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//===--- Context.h - Mechanism for passing implicit data --------*- C++-*-===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// Context for storing and retrieving implicit data. Useful for passing implicit
// parameters on a per-request basis.
#include "llvm/ADT/STLExtras.h"
#include "llvm/Support/Compiler.h"
#include <memory>
#include <type_traits>
namespace clang {
namespace clangd {
/// Values in a Context are indexed by typed keys.
/// Key<T> serves two purposes:
/// - it provides a lookup key for the context (each Key is unique),
/// - it makes lookup type-safe: a Key<T> can only map to a T (or nothing).
/// Example:
/// Key<int> RequestID;
/// Key<int> Version;
/// Context Ctx = Context::empty().derive(RequestID, 10).derive(Version, 3);
/// assert(*Ctx.get(RequestID) == 10);
/// assert(*Ctx.get(Version) == 3);
/// Keys are typically used across multiple functions, so most of the time you
/// would want to make them static class members or global variables.
template <class Type> class Key {
"Reference arguments to Key<> are not allowed");
constexpr Key() = default;
Key(Key const &) = delete;
Key &operator=(Key const &) = delete;
Key(Key &&) = delete;
Key &operator=(Key &&) = delete;
/// A context is an immutable container for per-request data that must be
/// propagated through layers that don't care about it. An example is a request
/// ID that we may want to use when logging.
/// Conceptually, a context is a heterogeneous map<Key<T>, T>. Each key has
/// an associated value type, which allows the map to be typesafe.
/// There is an "ambient" context for each thread, Context::current().
/// Most functions should read from this, and use WithContextValue or
/// WithContext to extend or replace the context within a block scope.
/// Only code dealing with threads and extension points should need to use
/// other Context objects.
/// You can't add data to an existing context, instead you create a new
/// immutable context derived from it with extra data added. When you retrieve
/// data, the context will walk up the parent chain until the key is found.
class Context {
/// Returns an empty root context that contains no data.
static Context empty();
/// Returns the context for the current thread, creating it if needed.
static const Context &current();
// Sets the current() context to Replacement, and returns the old context.
// Prefer to use WithContext or WithContextValue to do this safely.
static Context swapCurrent(Context Replacement);
struct Data;
Context(std::shared_ptr<const Data> DataPtr);
/// Same as Context::empty(), please use Context::empty() instead.
/// Constructor is defined to workaround a bug in MSVC's version of STL.
/// (arguments of std::future<> must be default-construcitble in MSVC).
Context() = default;
/// Copy operations for this class are deleted, use an explicit clone() method
/// when you need a copy of the context instead.
Context(Context const &) = delete;
Context &operator=(const Context &) = delete;
Context(Context &&) = default;
Context &operator=(Context &&) = default;
/// Get data stored for a typed \p Key. If values are not found
/// \returns Pointer to the data associated with \p Key. If no data is
/// specified for \p Key, return null.
template <class Type> const Type *get(const Key<Type> &Key) const {
for (const Data *DataPtr = this->DataPtr.get(); DataPtr != nullptr;
DataPtr = DataPtr->Parent.get()) {
if (DataPtr->KeyPtr == &Key)
return static_cast<const Type *>(DataPtr->Value->getValuePtr());
return nullptr;
/// A helper to get a reference to a \p Key that must exist in the map.
/// Must not be called for keys that are not in the map.
template <class Type> const Type &getExisting(const Key<Type> &Key) const {
auto Val = get(Key);
assert(Val && "Key does not exist");
return *Val;
/// Derives a child context
/// It is safe to move or destroy a parent context after calling derive().
/// The child will keep its parent alive, and its data remains accessible.
template <class Type>
Context derive(const Key<Type> &Key,
typename std::decay<Type>::type Value) const & {
return Context(std::make_shared<Data>(Data{
/*Parent=*/DataPtr, &Key,
llvm::make_unique<TypedAnyStorage<typename std::decay<Type>::type>>(
template <class Type>
derive(const Key<Type> &Key,
typename std::decay<Type>::type Value) && /* takes ownership */ {
return Context(std::make_shared<Data>(Data{
/*Parent=*/std::move(DataPtr), &Key,
llvm::make_unique<TypedAnyStorage<typename std::decay<Type>::type>>(
/// Derives a child context, using an anonymous key.
/// Intended for objects stored only for their destructor's side-effect.
template <class Type> Context derive(Type &&Value) const & {
static Key<typename std::decay<Type>::type> Private;
return derive(Private, std::forward<Type>(Value));
template <class Type> Context derive(Type &&Value) && {
static Key<typename std::decay<Type>::type> Private;
return std::move(*this).derive(Private, std::forward<Type>(Value));
/// Clone this context object.
Context clone() const;
class AnyStorage {
virtual ~AnyStorage() = default;
virtual void *getValuePtr() = 0;
template <class T> class TypedAnyStorage : public Context::AnyStorage {
static_assert(std::is_same<typename std::decay<T>::type, T>::value,
"Argument to TypedAnyStorage must be decayed");
TypedAnyStorage(T &&Value) : Value(std::move(Value)) {}
void *getValuePtr() override { return &Value; }
T Value;
struct Data {
// We need to make sure Parent outlives the Value, so the order of members
// is important. We do that to allow classes stored in Context's child
// layers to store references to the data in the parent layers.
std::shared_ptr<const Data> Parent;
const void *KeyPtr;
std::unique_ptr<AnyStorage> Value;
std::shared_ptr<const Data> DataPtr;
/// WithContext replaces Context::current() with a provided scope.
/// When the WithContext is destroyed, the original scope is restored.
/// For extending the current context with new value, prefer WithContextValue.
class LLVM_NODISCARD WithContext {
WithContext(Context C) : Restore(Context::swapCurrent(std::move(C))) {}
~WithContext() { Context::swapCurrent(std::move(Restore)); }
WithContext(const WithContext &) = delete;
WithContext &operator=(const WithContext &) = delete;
WithContext(WithContext &&) = delete;
WithContext &operator=(WithContext &&) = delete;
Context Restore;
/// WithContextValue extends Context::current() with a single value.
/// When the WithContextValue is destroyed, the original scope is restored.
class LLVM_NODISCARD WithContextValue {
template <typename T>
WithContextValue(const Key<T> &K, typename std::decay<T>::type V)
: Restore(Context::current().derive(K, std::move(V))) {}
// Anonymous values can be used for the destructor side-effect.
template <typename T>
WithContextValue(T &&V)
: Restore(Context::current().derive(std::forward<T>(V))) {}
WithContext Restore;
} // namespace clangd
} // namespace clang