist in the map.
111  /// Must not be called for keys that are not in the map.
112  template <class Type> const Type &getExisting(const Key<Type> &Key) const {
113  auto Val = get(Key);
114  assert(Val && "Key does not exist");
115  return *Val;
116  }
117 
118  /// Derives a child context
119  /// It is safe to move or destroy a parent context after calling derive().
120  /// The child will keep its parent alive, and its data remains accessible.
121  template <class Type>
123  typename std::decay<Type>::type Value) const & {
124  return Context(std::make_shared<Data>(Data{
125  /*Parent=*/DataPtr, &Key,
126  llvm::make_unique<TypedAnyStorage<typename std::decay<Type>::type>>(
127  std::move(Value))}));
128  }
129 
130  template <class Type>
131  Context
133  typename std::decay<Type>::type Value) && /* takes ownership */ {
134  return Context(std::make_shared<Data>(Data{
135  /*Parent=*/std::move(DataPtr), &Key,
136  llvm::make_unique<TypedAnyStorage<typename std::decay<Type>::type>>(
137  std::move(Value))}));
138  }
139 
140  /// Derives a child context, using an anonymous key.
141  /// Intended for objects stored only for their destructor's side-effect.
142  template <class Type> Context derive(Type &&Value) const & {
143  static Key<typename std::decay<Type>::type> Private;
144  return derive(Private, std::forward<Type>(Value));
145  }
146 
147  template <class Type> Context derive(Type &&Value) && {
148  static Key<typename std::decay<Type>::type> Private;
149  return std::move(*this).derive(Private, std::forward<Type>(Value));
150  }
151 
152  /// Clone this context object.
153  Context clone() const;
154 
155 private:
156  class AnyStorage {
157  public:
158  virtual ~AnyStorage() = default;
159  virtual void *getValuePtr() = 0;
160  };
161 
162  template <class T> class TypedAnyStorage : public Context::AnyStorage {
163  static_assert(std::is_same<typename std::decay<T>::type, T>::value,
164  "Argument to TypedAnyStorage must be decayed");
165 
166  public:
167  TypedAnyStorage(T &&Value) : Value(std::move(Value)) {}
168 
169  void *getValuePtr() override { return &Value; }
170 
171  private:
172  T Value;
173  };
174 
175  struct Data {
176  // We need to make sure Parent outlives the Value, so the order of members
177  // is important. We do that to allow classes stored in Context's child
178  // layers to store references to the data in the parent layers.
179  std::shared_ptr<const Data> Parent;
180  const void *KeyPtr;
181  std::unique_ptr<AnyStorage> Value;
182  };
183 
184  std::shared_ptr<const Data> DataPtr;
185 };
186 
187 /// WithContext replaces Context::current() with a provided scope.
188 /// When the WithContext is destroyed, the original scope is restored.
189 /// For extending the current context with new value, prefer WithContextValue.
190 class LLVM_NODISCARD WithContext {
191 public:
192  WithContext(Context C) : Restore(Context::swapCurrent(std::move(C))) {}
193  ~WithContext() { Context::swapCurrent(std::move(Restore)); }
194  WithContext(const WithContext &) = delete;
195  WithContext &operator=(const WithContext &) = delete;
196  WithContext(WithContext &&) = delete;
197  WithContext &operator=(WithContext &&) = delete;
198 
199 private:
200  Context Restore;
201 };
202 
203 /// WithContextValue extends Context::current() with a single value.
204 /// When the WithContextValue is destroyed, the original scope is restored.
205 class LLVM_NODISCARD WithContextValue {
206 public:
207  template <typename T>
208  WithContextValue(const Key<T> &K, typename std::decay<T>::type V)
209  : Restore(Context::current().derive(K, std::move(V))) {}
210 
211  // Anonymous values can be used for the destructor side-effect.
212  template <typename T>
214  : Restore(Context::current().derive(std::forward<T>(V))) {}
215 
216 private:
217  WithContext Restore;
218 };
219 
220 } // namespace clangd
221 } // namespace clang
222 
223 #endif // LLVM_CLANG_TOOLS_EXTRA_CLANGD_CONTEXT_H_
Key & operator=(Key const &)=delete
Values in a Context are indexed by typed keys.
Definition: Context.h:41
Context derive(Type &&Value) &&
Definition: Context.h:147
Context derive(const Key< Type > &Key, typename std::decay< Type >::type Value) &&
Definition: Context.h:132
static Context swapCurrent(Context Replacement)
Definition: Context.cpp:30
A context is an immutable container for per-request data that must be propagated through layers that ...
Definition: Context.h:70
WithContext replaces Context::current() with a provided scope.
Definition: Context.h:190
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:112
===– 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:122
WithContextValue(const Key< T > &K, typename std::decay< T >::type V)
Definition: Context.h:208
WithContextValue extends Context::current() with a single value.
Definition: Context.h:205
constexpr Key()=default
Context derive(Type &&Value) const &
Derives a child context, using an anonymous key.
Definition: Context.h:142